basics of structural design

Basis of Structural Design

Lecture notes

Aurel Stratan

Timişoara 2014

Contents

COURSE 1. INTRODUCTION TO STRUCTURES. STRUCTURAL MATERIALS.

COURSE 2. STRUCTURAL ACTION: CABLES AND ARCHES.

COURSE 3. STRUCTURAL ACTION: TRUSSES AND BEAMS.

COURSE 4. STRUCTURAL ACTION: PRESTRESSING. PLATE AND SHELL STRUCTURES.

COURSE 5. STRUCTURAL ACTION: CABLE STRUCTURES. MULTI-STOREY STRUCTURES.

COURSE 6. STRUCTURAL ACTION: FOUNDATIONS. GENERAL REMARKS ON STRUCTURAL ACTION

COURSE 7. THE PROCESS OF STRUCTURAL DESIGN. LOAD PATHS.

COURSE 8. DESIGN CODES: STRUCTURAL EUROCODES. CURRENT ROMANIAN CODES.

COURSE 9. ACTIONS ON STRUCTURES: PERMANENT LOADS, IMPOSED LOADS AND SNOW LOADS.

COURSE 10. ACTIONS ON STRUCTURES: WIND LOADS. OTHER LOADS.

COURSE 11. EN 1990: DESIGN REQUIREMENTS. THE LIMIT STATE METHOD.

COURSE 12. EN 1990: BASIC VARIABLES. THE PARTIAL FACTOR METHOD.

COURSE 13. EN 1990: THE PARTIAL FACTOR METHOD (CONT.).

COURSE 14. BUILDING DESIGN, CONSTRUCTION AND MAINTENANCE.

1


Course 1

Introduction to Structures

Structural Materials

Course notes are available for download athttp://www.ct.upt.ro/users/AurelStratan/

Structures

� Man-made structures

– buildings

– bridges

– dams

– masts

– drilling platforms

– ships aircrafts, etc.

� Natural structures

– skeleton of animals

– shell of snails

– spider's web

– tree trunk and branches, etc.

� Structure: something which carries weight or resists loads and forces, and which may form a protective cover or skeleton for an object or living thing.

2

Some structures can fail

� 12.02.2009. Mall under construction in Oradea



3





4


� 19.12.2008 – failure of a silo near Vinga

Design criteria

� Suitability for its function: a building should be designed and realised in a manner that will offer to its users a certain function

� Safety and serviceability:

– Structures should resist loads and other external actions without

collapse, protecting its inhabitants

– Structures should not develop excessive deformations and

cracks, nor vibrate alarmingly

� Aesthetics: buildings should be aesthetically pleasant, both individually and as a group

� Economy: generally, the above three criteria need to be fulfilled with a limited budget

– Cost to design and build a structure

– Maintenance cost during the planned life

5

Structural materials

� A building consists of the structure and other components used in order to protect and provide for building function and aesthetics (cladding, partitions, floors, etc.)

� Structural material is the one which is used in those parts of the structure which carry loads and give it strength and stiffness

� Properties of structural materials:

– strength

– stiffness

– ductility

.

.

.

deformation

Structural materials: properties

� Strength (ultimate stress): the stress (load per unit area of the cross-section) at which the failure takes place

– tension

– compression

� Stiffness: the resistance of an elastic body to deformation

� Ductility: capacity of the material to deform into the inelastic range without significant loss of its load-bearing capacity

ductility

stiffness

strength

force

deformation

6

Structural materials: ductility

ductile

force

deformation

brittle

force

deformation

� Ductile materials: able to deform significantly into the inelastic range

� Brittle materials:

– fail suddenly by cracking or splintering

– much weaker in tension than in compression


� "Traditional" materials: used by builders and engineers since the ancient times

� Stone and timber: occur naturally

� Bricks: man-made

– sun-dried clay/mud bricks - from 4500 B.C.

– fired bricks - from 3000 B.C.

– calcium silicate bricks

� Ancient concrete:

– lime mixed with stone and sand: early civ. of the Middle East

– "hydraulic cement" - lime, stone, sand and silicates: Romans

� Stone, bricks, ancient concrete:

– weak

– weaker in tension than in compression

� Stone and bricks masonry: units interconnected by even weaker mortar

7


� Timber:

– substantial tensile strength along the grain

– weak in compression and across the grain (difficult to realise

connections in tension)

� "Modern" materials: Portland cement concrete, steel, aluminium , etc.

� Portland cement concrete:

– mixture of Portland cement, water, aggregates

– weaker in tension

– brittle

� Steel (iron with low carbon content) and Aluminium (duraluminium alloy):

– strong in tension and compression

– ductile

Structural materials: strength

-2000

Very high-strength

prestressingwires

202Normal usePortland cement

concrete

Mo

de

rn

606High strength

355355Mild steel

Iron and steel

700700High strength steel

450450Aluminium alloy (dural)

-3.5Across grain

30120Along grainTimber (spruce)

CompressionTensile

606Brick

405Limestone

20040GraniteStone

Tra

ditio

nal

Ultimate strength σσσσu

(N/mm2)Material

8

Specific strength

� All structures have to support their own weight

� Can the size of a structure be increased indefinitely for it to be able to carry its own weight?

� Problem: how long a bar of uniform cross-section can be before it breaks due to its own weight?

� Equate the weight of the bar to its tensile strength:Weight = Tensile resistance

Specific strength

� Weight = Volume ×××× specific weight

W = A ×××× L ×××× ρρρρ ×××× g

� Tensile resistance = Area ×××× ultimate tensile strength

R = A ×××× σσσσu

� Equate weight to resistance:

W = R ⇒⇒⇒⇒ A ×××× L ×××× ρρρρ ×××× g = A ×××× σσσσu ⇒⇒⇒⇒

L = σσσσu / (ρρρρ ×××× g) = S = specific strength

� There is an absolute limit (= S) to the length that the bar can attain without breaking

� Larger a structure is, larger is the proportion of its own weight to the total load that can be carried by itself

� First to realise this: Galileo Galilei

9

Specific strength

� For structures subjected to tension/compression, as the size of an object increases, its strength increases with the square of the ruling dimensions, while the weight increases with its cube

� For each type of structure there is a maximum possible size beyond which it cannot carry even its own weight

� Consequences:

– it is impossible to construct structures of enormous size

– there is a limit to natural structures (trees, animals, etc.)

– larger a structure becomes, stockier and more bulky it gets

• large bridges are heavier in proportions than smaller ones

• bones of elephants are stockier and thicker than the ones of mice

– proportions of aquatic animals are almost unaffected by their size

(weight is almost entirely supported by buoyancy)

Specific strength

-26700-2000

Very high-strength

prestressingwires

90090202Normal usePortland cement

concrete

Mo

de

rn

2700270606High strength

45004500355355Mild steel

Iron and steel

80008000600600High strength steel

1700017000450450Aluminium alloy (dural)

-700-3.5Across grain

60002400030120Along grainTimber (spruce)

CompressionTensileCompressionTensile

3200320606Brick

1800225405Limestone

7000140020040GraniteStone

Tra

ditio

nal

Specific strength S (m)Ultimate strength σσσσu

(N/mm2)Material

10

Specific strength

� Stone, brick and concrete: used in compression

� Steel: used in tension

� Timber: excellent performance in terms of specific strength, especially in tension

� Aluminium: high specific strength

� Aircrafts must carry loads and must be capable of being

raised into the air under their own power ⇒⇒⇒⇒ materials with high specific strength

– wood was extensively used in early planes

– modern material: aluminium

Structural materials: stress-strain curves

� Stress-strain curves provide "at a glance" information on:

– strength

– stiffness

– ductility

� Elastic region

� Inelastic region

� Steel: elastic region is almost linear

� Stone, brick, concrete, aluminium:elastic region is not linear

11

Structural materials: stress-strain curves

� Steel and aluminium: excellent ductility

� Concrete, brick: brittle

� Modulus of elasticity: E = σσσσ / εεεε

� Unloading after loading in the elastic range ⇒⇒⇒⇒ NO permanent deformations

� Unloading after loading in the inelastic range ⇒⇒⇒⇒permanent deformations present

� Permanent deformations need to be avoided in structures

under service loads ⇒⇒⇒⇒ stresses should be kept in the elastic region under service loads

� factor of safety = ultimate strength / design stress

Structural materials: stiffness

� Excessive flexibility is undesirable in structures

– people dislike noticeable vibration and deflections in buildings

and bridges

– large vibrations and deflections can damage (brittle) non-

structural components (partitions, glazing, floors, etc.)

� Materials with large stiffness are generally desirable (steel is more advantageous than aluminium from this point of view)

� Elastic efficiency of materials:

– average stress in the bar:

σσσσ = A××××L××××ρρρρ××××g / (2A) = L××××ρρρρ××××g / 2

– extension of the bar under its own weight

δδδδ = σσσσ ×××× L / E = L2××××ρρρρ××××g / (2××××E) = L2 / (2××××M)

– specific modulus of the material - a measure of material stiffness

M = E / (ρρρρ××××g)the higher the value of M, the less it will extend under its own

weight

12

Structural materials: stiffness

� The extension δδδδ of a bar under its own weight is proportional to the square of the scale (a bar which is 10 times longer than a reference one will extend 102 = 100 times more than the reference one)

Structural materials: stiffness and ductility

Low ductility

2.80210 000

Very high-strength prestressingwires

Brittle1.1225 000Normal usePortland

cement concrete

Mo

de

rn

1.8040 000High strength

Large ductility

2.80210 000Mild steel

Iron and

steel

Moderate ductility

2.80210 000High strength steel

Ductile2.8070 000Aluminium alloy (dural)

--Across grainNA

3.0015 000Along grainTimber (spruce)

DuctilitySpecific modulus

M (m × × × ×105)

Modulus of elasticity E (N/mm2)

1.6030 000Brick

1.3530 000Limestone Brittle

1.5745 000GraniteStone

Tra

ditio

nal

Material

13

Structural materials: ductility

� Ductility is important for the "ultimate" behaviour of structures

� Most structures are designed to respond in the elastic range under service loads, but, given the uncertainties in real strength of material, behaviour of the structure, magnitude of loading, and accidental actions, a structure can be subjected to inelastic deformations

� A ductile material will sustain large deformations before collapsing, "warning" the people inside

� A ductile material allows for redistribution of stresses in statically indeterminate structures, which are able to support larger loads than in the case of a structure realised of brittle material

1


Course 2

Structural action: cables and arches


Structural action

� Structural action: the way in which a structure of a given type and configuration resists the loads acting on it

� Types of structures:

– Cables

– Arches

– Trusses

– Beams

– Plates and shells

– Frames

2

Cable / chain structures

� Cable and chains:

– excellent tensile strength

– no strength/stiffness in compression

– no strength/stiffness in bending

� Cable and chain structures exploit the benefits of high tensile strength of natural fibres and steel

� Especially useful in large-span structures

Cable / chain structures

� The form of a chain under its own weight?

� The form of a chain under equal loads applied in the pins?

3

A chain subjected to a single force

� The simplest chain structure:

– links connected by pins

– load W acts on the central pin

� Solution (equilibrium of node C):

– the pin C is acted by three forces: load W, and two tensile internal

forces T

– the vectors representing the three forces can be represented as a

a triangle of forces 012 (W=12, T=20, T=01)

– length of lines 20 and 01 gives the tensions in the chain

A chain carrying two vertical forces

� Weights W1 and W2 attached to pins D and E

� Tensions T1, T2 and T3 will be set up in three parts of the chain

� Problem: determine magnitudes of T1, T2 and T3 if deformed shape is known

� Solution (equilibrium of nodes D and E)

� Node D

– node D is acted by three forces:

load W1, and to tensile internal

forces T1 and T2

– the vectors representing the

three forces can be represented

as a a triangle of forces 012

(W1=12, T1=20, T2=01)

– length of lines 20 and 01 gives

the tensions in the chain

4

A chain carrying two vertical forces

� Node E

– node E is acted by three forces:

load W2, and to tensile internal

forces T2 and T3

– the vectors representing the

three forces can be represented

as a a triangle of forces 023

(W2=23, T2=02, T3=30)

– length of lines 02 and 30 gives

the tensions in the chain

� The two triangles can be combined to get a force diagram

A chain carrying four vertical forces

5

A chain carrying equal weight at each pin

� The chain hangs symmetrically about point C

� Each inclined line in the force diagram gives the magnitude and inclination of the force in the corresponding link

� Starting from the midspan, the slope of the links increases in proportion to the horizontal distance from

the midspan ⇒⇒⇒⇒ parabola

A chain carrying equal weight at each pin

� The slope at the sides: twice the average slope ⇒⇒⇒⇒tangents at the ends A and B will intersect at point F (GF=2GC)

� Considering the equilibrium of the chain as a whole, the chain is acted by the tensions T1, T16 and the total weight W.

� Provided the chain sag is known (GC), end tensions can be determined from triangle of forces 120

6

Deformed shape of a cable / chain

� Actual deformed shape of a cable or chain hanging under

its own weight: catenary (slightly ≠≠≠≠ from parabola)

� Parabola: the shape of a chain carrying uniform loads for each horizontal span

� Catenary:

– the shape of a chain hanging under its own weight

– weight of the chain per unit horizontal span increases toward the

sides due to increasing slope of the chain

� Parabola:

– easier to calculate

– differences between parabola and catenary negligible for small

spans

Arches

� The simplest chain structure (material working in tension):

� If the load direction is reversed (material working in compression)

⇒⇒⇒⇒ an arch is obtained

� Internal forces are the same in the two structures, but are compressive in the arch

7

Three-bar linear arch

Three-bar chain Three-bar arch

� Internal forces are the same in the two structures, but are compressive in the arch

� Linear arch (funicular shape) - the shape for which under loads acting on it (including its own weight), the thrust in the arch acts along the axis of members at all points

Three-bar linear arch

� The forces in an arch can be deduced from those in a chain of the same shape (first to be realised by Robert Hooke)

� An essential difference between a chain and an arch:

– a change in the relative values of loads W1 and W2 in a chain leads

to a new position of equilibrium

– a change in the relative values of loads W1 and W2 in a hinged

arch leads to collapse of the structure

� Collapse of the arch due to small changes of loading can be avoided by connecting the bars rigidly together

8

Arches: line of thrust

� Linear arch gives the smallest stresses

� Shape of the arch is not important for small arches: own weight has a small contribution to stresses in comparison with imposed (traffic) loads

� Shape of the arch is very important for large arches: own weight has a major contribution to stresses

Arches: forms

� Perfect arch: shape of catenary (example: Taq-e KisraPalace, Ctesiphon, Iraq - built 220 B.C.)

9

Arches: forms

� The first civilisation to make extensive use of arches: Romans

� Shape of Roman arches: semicircular

why?

� Circle - the easiest way to set out

Semicircular arch

� A cable takes a circular form when subjected to a uniform radial load

� A linear semicircular arch: loaded by uniform radial pressure

� Loading in bridges and buildings quite different from the condition above

10

Romanesque semi-circular arches and vaults

� Semi-circular arch used extensively in the Romanesque period

� Severe architectural restrictions:

– Romanesque barrel vault

requires continuous support

and makes the interior dark

when used for roofs

– groined arch: enables light to

enter from all sides but allows

only square bays to be covered

Gothic arches

� Gothic period - pointed arches

� Rectangular spans can be covered by varying the ratio of rise to span

11

Gothic arches

� A kink in an weightless cable implies a concentrated force at the kink, as well as a distributed load along the

two sides ⇒⇒⇒⇒ corresponding shape of linear Gothic arch

� This condition is not present in almost all Gothic arches, which requires support from the adjoining masonry

Gothic arches

� Correct use of pointed arch: Font Pedrouse viaduct in France

12

Arches: design

� A stone arch (no strength in tension) will fail when the thrust line reaches the extrados and intrados in four points, becoming a mechanism

Arches: design

� 19th century approach - avoid cracking (tensile stresses) under service loads - keep the thrust line within the middle third of the arch cross-section

13

Arches: design

� Thrusts at springings(reactions at supports) are inclined:

– vertical component

– horizontal

component

� Horizontal reactions tend to spread the

supports apart ⇒⇒⇒⇒buttresses can be used, especially for arches/vaults on high walls

Arches: buttresses

1


Course 3

Structural action: trusses and beams

Course notes are available for download at

http://www.ct.upt.ro/users/AurelStratan/

Arch

Linear arch supporting aconcentrated force: large

spreading reactions at supports

rafter

tie

Relieving of supportspreading: adding a tie

between the supports

→→→→ Truss

2

Truss forces

� Truss members connected by

pins: axial forces

(direct stresses) only

� Supports:

– one pinned, allowing free rotations

due to slight change of truss shape

due to loading

– one roller bearing support ("simple

support") - allowing free rotations

and lateral movement due to

loading and change in temperature

� Forces in the truss:

– tie is in tension (+)

– rafters are in compression (-)

- (C) - (C)

+ (T)

Truss forces

� If more forces are present within the length of the rafter ⇒⇒⇒⇒bending stresses

� To avoid bending stresses, diagonal members and vertical

posts can be added

� More diagonals and posts can be added for larger spans in order to avoid bending stresses

-

+ +

-

---

-+

3

Alternative shape of a truss

� For a given loading find out the shape of a linear arch

(parabolic shape)

� Add a tie to relieve spreading of supports

� Highly unstable shape

Alternative shape of a truss

� Add web bracing (diagonals and struts) in order to provide stability for the pinned upper chord

members

� If the shape of the truss corresponds to a linear arch web members are

unstressed, but they are essential for stability of the truss

� Reverse bowstring arches:

– advantage: longer members

are in tension

– disadvantage: limited

headroom underneath

4

Truss shapes

� Curved shape of the arch: difficult to fabricate ⇒⇒⇒⇒ trusses

with parallel chords

� Trusses with parallel chords: web members (diagonals

and struts) carry forces whatever the loads

� Pratt truss:

– top chord in compression

– bottom chord and diagonals in tension

– economical design as

longer members

(diagonals) are in tension

Truss shapes

� Howe truss:


– bottom chord in tension

– diagonals in compression

� Warren truss:


– bottom chord in tension

– diagonals in tension and

compression

– economy of fabrication: all

members are of the same

length and joints have the

same configuration

5

Truss joints

� Pinned joints ⇒⇒⇒⇒ statically

determinate structures ⇒⇒⇒⇒ member

forces can be determined from equilibrium only

� Rigid joints ⇒⇒⇒⇒ small bending

stresses will be present, but which

are negligible due to the triangular

shape

� Traditionally trusses are designed

with pinned joints, even if members are connected rigidly between them

Space trusses

� The most common plane truss consists of a series of triangles

� The corresponding shape in three dimensions: tetrahedron (a)

� The truss at (b) is a true space truss

– theoretically economical in material

– joints difficult to realise and

expensive

� Two plane trusses braced with

cross members are usually preferred

6

Statically indeterminate trusses

� Indeterminate trusses:

large variety

� Example (a): cross

diagonals in the middle

panel, so that one of

the diagonals will

always be in tension

� Example (b): Sydney

Harbour Bridge,

Australia - both

supports pinned

Beams

� Beam: a structure that supports loads through its ability to resist bending stresses

� Leonardo da Vinci (1452-1519): the strength of a timber beam is proportional to the square of its depth

� Leonhard Euler and Daniel Bernoulli were the first to put together a useful theory around 1750

7

Beams: analogy with trusses

� Forces in a

Pratt truss

loaded by a unit central force

� Forces in a

Howe truss

� Forces in a truss with double diagonals(reasonable estimate)


� Chords:

– The forces in the top and bottom chord members in any panel are

equal, but of opposite signs, and they increase with the distance

from the nearest support

– Chords have to resist the bending moment, proportional to the

distance from the nearest support

� Diagonals:

– The forces in the diagonal members are equal, but opposite in

sign, and have the same values in all panels

– Diagonals have to resist the shear forces, the same in all panels

8


� Bending and shear deformations in a truss

Steel plate girder

� Steel plate girder: heavy flanges and thin web welded together, and reinforced by transversal stiffeners

� Unit vertical force at the midspan

� Top flange: in compression

� Bottom flange: tension

� Web: shear, with principal tension and compression

stresses similar to those in a truss

� After web buckling, only tensile loads are

resisted by the web, plate girder acting as a

Pratt truss

9

Beams: bending action

� Top flange in compression

� Bottom flange in tension

� Normal stress proportional to distance from the neutral

plane

� Simplifications:

– Thin web, thick flanges ⇒⇒⇒⇒ web has a small contribution to the

bending resistance (ignore it)

– Normal stress can be considered uniform on flanges

linear variation of

normal stress


� Moment resistance

– Idealised double T beam: M = σσσσ⋅⋅⋅⋅A⋅⋅⋅⋅d/2

– Rectangular beam of the same area and depth:

M = σσσσ⋅⋅⋅⋅b⋅⋅⋅⋅d2/6 = σσσσ⋅⋅⋅⋅A⋅⋅⋅⋅d/6

� The best

arrangement of material for bending

resistance:

away from the neutral axis

−−−−σσσσ

++++σσσσ

d

F = σσσσ ·(A/2)

F = σσσσ ·(A/2)

d M = σσσσ ·A·d/2

A/2

A/2

−−−−σσσσ

++++σσσσ

d

b

F = σσσσ ·(0.5d·b/2)

F = σσσσ ·(0.5d·b/2)

2d

/3 M = σσσσ ·A·d/6A

10


� Examples of efficient location of material for bending

resistance

– light roof beams (trusses)

– hot-rolled and welded girder


� Examples of efficient location of material for bending resistance

– panel construction

11



resistance

– corrugated steel sheet


� Examples of efficient location of material for bending resistance

– castellated joist

12



resistance

– columns requiring bending resistance in any direction: tubular

sections

Beams: shear stresses

� Simply supported beam of uniform rectangular cross-section loaded by a concentrated central force W:

– can carry a moment M = σσσσ⋅⋅⋅⋅b⋅⋅⋅⋅d2/6

– has a deflection δδδδ

� If the beam is cut in two parts along the neutral plane:

– sliding takes place between the two overlapped beams

– the two overlapped beams can carry a moment

M = 2⋅⋅⋅⋅[σσσσ⋅⋅⋅⋅b⋅⋅⋅⋅(d/2)2/6] = σσσσ⋅⋅⋅⋅b⋅⋅⋅⋅d2/12, half of the uncut beam

– the deflection of the two overlapped beams is 4δδδδ

13

Beams: shear stresses

� In the uncut beam stresses should be present along the

neutral plane to prevent sliding of the lower and upper

halves of the beam: shear stresses

� Smaller stresses would be required to keep the unity of

action if the beam were cut above the neutral plane

� Shear stresses

– parabolic variation in a rectangular cross-section

– carried mainly by the web, on which they can be considered to be

constant for a steel double T beam

Structural shapes

� Simply supported beam subjected to a uniformly distributed load

� The "perfect" use of material for bending resistance in a beam with

idealised double T cross-section (M = σσσσ⋅⋅⋅⋅A⋅⋅⋅⋅d/2):

parabolic variation of height

A/2

A/2

14

Structural shapes

� Simply

supported truss

subjected to a uniformly

distributed load

� The "perfect" use

of material for

"bending" action:

parabolic variation of height

Structural shapes

� Bridge with a simply supported central span and two cantilevered sides

� The shape of the truss must resemble the bending moment diagram in order to make efficient use of material

in upper and bottom chords

� Quebec

railway bridge

15

Structural shapes

� Forth

bridge,

Scotland

� Angel

Saligny bridge, Romania

1


Course 4

Structural action:

- prestressing

- plate and shell structures


Prestressing

� Prestressing: setting up an initial state of stress, that makes the structure work better than without it

� Examples:

– wall plugs

– spider's web

– bicycle wheel

� Main use in structural engineering: prestressed concrete

2

Prestressing examples: wall plug

� A hole in the wall is filled with a wooden or plastic plug

� The screw driven into the plug squeezes the plug against the sides of the hole, generating compressive stresses in the plug and in the wall around it

� Compressive prestressing generates frictional resistance to pulling out the screw

Prestressing examples: spider's web

� Spider's web threads: high tensile, but no compressive resistance

� Spider pulls its threads tight, creating a tensile prestressing

� A load in the centre of the web produces compressive forces in the threads below it

� Without the tensile prestress, the lower part of the web would go slack, being more prone to collapse

3

Prestressing examples: bicycle wheel

� Wire spokes are strong in tension but weak in compression (due to buckling)

� Spokes must be kept in tension

� When the wheel is assembled, spokes are tightened up uniformly by the turnbuckles at the rim

� Under a downward load on the wheel, the spokes in the lower part of the wheel tend to be subjected to compression

� Tensile prestress in the spokes must be higher than the compression force to keep all the spokes in tension


� Other types of loading on the wheel: due to braking and due to taking a sharp corner

� Forces due to braking:

– could not be resisted if the spokes were arranged radiating from

the centre of the hub

– spokes are set at an angle to the radii, each pair forming a

triangulated system which is able to generate tensile and

compression forces which oppose the braking force

– tensile prestress ensures that all spokes are in tension

and active

4


� Forces due to cornering:

– force is imposed on the wheel at right

angles to its plane

– the spokes are inclined with respect to the

plane of the wheel, forming a triangulated

system, which resists the forces due to

cornering

– tensile prestress ensures that all spokes

are in tension and active

Other prestressing examples

� Pneumatic tire of cycle wheel

� Inflated membranes for storage spaces and sport halls

– air pressure inside is maintained above the atmospheric pressure

by blowers

– fabric of the membrane permanently in tension

5

Other prestressing examples

� A set of books: no tensile resistance between the volumes

� The books can be moved if a pressure is applied at the middepth:

– the row of books act as a simply

supported beam

– the pressure overcomes the tensile

stress in the lower part due to own

weight of the books, enabling them to act

as a unit

� The books can be moved with lower pressure if it is applied somewhat lower than the middepth: an upward moment is introduced, which counteracts the downward moment due to own weight of the books

Reinforced concrete beams

� Concrete: weak in tension

� When loading is applied on a simply supported beam, the concrete cracks at the tension side:

– Concrete active in compression

– Steel reinforcement active in

tension

– Only a small part of the concrete

cross-section resists the applied

loading

6

Prestressed concrete beams

� Concrete is kept in compression by cables or rods

� The whole concrete cross-section can be considered in design

� Substantial economy in material

� If prestressing is applied in the centroid of the cross-section:

– by choosing correctly the

prestressing force, the entire cross-

section can be kept in compression

– a large stress is present at the

compression side


� Position of prestressing force: important

� If prestressing is applied at 1/3 of the beam depth from the bottom face:

– a negative moment due to eccentric

prestressing counteracts the

positive bending moment due to

applied moment

– the pestressing force needed to

keep the entire cross-section in

compression can be reduced

– the stress at the compression side is

reduced ⇒⇒⇒⇒ the required concrete

strength can be reduced

7


� Bending moment due to dead weight in a simply supported beam: parabolic shape

� The best arrangement of the prestressing tendons?

⇒⇒⇒⇒ a parabolic shape along the beam, in order to generate bending moment M=F⋅⋅⋅⋅e counteracting the bending moment due to dead load


8

Prestressed concrete

� Type of prestress:

– Posttensioning: the prestressing force is applied after concrete

has been cast and has set, through tendons located in holes left

in concrete elements. The prestress is retained due to anchorage

of steel tendons at the end of the element.

– Pretensioning: prestressing wires are stretched over a long

length and the concrete is cast around them in steel forms. The

prestress is retained due to the bond between the concrete and

the steel wires.

� Problems related to prestressing:

– When the concrete sets up, it shrinks, leading to loss of

prestressing (in the case of pretensioning)

– Concrete shortens in time (creep) after it sets up due to

compression acting on it, leading to loss of compression

– High strength steel required for prestressing, in order to reduce

the loss of prestress due to shrinkage and creep

– Higher strength concrete is needed to resist higher compression

and to reduce the contraction due to creep and shrinkage

Plates

� Plates: a flat surface element that acts in bending in order to resist out of plane loading

� The simplest plate: a flat slab spanning between two supports

� It may appear to behave like a wide beam, but it is not as simple as that

9

One-way plates

� When a narrow beam bends, the material in the lower half of the beam extends longitudinally ⇒⇒⇒⇒ it contracts in the transversal direction due to Poisson effect (µµµµ times the longitudinal strain)

� The material in the upper half of the beam contractslongitudinally ⇒⇒⇒⇒it expands in the transversal direction

� An anticlastic curvature of the beam in the transversal direction equal with µµµµ times the longitudinal curvature

One-way plates

� In plates the anticlastic curvature is suppressed due to large dimension in the transversal direction (the deflected shape is almost cylindrical, except near the free edges)

� At any point of the beam there is a transverse bending moment equal to µµµµtimes the spanwise bending moment

� Suppression of the transverse curvature induces an additional spanwise curvature

� In one-way plates reinforcement is needed in both spanwise and transverse direction

10

Two-way plates

� Two-way plates simply supported on all four sides: complicated interaction between the two ways in which a load is supported

� If a slab is more than about 4 times as long as it is wide, the bending moment at the center of the plate is almost the same as in a one-way plate supported on longer edges. Why? ⇒⇒⇒⇒

� Stiffer structural action (bending in the short direction) attracts larger forces

Stiffness in structural action

� A straight bar of length L and rectangular cross-section can support a concentrated force P in two ways:

– as a column acting in compression

– as a cantilever acting in bending

� In the column the stress σσσσ1 is axial and uniform

� In the cantilever the stress σσσσ2 has a linear variation along the bar and across the cross-section ⇒⇒⇒⇒the material is far less efficient

11

Stiffness in structural action

� Column is much stronger than the beam: σσσσ2/σσσσ1 = 6(L/h)for L/h=20 ⇒⇒⇒⇒ σσσσ2/σσσσ1 = 120

� Column is much stiffer than the beam: δδδδ2/δδδδ1 = 4(L/h)2

for L/h=20 ⇒⇒⇒⇒ δδδδ2/δδδδ 1 = 1600 (P=k∙δδδδ ⇒⇒⇒⇒ k1/k2 = 1600)

� If the beam and the column are used in conjunction to support the load P:

– the two members deflect by the

same ammount δδδδ

– P=k∙δδδδ ⇒⇒⇒⇒ P1=k1∙δδδδ1; P2=k2∙δδδδ2. If the deflection

is the same for the two members δδδδ1=δδδδ2 ⇒⇒⇒⇒

P1/k1 = P2/k2; P1/P2=k1/k2 = 1600

– the column carries a load of (1600/1601)P

– the beam carries a load of (1/1601)P

� Of the two alternative modes of action open to this structure, it chooses the column compression, because it is stiffer

Membrane action

� Some structures can support loads only in bending.Example: simply supported beam

� Uniform loading:

– the neutral axis becomes curved

– roller support moves slightly toward the other end of the beam

12

Membrane action

� A beam pinned at both ends

� Uniform loading:

– the neutral axis becomes curved

– horizontal movement of the support is prevented ⇒⇒⇒⇒ longitudinal

tension H develops ⇒⇒⇒⇒ the beam begins to support load as a

slightly curved cable or catenary

Membrane action

� The catenary action is much stiffer than bending

� Beam action: stiffness remains constant

� Catenary action: stiffness increases with the square of the deflection

� As the load increases, the portion of the load carried axially (w1), as catenary, increases rapidly

� It can be shown that w1/w2 = 3.33(δδδδ/h)2

w2 - the portion of the loading carried through bending.When the deflection δδδδ ammounts to twice the depth of the beam, w1/w2 = 13.33, so that the catenary action ammounts to 13.33/14.33 = 0.93 of the total resistance to load

� Membranes: surface elements in which loading is resisted through direct (axial) stresses

13

Shells

� Shells: surface elements resisting loading through bending and membrane action

� Examples:

– dome

– human skull

– turtle's armour

– bird egg

Shells

� Bird's egg: weak under a concentrated loading (breaking against a cup's rim) but strong under distributed loading (squeezing between ends with palms)

– distributed loading resisted through membrane action (stronger)

– concentrated loading resisted through bending action (weaker)

� Domes:

– used since ancient times

– capable of resisting through membrane

action a variety of distributed loading

14

Dome: structural action

� The shape of a cable changes as the shape of the applied loading changes

� The same behaviour if a set of cables are hanged around a circular perimeter

– uniform loading: "bowl" shape

– larger loading toward the supports: the

"bowl" bulges toward supports and the

bottom rises slightly

– a different shape of the cable is needed in

order to resist the applied loading

through axial action only


� If a series of circumferential cables are added, capable of resisting both tension and compression

� When the load changes, the circumferential cables prevent the dome from changing its shape:

– circumferential cables near the rim are

put into tension

– those near the bottom are put into

compression

15


� A system formed by using enough cables in order to obtain a surface approximates a thin-shelled dome

� Such a structures is capable of carrying a variety of distributed loading through membrane action (stresses which are uniformly distributed over the thickness of the shell)

� A shell is capable of resisting loads either through bending stresses or direct (membrane) stresses

� Membrane action is "preferred" by the dome, as it is much stiffer for this action

� Ideally, for a membrane action to take place in a shell, it must be thin and its shape should be similar to that assumed by a flexible membrane under the same loading


� The heaviest load in many domes is their own weight

� In a hemispherical dome of a uniform thickness,

– the stresses σσσσ1 in the direction of meridians are compressive

throughout

– the circumferential stresses σσσσ2 are tensile near the rim: tensile

reinforcement needed to resist them

16

Shells: hyperbolic paraboloid

� Rectangular area to be covered: (a) taking a portion of a sphere and arching it between supports

� Rectangular area to be covered: (b) hyperbolic paraboloid - can be obtained by taking a rectangular grid of straight lines and lifting one of the corners, so that the lines would remain straight

� A flat surface becomes a curved one, known as hyperbolic paraboloid

� Lines drawn diagonally are parabolas, humped in one direction and sagging in the other direction


� Constructional advantage that elaborate formwork is not needed

� Hyperbolic paraboloid supports loads by tension/compression, as opposed to a plate, acting in bending

� Given the opportunity, a structure will support loads by direct tension and compression rather than bending

17


1


Course 5

Structural action:

- Cable structures

- Multi-storey structures


Cable structures

� Cables - good resistance in tension, but no strength in compression

� Tent:

– a cable structure consisting of a waterproofing membrane

supported by ropes or cables and posts

– cables must be maintained in tension by prestressing in order to

avoid large vibrations under wind forces and avoid collapse

2

Cables: roof structures

� Cables in a cable-supported roof must be maintained in tension -easily achieved if the roof is saddle-shaped

� Example: hyperbolic paraboloid, with curvatures in opposite senses in directions at right angles

– cables hung in direction BD

– a second set of cables placed over

them, parallel to direction AC and put in

to tension

– cables from the second set press down

on those from the first one, putting them

into tension as well ⇒⇒⇒⇒ fully-tensioned network

Cables: roof structures

� One of the first doubly curved saddle-shaped cable supported roof was the Dorton Arena in Raleigh, North Carolina, built in 1952

� The building has dimensions of 92 m x 97 m

� The roof is suspended between two parabolic arches in reinforced concrete intercrossing each other, and supported by columns

� The cable network consists of 47 prestressed cables with diameter varying from 19 mm to 33 mm

3

Suspension bridges

� Suspension bridges: the earliest method of crossing large gaps

� Early bridges realised from a walkway suspended from hanging ropes of vines

� To walk a lighter bridge of this type at a reasonable pace requires a particular gliding step, as the more normal walking step will induce travelling waves that can cause the traveller to pitch (uncomfortably) up and down or side-to-side.

Suspension bridges

� Suspension bridge realised following the simple design of early bridges:

– cables (catenaries)

– light deck

– hangers suspending the deck on catenaries

� Lack of stability in high winds

� Very flexible under concentrated loads, as the form of the cable will adapt to loading form

4

Suspension bridges

� Capilano Suspension Bridge, Canada

Suspension bridges

� Improved behaviour under traffic and wind loads: stiffening trusses at the level of the deck, that distributes concentrated loads over greater lengths

� Alternatively: restrain vertical movement of the catenaries by inclined cables attached to the top of the towers or inclined struts below the deck

5

Suspension bridges

� The Akashi-Kaikyo Bridge, Japan: 1991 m span

Suspension bridges

� Golden Gate Bridge, California, USA: 1280 m span

6

Suspension bridges

� Brooklyn Bridge, USA (the largest from 1883 until 1903): 486 m span

Suspension bridges: famous collapse

� Tacoma Narrows Bridge, USA, collapsed on November 7, 1940 due to wind-induced vibrations. It had been open for traffic for a few months only before collapsing.

7

Cable-stayed bridges

� A cable-stayed bridge consists of one or more piers, with cables supporting the bridge deck

� Basic idea: reduce the span of the beam (deck) several times compared to the clear span between the piers

� Steel cable-stayed bridges are regarded as the most economical bridge design for spans ranging between 200 and 400 m

� Shorter spans: truss or box girder bridges

� Larger spans: suspension bridges

Cable-stayed bridges

� Reducing the span of a beam greatly improves the maximum stress and deflection

8

Cable-stayed bridges: examples

� Rio-Antirio bridge in Greece. Longest span: 560 m. Total length: 2,880 m.

Cable-stayed bridges: examples

� The Millau Viaduct, France. Longest span: 342 m. Total length: 2,460 m.

9

Multi-storey buildings

� Why multi-storey buildings?

– large urban population

– expensive land

� Multi-storey buildings make more efficient use of land: higher the building (more storeys) - larger the ratio of the building floor area to the used land area

� Technological competition (very high buildings)

� Until the end of the 18th century most buildings of several storeys in the Western world were made of:

– continuous walls of brick or stone masonry supporting the roof

– floors from timber beams

� The same structural system used in the Roman city of Herculaneum

Multi-storey buildings: beginnings

� Beginning of the 19th century - forefront of the industrial revolution in England:

– demand for large factory buildings of several storeys and large

clear floor areas

– cast iron available in bulk

– cast iron columns used instead of bearing walls and cast iron

beams instead of timber floor joists

� Elevator invented in USA in 1870, enabling much taller office and apartment buildings to be constructed

� Most multi-storey buildings in USA were still making use of masonry walls instead of columns

10

Multi-storey buildings: masonry

� Monadnock building in Chicago

� Built between 1889 and 1891

� 16 storeys, 60 m high

� Tallest masonry building until today

� Walls at the ground floor: almost 1.80 m thick, occupying more than one-fifth of the width of the building

� Wall thickness: rule of thumb - 0.3m3 of exterior walls for each square meter of floor

Multi-storey buildings: skeleton frames

� Home Insurance Building

� Built in 1884 and demolished in 1931

� 10 storeys, 42 m high

� Considered to be the first skyscraper

� Exterior masonry walls

� Cast-iron columns

� Wrought-iron beams

� One of the first to make use of steel skeleton frame instead of masonry walls

⇒⇒⇒⇒ significant reduction of dead weight (1/3 of that of a masonry building)

11

Multi-storey buildings: skeleton frames

� Steel skeleton frames

– loads carried by a steel frame composed of columns and beams

rigidly connected between them

– large clear spaces

Traditional load-bearing wall construction

� Outside load-bearing wall support:

– dead weight of the walls

and floors above

– live loads on the floors

– horizontal forces due to

wind pressure

� Columns support gravity loads only

� To avoid tension on the brick walls, the resultant force must lie in the middle third of the

thickness of the wall ⇒⇒⇒⇒very thick walls in the lower storeys

12

Load-bearing wall construction

� In modern load-bearing wall construction, lateral forces due to wind are resisted by walls aligned in the direction of the wind

� Such walls are much more effective, because they have a much larger moment resistance

� Transverse walls acts as vertical cantilevers against lateral forces

� In modern construction,load-bearing walls are from reinforcedconcrete

Multi-storey buildings: gravity and lateral loads

� The load-bearing walls must be in the same position in plan to act as a vertical cantilever

� In order to provide clear floor spaces, doors, corridors, lift wells and staircases

� Most buildings realised as a combination of:

– load-bearing walls resisting lateral forces

– frames resisting gravity loads

load-bearing walls

or braced framesload-bearing walls

or braced frames

frames resisting

vertical loads only

frames resisting

vertical loads only

load-bearing walls

for lateral loads

frames resisting

vertical loads only

13

Multi-storey buildings: gravity and lateral loads

� Lateral forces on external cladding are transmitted to the bearing walls

– directly, through external cladding

– indirectly, via floors

� Floors must be stiff and strong in their plane in order to allow lateral forces acting on gravity frames to be transmitted to load-bearing walls

� Usually floors are realised from cast in place reinforced concrete to give a monolithic slab over full plan of the building

F F

stiff floor flexible floor

Multi-storey buildings: types of structures

� As the height of the building increases, the more important are wind and earthquake loads in comparison with gravity loading

– In a multi-storey building, acting as a vertical cantilever, bending

stresses at the base increase with the square of its height

– Wind loading increases with the height

– Earthquake loading increases with building weight

� Reinforced concrete structures:

– reinforced concrete frames

– load-bearing walls

� Steel structures:

– moment-resisting frames

– braced frames

14

Multi-storey buildings: types of steel structures

� Moment-resisting frames resist lateral loads through flexural strength of members

– clear spaces, but

– large deformations of the structure

– large stresses due to bending

� Braced frames resist lateral loads through direct (axial) stresses in the triangulated system

– obstruction of clear spaces, but

– small deformations (rigid structure)

– smaller stresses due to more efficient

structural behaviour

Multi-storey buildings: braced steel frames

� Concentrically braced frames with diagonal bracing

� Concentrically V-braced frames

� Eccentrically braced frames

15

Multi-storey buildings: steel structural systems


� Braced frame efficient in reducing lateral deformations at the lower storeys, but becomes inefficient at upper storeys due to overall cantilever-like effect

� Moment-resisting frame: uniform "shear-like" deformations

� Combined moment-resisting frame and braced frame: more rigid overall behaviour due to interaction between the two systems

16


� Braced frame with central braced span:

– inner columns: large axial stresses due to truss action

– outer columns: small axial stresses

� Outrigger truss: outer columns are "involved" into the truss-like action (axial stresses) through the outrigger truss


� Exterior framed tube: closely spaced columns at the exterior of the building, rigidly connected to deep beams

� Acting like a giant rectangular steel hollow section

� Shear-lag effect - non-uniform stresses on web and flanges: middle sections are not very stressed

17


� Exterior framed tube: World Trade Center, New-York



18




� Bundled framed tube: combination of multiple tubes to reduce the shear lag effect

� Sears Tower, Chicago

19


� Exterior diagonal tube: giant truss-like behaviour


� Exterior diagonal tube: John Hancock Center, Chicago

1


Course 6

Structural action:

- Foundations

- General remarks on structural action


Foundations

� Most structures invariably rest on the ground

� The best solution would be to place the supports of a

structure on solid rock, but this is seldom possible

� In most cases solid rocks lies deep in the ground, with softer and weaker soil layers above it

� Relatively high stresses in the superstructure have to be safely transferred to the much softer and weaker soil.

This is done through foundations

2

Types of foundations

� Isolated footing

– Top soil layer is removed and a block of

concrete, wider than the one which rests

on it, is placed on the ground

– Plan dimensions of the isolated footing

need to be larger than the ones of the

column, in order to have lower stresses

at the foundation-soil interface

– Foundation dimensions should be large

enough to allow stresses acting on the

soil to be smaller than the soil strength

� Continuous footing: when the

structural member to be supported

by the foundation is a wall, the footing is realised continuously below the wall, following the concept of the isolated footing


� Raft foundation:

– When the soil is very poor, larger

area is required for the foundation,

which extends over the full plan

dimension of the building

– Raft foundations were developed by

Romans, who built them from

hydraulic concrete several metres

deep

– Modern raft foundations are much

thinner, as they area realised from

reinforced concrete

– Raft foundations can be

constructed as a series of boxes,

with the walls in the basement

contributing to the strength of the

foundation and enabling thinner

slab

3


� Isolated and continuous footings, and raft foundations

are shallow foundations:

– placed relatively close to the surface of the ground

– loads are transferred from the building to the soil by providing

large enough area of the foundation in order to reduce stresses

below the ones allowed by the strength of the soil


� Pile foundations:

– Soil properties get better as the depth

increases. When the soil near the surface is

very poor, pile foundations can be used.

– Pile foundations are made of tree trunks (in

old times), steel or reinforced concrete (in

modern times)

– Loads are transferred to the soil through

shear stresses between the pile shaft and

the soil (major contribution) and

compression stresses at the bottom of the

pile (minor contribution)

– Piles are long, enabling them to reach

stronger and stiffer soil layers, or even

solid rock

– First pile foundations date back to Neolithic

period, and were made of tree trunks

4


� Pile foundations:

– Without pile foundations, cities like Venice

and Amsterdam, located due to strategic

and economic reasons on marshes could

not have been developed at all

– Wooden piles were usually of oak or, in the

sea, of greenheart from Central America,

which is particularly resistant to marine

borers

– Pile foundations can be installed by either

driving them into the ground (wooden, steel

and precast concrete) or drilling a shaft and

filling it with concrete

� Piles are deep foundations, in which

loads are transferred to the soil by reaching deeper and stronger soil layers.


� Cofferdam foundations

– Cofferdam is an enclosure beneath

the water constructed to allow

water to be displaced by air for the

purpose of creating a dry work

environment

– Were developed by Romans and

remained mainly unchanged until

the early 19th century

– Pneumatic caissons were then

invented, allowing underwater

foundations to be excavated,

keeping the water out by air

pressure. Difficult and expensive

to operate.

5

Failure of foundations

� Complete failures of foundations are rare, though they

may happen

– Example: Transcona grain silo, Winipeg , Canada. In October

1913, this grain silo started to tip over. It was loaded with over a

million bushels of wheat and was newly built. It continued to sink

slowly for over 12 hours until finally it was at an angle of 30

degrees from vertical but still intact. The wheat was emptied from

the bins, and work began to right it. By tunelling underneath it,

they built new foundations down to the bedrock and then pushed

it back into position. It is still in use today

Failure of foundations

� Complete failure of foundations are rare, though they may happen

– Example: Tilting of apartment buildings at Kawagishi-Cho,

Niigata, produced by liquefaction of the soil during the 1964

Niigata Earthquake

6

Foundation soil behaviour

� The biggest problem of foundations is soil settlement,

especially the differential settlement, of various parts of a

structure, leading to cracking and distortion of the superstructure

� Soil can vary greatly in composition from one point to

another, even under the same structure

� Soil properties are greatly affected by ground water

� Soil consists of a mass of solid particles (soil skeleton) of

sand and/or clay, more or less loosely packed, and the

spaces between them filled with water

� In an undisturbed soil the weight of the earth above is carried by solid particles, and the water in pores is at normal pressure of water at that level below the water table


� Soil skeleton is much more compressible than water, and when an additional load (e.g. from a building) comes onto the ground,

– At first, the additional compressive stress in the soil is carried

entirely by water because it is stiffer than the soil skeleton

– The pore pressure increases and it is squeezed out sideways

from under the foundation

– Pore water pressure drops gradually back to normal values at that

depth, as the soil skeleton is compressed enough to carry itself

the loads

� In fine clays the water escapes slowly and the process of consolidation under a foundation can take many years

7


� Problems due to settlement can arise when:

– Soil property changes at different points under the same structure

– When construction of the building proceeds fast (as is the case in

modern times)

– When an additional heavy load (e.g. a tower in old times) is added

after the bulk of the structure is completed and has settled

– Ground water is pumped out. Notorious instances: Venice and

Mexico-city

� Example: Venice

– Water supply in Venice originally came from mainland

– Starting from 1910, this was increasingly supplemented from

boreholes up to 300 m deep

– General subsidence of buildings (100-200 mm) ⇒⇒⇒⇒ extremely damaging to buildings as walls of most Venetian houses start at

only about 1 m above average sea level


� Example: Mexico-city:

– Most of the city is built on a soft bed 30-40 m deep of a dried-up

lake

– Building settlement reached constant levels and was not a

problem

– In the 19th century pumping started from deep wells to

supplement water supply

– Today the ground level in the centre of the city is more than 6 m

lower than it was in 1900

– Old buildings, sewers and water pipes much affected

8

General remarks on structural action

� [1] Structures support loads in

the most direct way open to

them

– bowstring truss: if the top chord

has the right shape for the given

loading, loads pass directly to the

support, ignoring the web

members

– a lateral load at the top of a

triangular tower is transmitted

down the two main members while

the inner bars are unstressed


� [1] Structures support loads in the most direct way open to them

– (a) the load applied at the top of a

column in the frame from the

figure goes directly to the

foundation through the column,

while the rest of structure is

virtually unstressed

– (b) if the direct path is interrupted,

the load path is much more

complicated, and the stresses and

deflections are greatly increased

– Rule: provide paths as simple and

as direct as possible for the loads

to pass to the supports

9


� Characteristics of a well-designed structure:

– elements are few and well-disposed

– their function is obvious, and

– the whole effect inspires confidence

� well conceived structure

� ill-conceived structure


� [2] The larger the structure,

– the more important is the own weight of the structure in

comparison with other loads

– the more important is that structural elements be arranged as

efficiently as possible

� Example: simply supported beam bridge

– moment larger at the midspan

– provide more material at the midspan

to increase the moment resistance

– larger loads at the midspan

– larger moments

– inefficient structural configuration

– Bridges using simply-supported beams are most often of

constant cross-section and are used for small spans only

Mmax

10


� Example: cantilever

bridge

– the moment due to dead

weight is largest at the

support

– the material must be

concentrated at the

supports

– a load near the support

produces only a small

increase in moment

– efficient structure for large

bridges


� [3] Statically indeterminate structures support loads in the stiffest mode open to them

– very often, load paths can take two alternatives: direct

tension/compression or bending

– a thin plate loaded transversally supports loads by bending but

direct (membrane) action develops rapidly as the plate deflects

– thin shells support transverse loads as far as possible by

compressive membrane forces rather than bending

11


� [3] Statically indeterminate structures support loads in

the stiffest mode open to them

– sometimes simple change in a structure allows loads to be

carried in a more efficient way:

• portal frame with a concentrated load at the ridge develops bending stresses

• if a tie is inserted between the two sides of the eaves level. The two rafters and the tie form a triangulated structure. The loads are

transmitted through compression in the rafters, tension in ties,compression in beams, with negligible bending.


� [4] Direct tension is preferable to direct compression

– it is rarely possible to avoid compression

– even in predominantly tension structures as suspension bridges,

tensile forces in cables must be balanced by compressive forces

in towers

– minimize the loss of efficiency due to compression by:

• keeping the compressive members short

• use a material (e.g concrete) with lower strength, and therefore more stocky members less prone to instability

� [5] In statically indeterminate structures, the stiffer elements will attract larger forces

– Example: portal frames are often

haunched near the corners

• further increase of bending moments at the corners though actual stresses reduce due increase of cross-section

1


Course 7

The process of structural design

Load paths


References

� ESDEPhttp://www.esdep.org/members/master/wg01b/l0100.htm

� Load path visual aids: http://www.ce.jhu.edu/steel/aisc_web/Visual_Aids/Animations/building_design/index.html

� Tributary Areas and Load Diagrams by T. Bartlett Quimbyhttp://www.bgstructuralengineering.com/BGSMA/BGTribArea/BGSMA_TA_02.htm

2

Structures

� Civil engineering structures: how to design?

Design objectives

� Design is "the process of defining the means of manufacturing a product to satisfy a required need"

– conceptual ideas,

– study of human intentions,

– detailed technical and

manufacture stages

� In structural design, prime objectives are to ensure the best possible:

– intended function of the construction,

– safe construction system,

– imaginative and delightful solution,

– completed on time and to the original budget cost

� These points could possibly be satisfied by either:

– simply making an exact copy of a previous artefact, or,

– "re-inventing the wheel", by designing every system and

component afresh.

ideas and studies communicated with drawings, words and models

3

Design objectives

� Re-using an existing design:

– not always suitable (e.g. need for larger spans, larger loads,

different function, etc.)

– need to improve a deficient design

– new materials

– boring

� Completely new design:

– not enough time - new theoretical ideas and innovations

invariably takes much time

– potential risks and errors

� The middle way:

– The solutions will incorporate components from both of

fundamental principles and recent developments

– However, throughout the Design Process it is prudent to maintain

a clear grasp of final objectives and utilise relatively simple

technical means and solutions

Design approach

� Many design methods exist

� Suggested approach:

– Recognise that a challenge

exists and clearly define the

overall objectives for a design.

– Research around the task and

investigate likely relevant

information (Analysis).

– Evolve possible solutions to

the task (Synthesis).

– Decide on, and refine, the best

solution (Evaluation),

establishing clear priorities for

action (in terms of

manufacture, construction,

operation and maintenance).

– Communicate decisions to

others involved in the task.

4

Design process

� The design process is highly complex, as all factors in the design are interdependent to a greater or lesser degree

� All factors and combinations must be explored comprehensively from idea to detail, with many compromises having to be finely balanced to achieve a feasible solution

Development of a structural system: example

� Case study: two-lorry garage building with an office, toilet and tea room

5

Development of a structural system: garage

� [1] Pose an initial concept that may well satisfy the functions

– start by looking at the functions (performance) required and their

relationships

– generate a 'bubble' (or flow) diagram of relationships between

different functional areas to decide possible interconnections and

locations

– find suitable plan areas and minimum clear heights of each three-

dimensional 'volume of space'

– a possible plan layout may then

be indicated, noting any

particular complications of the

site, e.g. plan shape, proximity

of old buildings, slope or

soil consistency

– many other plan arrangements

will be possible and should be

considered quickly at this phase


– The requirements

of each 'volume of

space' and its

interfaces must be

examined for all

functional, cost

and aesthetic

criteria, e.g. what

structural applied

live loads must be

resisted; what

heating, ventilating,

lighting and

acoustic

requirements are

likely to be desired


6


– The main criteria

can easily be

recognised and

then followed up

and tested by

numerical

assessment.

– Incompatibilities

may be 'designed

out' by re-

arranging the

planned spaces or

making other

compromises.



– Prepare a set of initial

assumptions for possible

materials and the

structural system

compatible with the

'volumes of space'

• 'Frame'

• 'Planar' or

• 'Membrane'

– Previous knowledge and

understanding of actual

constructions or

structural theory, as well

as the current availability

of materials and skills


7


– Hypothesis: choose steel 'frame' system

– Elucidate and test your ideas by making quick 3D

sketches, or simple physical models, to explore

the likely compatibility and aesthetic impact

– A range of patterns viewed at different distances

from, all around, and inside the buildings must

be developed

• Long range - the skyline silhouette or "landscape"

pattern

• Middle distance - when the whole built object can be seen

• Close up - when a detail is clearly seen

• Very close - when the texture of the materials can be seen



– All principal specialists (architects, engineers for

structure and environmental services, and also

major suppliers and contractors) collaborate and

communicate freely with each other - also with

the client - at this conceptual design phase

– Be prepared to modify the concept readily and

work quickly

– Timescale for an initial structural design

concept: minutes. But hours will be needed for

discussion and communication with others in

researching an initial complete design idea


8


• Profiled steel decking roof: the rainwater should run to the sides

• Manufacturers' data table

will indicate both the slope angle to be provided (4°- 6°minimum) and the secondary beam (purlin) spacing required.

� [2] Recognise the main structural systems and estimate the necessary strength and stiffness

– Consider the applied live loads from roofs, floors or walls, and

trace the 'load paths' through the integral 3D array of elements to

the foundations


• The purlins must be supported, by a sloped main beam or truss, usually spanning the shorter direction in plan, and supported by columns

stabilised in three dimensions

• Wind loads on the longer side of the building can be resisted by cladding that spans directly to the main columns, or onto sidewall rails spanning between

columns




the foundations

9


• The columns could resist overturning by:

– cross-bracing (in this case

the large entry door would be impeded).

– rigidly fixing the columns to the foundation bases;

can the soil resist the extra overturning effect at the base?

– or rigidly fixing the tops of

the columns to the main beams (creating 'portals') and giving smaller, cheaper "pin" base foundations.




the foundations


• Wind loads on the open short side of the building can be resisted by the opening door spanning top to bottom, or side to side

• At the closed short side the wind loads can be resisted by cladding that either spans directly between secondary end wall columns, or onto rails to these columns




the foundations

10


• Roof bracings may be required to transfer the horizontal longitudinal forces due to wind and earthquake to the top of the columns along

longitudinal walls

• These forces need to

reach the foundation ⇒⇒⇒⇒vertical braces along the longitudinal walls




the foundations



– Identify the prime force actions (compression C; tension T;

bending B) in the elements and the likely forms of overall and

element deflections for all applied loadings both separately and

when combined.

– Have the elements drawn to an approximate scale

– At this phase the structural design becomes more definite and

takes longer. Timescale: minutes.

11


� [3] Assess loads accurately and estimate sizes of main elements

– Evaluate loads

– Estimate action effects (forces and bending moments in structural

members)

– Estimate beam sizes (checking bending strength

and stiffness). Structure/service duct or

pipe integration may require beams to

be as shallow as possible, or

deeper and with holes in the web.

– Likely jointing methods must

be considered carefully: is

the beam to be simply

supported or fully continuous

and what are the fabrication,

erection and cost implications?

– Structural calculations are now being performed and the time

involved is more significant. Timescale: minutes/hours.


� [4] Full structural analysis, using estimated element sizes with suitable modelling of joints, related to actual details

– The structural calculations and details are now progressing.

Timescale: hours/days.

– Iteration of phases 1-4 above will undoubtedly be required, in

particular to ensure that the early structural decisions are

compatible with the subsequent investigations concerning the

functional, environment, cost and aesthetic aspects.

12


� [5] Communicate Design Intentions through Drawings and Specifications

– The structural design is now being finalised.

Timescale: days/weeks.


� [6] Supervise the Execution Operation

– The design ideas are now being put into operation. Timescale:

weeks/months

13


� [7] Conduct Regular Maintenance

– This is the operation phase. Timescale: years/decades

1


Course 8

Design codes:

- Structural Eurocodes

- Current Romanian codes


Code of Hammurabi

� The Code of Hammurabi is a well-preserved ancient law code, created about 1760 BC in ancient Babylon.

� It was enacted by the sixth Babylonian king, Hammurabi.

� Only one example of the Code survives today, inscribed on a basalt stone stele. Originally, several stelae would have been displayed in temples around the empire.

� The text has been broken down by translators into 282 laws, but this division is arbitrary, since the original text contains no divisional markers

2

Code of Hammurabi (excerpt)

...

� § 228. If a builder build a house for a man and complete it, (that man) shall give him two shekels of silver per SAR of house as his wage.

� § 229. If a builder build a house for a man and do not make its construction firm, and the house which he has built collapse and cause the death of the owner of the house, that builder shall be put to death.

� § 230. If it cause the death of a son of the owner of the house, they shall put to death a son of that builder.

� § 231. If it cause the death of a slave of the owner of the house, he shall give to the owner of the house a slave of equal value.

� § 232. If it destroy property, he shall restore whatever it destroyed, and because he did not make the house which he built firm and it collapsed, he shall rebuild the house which collapsed at his own expense.

� § 233. If a builder build a house for a man and do not make its construction meet the requirements and a wall fall in, that builder shall strengthen that wall at his own expense.

Building codes

� Many of the restrictions encountered in building design are imposed by legal regulations

� The most important ones for structural engineers are building codes, which represent a set of regulations regarding:

– principles of structural design

– guidance in evaluation of loads on structures

– specific design provisions for different type of structures (steel structures, reinforced concrete structures, foundations, etc.) and building components (electrical system, HVAC, plumbing, etc.)

� In general, building-code requirements are the minimum needed for public protection.

� Often, however, architects and engineers must design more conservatively, to meet the client’s needs, produce a more efficient building system, or take into account conditions not covered fully by code provisions.

3

Forms of building codes

� Codes can often be classified as specifications type or performance type

� Specifications type codes:

– names specific materials for specific uses and specifies minimum or maximum dimensions,

– for example, "a brick wall may not be less than 40 cm thick".

� Performance type codes:

– Specifies required performance of a construction but leaves materials, methods, and dimensions for the designers to choose.

– Performance-type codes are generally preferred, because they give designers greater design freedom in meeting clients’ needs, while satisfying the intent of the code.

� Most codes are rather a mixture of specifications and performance type. The reason for this is that insufficient information is currently available for preparation of an entire enforceable performance code.

The Eurocodes: why?

� European continent was traditionally divided in many countries, each with its own building design code ⇒⇒⇒⇒ firms were constrained to design/fabricate/construct for their own country market

� With the advent of the European Union, it has developed a single market through a standardised system of laws which apply in all member states, guaranteeing the freedom of movement of people, goods, services and capital

� The objective of the European Commission is for “the Eurocodes to establish a set of common technical rules for the design of buildings and civil engineering works which will ultimately replace the differing rules in the various Member States”.

4

What are the Eurocodes?

� The Eurocodes are a set of European Standards (EN) for the design of buildings and other civil engineering works and construction products

� The Eurocodes are produced by the Comité Européen de Normalisation (CEN).

� The Eurocodes embody National experience and research output together with the expertise of CEN Technical Committee 250 (CEN/TC250) and of International Technical and Scientific Organisations and represent a "world-class standard for structural design".

� Publication of the Eurocodes was completed in 2007. Following CEN rules, the Eurocodes can be used in parallel with National Standards until 2010, when all conflicting National Standards should be withdrawn.

What are the Eurocodes?

� The Eurocodes cover in a comprehensive manner:

– all principal construction materials (concrete, steel, timber, masonry and aluminium),

– all major fields of structural engineering (basis of structural design, loading, fire, geotechnics, earthquake, etc.) and

– a wide range of types of structures and products (buildings, bridges, towers and masts, silos, etc).

� The verification procedure in the Eurocodes is based on the limit state concept used in conjunction with partial safety factors. The Eurocodes allow also for design based on probabilistic methods as well as for design assisted by testing, and provide guidance for the use of these methods.

5

The Eurocodes suite

The Eurocodes suite is made up by 10 European Standards for structural design

� EN 1990 Eurocode: Basis of structural design

� EN 1991 Eurocode 1: Actions on structures

� EN 1992 Eurocode 2: Design of concrete structures

� EN 1993 Eurocode 3: Design of steel structures

� EN 1994 Eurocode 4: Design of composite steel and concrete structures

� EN 1995 Eurocode 5: Design of timber structures

� EN 1996 Eurocode 6: Design of masonry structures

� EN 1997 Eurocode 7: Geotechnical design

� EN 1998 Eurocode 8: Design of structures for earthquake resistance

� EN 1999 Eurocode 9: Design of aluminium structures

Links between the Eurocodes

EN 1990Structural safety, serviceability and durability

EN 1991Actions on structures

EN 1992 EN 1993 EN 1994

EN 1995 EN 1996 EN 1999

Design and detailing

EN 1997Geotechnical design

EN 1998Seismic design

6

EN Eurocode Parts and Packages

� Each of the codes (except EN 1990) is divided into a number of Parts covering specific aspects of the subject

� In total there are 58 EN Eurocode parts distributed in the ten Eurocodes (EN 1990 – 1999)

� All of the EN Eurocodes relating to materials have a Part 1-1 which covers the design of buildings and other civil engineering structures and a Part 1-2 for fire design

� The EN Eurocode Parts have been grouped into Packages, each of which must be published before the implementation of that set of EN Eurocodes may begin

EN 1990: Basis of structural design

� EN 1990 establishes Principles and Requirements for the safety, serviceability and durability of structures, describes the basis for their design and verification and gives guidelines for related aspects of structural reliability

� EN 1990 is intended to be used in conjunction with EN 1991 to EN 1999 for the structural design of buildings and other civil engineering works, including geotechnical aspects, structural fire design, situations involving earthquakes, execution and temporary structures. For the design of special construction works (e.g. nuclear installations, dams, etc.), other provisions than those in EN 1990 to EN 1999 might be necessary.

7


� EN 1990 is applicable for the design of structures where other materials or other actions outside the scope of EN 1991 to EN 1999 are involved.

� EN 1990 is applicable for the structural appraisal of existing construction, in developing the design of repairs and alterations or in assessing change of use.

� EN 1990 may be used, when relevant, as a guidance document for the design of structures outside the scope of the Eurocodes EN 1991 to EN 1999, for:

– assessing other actions and their combinations;

– modelling material and structural behaviour;

– assessing numerical values of the reliability format.

EN 1991: Actions on structures

� EN 1991 (Eurocode 1) provides comprehensive information on all actions that should normally be considered in the design of buildings and other civil engineering works, including some geotechnical aspects.

� EN 1991 is intended to be used in conjunction with EN 1992 to EN 1999 for the structural design of buildings and other civil engineering works.

� EN 1991 is divided into four main parts:

– Part 1: General actions

– Part 2: Traffic loads on bridges

– Part 3: Actions induced by cranes and machinery

– Part 4: Silos and tanks

8

EN 1991: Actions on structures

EN 1991-1-1:2002 Eurocode 1: Actions on structures - Part 1-1: General actions -Densities, self-weight, imposed loads for buildings

EN 1991-1-2:2002 Eurocode 1: Actions on structures - Part 1-2: General actions -Actions on structures exposed to fire

EN 1991-1-3:2003 Eurocode 1: Actions on structures - Part 1-3: General actions -Snow loads

EN 1991-1-4:2005 Eurocode 1: Actions on structures - Part 1-4: General actions -Wind actions

EN 1991-1-5:2003 Eurocode 1: Actions on structures - Part 1-5: General actions -Thermal actions

EN 1991-1-6:2005 Eurocode 1: Actions on structures - Part 1-6: General actions -Actions during execution

EN 1991-1-7:2006 Eurocode 1: Actions on structures - Part 1-7: General actions -Accidental actions

EN 1991-2:2003 Eurocode 1: Actions on structures - Part 2: Traffic loads on bridges

EN 1991-3:2006 Eurocode 1: Actions on structures - Part 3: Actions induced by cranes and machinery

EN 1991-4: 2006 Eurocode 1: Actions on structures - Part 4: Silos and tanks

EN 1992: Design of concrete structures

� EN 1992 (Eurocode 2) applies to the design of buildings and other civil engineering works in plain, reinforced and prestressed concrete.

� Eurocode 2 is concerned with the requirements for resistance, serviceability, durability and fire resistance of concrete structures.

– Part 1-1 gives a general basis for the design of structures in plain, reinforced and prestressed concrete

– Part 1-2 deals with the design of concrete structures for the accidental situation of fire exposure

– Part 2 gives a general basis for the design and detailing of bridges in reinforced and prestressed concrete

– Part 3 covers additional rules for the design of concrete structures for the containment of liquids or granular solids and other liquid retaining structures

9

EN 1992: Design of concrete structures

EN 1992-1-1:2004 Eurocode 2: Design of concrete structures -Part 1-1: General rules and rules for buildings

EN 1992-1-2:2004 Eurocode 2: Design of concrete structures -Part 1-2: General rules - Structural fire design

EN 1992-2:2005 Eurocode 2: Design of concrete structures -Part 2: Concrete bridges - Design and detailing rules

EN 1992-3:2006 Eurocode 2: Design of concrete structures -Part 3: Liquid retaining and containment structures

EN 1993: Design of steel structures

� EN 1993 (Eurocode 3) applies to the design of buildings and other civil engineering works in steel.

� EN 1993 is concerned with requirements for resistance, serviceability, durability and fire resistance of steel structures.

� EN Eurocode 3 is wider in scope than most of the other design EN Eurocodes due to the diversity of steel structures, the need to cover both bolted and welded joints and the possible slenderness of construction.

� EN 1993 has 20 parts covering common rules, fire design, bridges, buildings, tanks, silos, pipelined piling, crane supported structures, towers and masts, chimneys etc.

10


EN 1993-1-1:2005 Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for buildings

EN 1993-1-2:2005 Eurocode 3: Design of steel structures - Part 1-2: General rules - Structural fire design

EN 1993-1-3:2006 Eurocode 3: Design of steel structures - Part 1-3: General rules - Supplementary rules for cold-formed members and sheeting

EN 1993-1-4:2006 Eurocode 3: Design of steel structures - Part 1-4: General rules - Supplementary rules for stainless steels

EN 1993-1-5:2006 Eurocode 3: Design of steel structures - Part 1-5: General rules - Plated structural elements

EN 1993-1-6:2007 Eurocode 3: Design of steel structures - Part 1-6: Strength and stability of shell structures

EN 1993-1-7:2007 Eurocode 3: Design of steel structures - Part 1-7: Strength and stability of planar plated structures subject to out of plane loading

EN 1993-1-8:2005 Eurocode 3: Design of steel structures - Part 1-8: Design of joints

EN 1993-1-9:2005 Eurocode 3: Design of steel structures - Part 1-9: Fatigue


EN 1993-1-10:2005 Eurocode 3: Design of steel structures - Part 1-10: Material toughness and through-thickness properties

EN 1993-1-11:2006 Eurocode 3: Design of steel structures - Part 1-11: Design of structures with tension components

EN 1993-1-12:2007 Eurocode 3: Design of steel structures - Part 1-12: General - High strength steels

EN 1993-2:2006 Eurocode 3: Design of steel structures - Part 2: Steel bridges

EN 1993-3-1:2006 Eurocode 3: Design of steel structures - Part 3-1: Towers, masts and chimneys – Towers and masts

EN 1993-3-2:2006 Eurocode 3: Design of steel structures - Part 3-2: Towers, masts and chimneys – Chimneys

EN 1993-4-1:2007 Eurocode 3: Design of steel structures - Part 4-1: Silos

EN 1993-4-2:2007 Eurocode 3: Design of steel structures - Part 4-2: Tanks

EN 1993-4-3:2007 Eurocode 3: Design of steel structures - Part 4-3: Pipelines

EN 1993-5:2007 Eurocode 3: Design of steel structures - Part 5: Piling

EN 1993-6:2007 Eurocode 3: Design of steel structures - Part 6: Crane supporting structures

11

EN 1994: Design of composite steel and concrete structures

� EN 1994 (Eurocode 4) applies to the design of composite structures and members for buildings and other civil engineering works

� EN 1994 is concerned with requirements for resistance, serviceability, durability and fire resistance of composite structures.

� EN 1994 is intended to be used in conjunction with:

– EN 1992: Eurocode 2 - Design of concrete structures and

– EN 1993: Eurocode 3 - Design of steel structures

EN 1994: Design of composite steel and concrete structures

EN 1994-1-1:2004 Eurocode 4: Design of composite steel and concrete structures – Part 1-1: General rules and rules for buildings

EN 1994-1-2:2005 Eurocode 4: Design of composite steel and concrete structures – Part 1-2: General rules - Structural fire design

EN 1994-2:2005 Eurocode 4: Design of composite steel and concrete structures – Part 2: General rules and rules for bridges

12

EN 1995: Design of timber structures

� EN 1995 (Eurocode 5) applies to the design of buildings and other civil engineering works in timber (solid timber, sawn, planed or in pole form, glued laminated timber or wood-based structural products) or wood-based panels jointed together with adhesives or mechanical fasteners

EN 1995-1-1:2004 Eurocode 5: Design of timber structures -Part 1-1: General - Common rules and rules for buildings

EN 1995-1-2:2004 Eurocode 5: Design of timber structures -Part 1-2: General - Structural fire design

EN 1995-2:2004 Eurocode 5: Design of timber structures -Part 2: Bridges

EN 1996: Design of masonry structures

� EN 1996 (Eurocode 6) applies to the design of buildings and other civil engineering works, or parts thereof, in unreinforced, reinforced, prestressed and confined masonry.

� The execution is covered to the extent that is necessary to indicate the quality of the construction materials and products that should be used and the standard of workmanship on site needed to comply with the assumptions made in the design rules.

EN 1996-1-1:2005 Eurocode 6: Design of masonry structures - Part 1-1: General rules for reinforced and unreinforced masonry structures

EN 1996-1-2:2005 Eurocode 6: Design of masonry structures - Part 1-2: General rules -Structural fire design

EN 1996-2:2006 Eurocode 6: Design of masonry structures - Part 2: Design considerations, selection of materials and execution of masonry

EN 1996-3:2006 Eurocode 6: Design of masonry structures - Part 3: Simplified calculation methods for unreinforced masonry structures

13

EN 1997: Geotechnical design

� EN 1997 (Eurocode 7) applies to the geotechnical aspects of the design of buildings and other civil engineering works

� Numerical values of actions on buildings and other civil engineering works to be taken into account in design are provided in EN 1991 for the various types of construction, whereas actions imposed by the ground, such as earth pressures and by ground water, shall be calculated according to the rules of EN 1997.

EN 1997-1:2004 Eurocode 7: Geotechnical design - Part 1: General rules

EN 1997-2:2007 Eurocode 7: Geotechnical design - Part 2: Ground investigation and testing

EN 1998: Design of structures for earthquake resistance

� EN 1998 Eurocode 8 applies to the design and construction of buildings and other civil engineering works in seismic regions.

� Its purpose is to ensure that in the event of earthquakes

– human lives are protected;

– damage is limited;

– structures important for civil protection remain operational.

� The random nature of the seismic events and the limited resources available to counter their effects are such as to make the attainment of these goals only partially possible and only measurable in probabilistic terms.

14


� The extent of the protection that can be provided to different categories of buildings, which is only measurable in probabilistic terms, is a matter of optimal allocation of resources and is therefore expected to vary from country to country, depending on the relative importance of the seismic risk with respect to risks of other origin and on the global economic resources.

� Special structures, such as nuclear power plants, offshore structures and large dams, are beyond the scope of EN 1998.


� EN 1998 contains only those provisions that, in addition to the provisions of the other relevant EN Eurocodes, must be observed for the design of structures in seismic regions. It complements in this respect the other EN Eurocodes.

EN 1998-1:2004 Eurocode 8: Design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings

EN 1998-2:2005 Eurocode 8: Design of structures for earthquake resistance – Part 2: Bridges

EN 1998-3:2005 Eurocode 8: Design of structures for earthquake resistance – Part 3: Assessment and retrofitting of buildings

EN 1998-4:2006 Eurocode 8: Design of structures for earthquake resistance – Part 4: Silos, tanks and pipelines

EN 1998-5:2004 Eurocode 8: Design of structures for earthquake resistance – Part 5: Foundations, retaining structures and geotechnical aspects

EN 1998-6:2005 Eurocode 8: Design of structures for earthquake resistance – Part 6: Towers, masts and chimneys

15

EN 1999: Design of aluminium structures

� EN 1999 Eurocode 9 applies to the design of buildings and other civil engineering and structural works in aluminium.

� EN 1999 is concerned with requirements for resistance, serviceability, durability and fire resistance of aluminium structures.

EN 1999-1-1:2007 Eurocode 9: Design of aluminium structures - Part 1-1: General structural rules

EN 1999-1-2:2007 Eurocode 9: Design of aluminium structures - Part 1-2: Structural fire design

EN 1999-1-3:2007 Eurocode 9: Design of aluminium structures - Part 1-3: Structures susceptible to fatigue

EN 1999-1-4:2007 Eurocode 9: Design of aluminium structures - Part 1-4: Cold-formed structural sheeting

EN 1999-1-5:2007 Eurocode 9: Design of aluminium structures - Part 1-5: Shell structures

EU legislation for construction works and products

� The Construction Products Directive (Council Directive 89/106/EEC) aim is to breakdown artificial barriers to trade throughout the European Union and is intended for products placed on the market

� According to the Construction Products Directive, construction products suitable for construction works need to satisfy the following six essential requirements as appropriate:

– mechanical resistance and stability

– safety in case of a fire

– hygiene, health and the environment

– safety in use

– protection against noise

– energy economy and heat retention

16

EU legislation for construction works and products

� The Eurocodes provide common design methods, expressed in a set of European standards, which are intended to be used as reference documents for Member States to:

– prove the compliance of building and civil engineering works or parts thereof with Essential Requirement n°1 Mechanical resistance and stability (including such aspects of Essential Requirement n°4 Safety in use, which relate to mechanical resistance and stability) and a part of Essential Requirement n°2 Safety in case of fire, including durability

– express in technical terms , these Essential Requirements applicable to the works and parts thereof;

– determine the performance of structural components and kits with regard to mechanical resistance and stability and resistance to fire, insofar as it is part of the information accompanying CE marking

Implementation of the Eurocodes

� When an EN Eurocode Part is made available by CEN (Date of Availability), National Authorities and National Standards Bodies should:

– translate the Eurocode Part in authorised national languages

– set the Nationally Determined Parameters to be applied on their territory

– publish the National Standard transposing the EN Eurocode Part and the National Annex

– adapt their National Provisions so that the EN Eurocode Part can be used on their territory

– promote training on the Eurocodes

17


� The implementation of an EN Eurocode Part has three phases:

– Translation period (max 1 year). The National Standards Bodies may start the translation of a Eurocode Part in authorised national languages at the latest at the Date of Availability.


– National Calibration period (max 2 years). The Member States should fix the Nationally Determined Parameters. At the end of this period, the national version of the EN Eurocode Part with the National Annex will be published by the National Standards Bodies. Also, the Member States should adapt the National Provisions so that the Eurocode Part can be used on their territory.

18


– Coexistence period. During the coexistence period, which starts at the end of the National Calibration period, the Eurocode Part can be used, just as the presently existing national system can also be used. The coexistence period of a Eurocode Package will last up to a maximum time of three years after the national publication of the last Part of a Package. Member States shall make sure that all the Parts of the related Package can be used without ambiguity on their territory by adapting their National Provisions as necessary.


� All conflicting National Standards in a Package should be withdrawn a maximum of 5 years after the Date of Availability of the last available Part in the Package

� Following CEN rules, the Eurocodes can be used in parallel with National Standards until 2010, when all conflicting National Standards will be withdrawn

19

National Standards

� The National Standard transposing the EN Eurocode Part, when published by a National Standards Body, will be composed of the EN Eurocode text preceded by the National Title page and by the National Foreword and generally followed by the National Annex

� The National Standards Bodies should normally publish the National Annex, on behalf of and with the agreement of the competent National Authorities

National Standards: National Annex

� The National Annex (NA) may contain directly, or by reference to specific provisions,

– information on the Nationally Determined Parameters (NDPs) to be used for the design of buildings and other civil engineering works to be constructed in the country concerned.

– It may also contain decisions on the application of informative annexes and reference to non-contradictory complementary information

� A National Annex is not necessary if a Eurocode Part contains no choice open for Nationally Determined Parameters, or if a Eurocode Part is not relevant for the Member State (e.g. EN 1998 for seismic design for some countries)

20

National Standards: NDPs

� Eurocodes "recognise the responsibility of regulatory authorities in each Member State and have safeguarded their right to determine values related to safety matters at national level where these continue to vary from State to State".

� National choice is provided by the Eurocodes with sets of recommended values, classes, symbols and alternative methods to be used as NDPs.

� The NDPs account for possible differences in geographical or climatic conditions, or in ways of life, as well as different levels of protection that may prevail at national, regional or local level.

� When the Eurocodes are used for the design of construction works, or parts thereof, the NDPs of the Member State on whose territory the works are located shall be applied.

Current Romanian codes

� Standards issued and maintained by ASRO - Asociaţia de Standardizare din România (Romanian Standards Association): "SR-EN“, "STAS", "SR"

� Application of standards issued by ASRO are optional (law 39/1998 and 355/2002). Application of standards can become compulsory only when stipulated by a technical regulation issued by an authority

� Authority - Ministry of Regional Development and Public Administration (Ministerul Dezvoltarii Regionale si Administratiei Publice - MDRAP)

� MDRAP issues a list of technical regulations in force, most available in "Buletinul Constructiilor" published by URBAN-INCERC (Institutul Naţional de Cercetare-Dezvoltare în Construcţii, Urbanism şi Dezvoltare Teritorială Durabilă)

� Current design codes in Romania are in continuous evolution, as new European standards are implemented

21

Some of the current Romanian codes

� CR 0-2012: “Cod de proiectare. Bazele proiectării construcţiilor”

� CR 1-1-3/2012: ”Cod de proiectare. Evaluarea acţiunii zăpezii asupra construcţiilor”

� CR 1-1-4/2012: “Cod de proiectare. Evaluarea acţiuniivântului asupra construcţiilor”

� P 100-1/2013: “Cod de proiectare seismică – Partea I –Prevederi de proiectare pentru clădiri”

1


Course 9

Actions on structures: permanent loads, imposed loads and snow loads


EN 1990: Classification of loads

Actions are classified by their variation in time as follows:– permanent actions (G), e.g. self-weight of structures, fixed

equipment and road surfacing, and indirect actions caused by shrinkage and uneven settlements;

– variable actions (Q), e.g. imposed loads on building floors, beams and roofs, wind actions or snow loads;

– accidental actions (A), e.g. explosions, or impact from vehicles.

Actions can also be classified– by their origin, as direct or indirect,

– by their spatial variation, as fixed or free, or

– by their nature and/or the structural response, as static or dynamic.

2


Permanent action is one that is likely to act throughout a given reference period and for which the variation in magnitude with time is negligible, or for which the variation is always in the same direction (monotonic) until the action attains a certain limit value

Variable action is one for which the variation in magnitude with time is neither negligible nor monotonic

Accidental action is usually of short duration but of significant magnitude, that is unlikely to occur on a given structure during the design working life


Certain actions, such as snow loads, may be considered as either accidental and/or variable actions, depending on the site location

Actions caused by water may be considered as permanent and/or variable actions depending on the variation of their magnitude with time

Direct action: a set of forces (loads) applied to the structure

Indirect action: a set of imposed deformations or accelerations caused for example, by temperature changes, moisture variation, uneven settlement or earthquakes

3


A fixed action is one that has a fixed distribution and position over the structure or structural member such that the magnitude and direction of the action are determined unambiguously for the whole structure or structural member if this magnitude and direction are determined at one point on the structure or structural member

A free action is one that may have various spatial distributions over the structure

An action should be described by a model, its magnitude being represented in the most common cases by one scalarNOTE: For some actions and some verifications, a more complex representation of the magnitudes of some actions may be necessary.

Permanent actions: EN 1991-1-1

The self-weight of construction works is classified as a permanent fixed action

Permanent action is one which is likely to act throughout a given reference period and for which the variation in magnitude with time is negligible, or for which the variation is always in the same direction (monotonic) until the action attains a certain limit value

Examples of permanent actions: – self-weight (or dead load) of structures,

– fixed equipment and road surfacing,

– and indirect actions caused by shrinkage and uneven settlements

4


Normative references: EN 1991-1-1: Eurocode 1: Actions on structures - Part 1-1: General actions - Densities, self-weight, imposed loads for buildings

The total self-weight of structural and non-structural members should be taken into account in combinations of actions as a single action.

The self-weight of new coatings and/or distribution conduits that are intended to be added after execution should be taken into account in design situations.

The source and moisture content of bulk materials should be considered in design situations of buildings used for storage purposes.


The self-weight of the construction works should be represented in most cases by a single characteristic value and be calculated on the basis of the nominal dimensions and the characteristic values of the densities.

The self weight of the construction works includes the structure and non-structural elements including fixed services as well as the weight of earth and ballast.

Non-structural elements include:– roofing;

– surfacing and coverings;

– partitions and linings;

– hand rails, safety barriers, parapets and kerbs;

– wall cladding;

– suspended ceilings;

– thermal insulation;

– fixed services.

5


– roofing;

– surfacing and coverings;


– partitions and linings;

– hand rails, safety barriers, parapets and kerbs;

6


– wall cladding;

– suspended ceilings;


– thermal insulation;

– fixed services

Fixed services include:– equipments for lifts and moving

stairways;

– heating, ventilating and air conditioning (HVAC) equipment;

– electrical equipment;

– pipes without their contents;

– cable trunking and conduits.

7


Characteristic values of self-weight are determined using – nominal dimensions (from architectural plans and details) and

– characteristic values of densities (obtained from Annex A to EN 1991-1-1 or manufacturer)


For manufactured elements such as flooring systems, facades and ceilings, lifts and equipment for buildings, data may be provided by the manufacturer

For determining the effect of the self-weight due to movable partitions, an equivalent uniformly distributed load shall be used and added to the imposed load

8

Self-weight: example

CARPET FLOOR

LEVELING MORTAR

RAISED FLOOR SYSTEM

REINFORCED CONCRETE SLAB

Thickness,mm

Specificweight,kN/m3

Weight,kN/m2

CARPET FLOOR ON RAISEDFLOOR SYSTEM

0.40

LEVELING MORTAR 30 21.0 0.63REINFORCED CONCRETE SLAB 150 25.0 3.75

TOTAL 4.78

Imposed loads on buildings - EN 1991-1-1

Imposed (or live) loads on buildings are those arising from occupancy, including: – normal use by persons;

– furniture and moveable objects (e.g. moveable partitions, storage, the contents of containers);

– vehicles;

– anticipating rare events, such as concentrations of persons or of furniture, or the moving or stacking of objects which may occur during reorganization or redecoration

Imposed loads shall be classified as variable free actions

The imposed loads are modelled by uniformly distributed loads, line loads or concentrated loads or combinations of these loads.

For the determination of the imposed loads, floor and roof areas in buildings should be sub-divided into categories according to their use.

9

Imposed loads on buildings - EN 1991-1-1

Heavy equipment (e.g. in communal kitchens, radiology rooms, boiler rooms etc) are not included in the loads given in EN 1991-1-1. Loads for heavy equipment should be agreed between the client and/or the relevant Authority.

Generally, imposed loads are considered as uniformly distributed. To ensure a minimum local resistance of the floor structure a separate verification shall be performed with a concentrated load. The concentrated load shall be considered to act at any point on the floor (over an area with a shape which is appropriate to the use and form of the floor)

qk Qk

Imposed loads on buildings: Categories

Areas in residential, social, commercial and administration buildings are divided into categories according to their specific uses

Dynamic effects shall be considered where it is anticipated that the occupancy will cause significant dynamic effects

10

Imposed loads on buildings: load values

Characteristic values qk for uniformly distributed load and Qk for concentrated load are assigned to each category. Recommended values are underlined.

Imposed loads on buildings: load values

Romanian National Annex to SR EN 1991-1-1:2004

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Imposed loads on buildings

Where necessary qk and Qk should be increased in the design (e.g. for stairs and balconies depending on the occupancy and on dimensions). Where no value is specified in the code, informatively, the loads on stairs and balconies can be increased by 1.0 kN/m2.

Imposed loads on buildings: movable partitions

Provided that a floor allows a lateral distribution of loads, the self-weight of movable partitions may be taken into account by a uniformly distributed load qk which should be added to the imposed loads of floors. This defined uniformly distributed load is dependent on the self-weight of the partitions as follows:– for movable partitions with a self-weight ≤ 1.0 kN/m wall length: qk =0.5 kN/m2

– for movable partitions with a self-weight ≤ 2.0 kN/m wall length: qk =0.8 kN/m2;

– for movable partitions with a self-weight ≤ 3.0 kN/m wall length: qk =1.2 kN/m2

Heavier partitions should be considered in the design taking account of:– the locations and directions of the partitions;

– the structural form of the floors

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Imposed loads are free actions: – the most unfavourable spatial distribution shall be considered

– in practice, several "chessboard" distributions are considered in addition to the uniform distribution

uniform distribution chessboard distribution 1 chessboard distribution 2


EN 1991-1-1 contain provisions for calculation of characteristic values of loads for the following types of use of buildings:– Residential, social, commercial and administration areas

– Areas for storage and industrial activities (including actions induced by forklifts, actions induced by transport vehicles)

– Garages and vehicle traffic areas (excluding bridges)

– Roofs

Additionally, horizontal loads on parapets and partition walls acting as barriers need to be considered in design.

Normative references: EN 1991-1-1: Eurocode 1: Actions on structures - Part 1-1: General actions - Densities, self-weight, imposed loads for buildings

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Snow load: normative references

Normative references– EN 1991-1-3: Eurocode 1 - Actions on structures -

Part 1-3: General actions - Snow loads

– CR 1-1-3/2012: Cod de proiectare. Evaluarea acţiunii zăpezii asupra construcţiilor

EN 1991-1-3 and CR 1-1-3-2012 give guidance to determine the values of loads due to snow to be used for the structural design of buildings and civil engineering works

Snow load: special cases

The two codes does NOT give guidance on specialist aspects of snow loading, for example:– impact snow loads resulting from snow sliding off or falling from

a higher roof;

– the additional wind loads which could result from changes in shape or size of the construction works due to the presence of snow or the accumulation of ice;

– loads in areas where snow is present all year round;

– ice loading;

– lateral loading due to snow (e.g. lateral loads exerted by drifts);

– snow loads on bridges.

In regions with possible rainfalls on the snow and consecutive melting and freezing, snow loads on roofs should be increased, especially in cases where snow and ice can block the drainage system of the roof

14

Snow load: classification and characteristics

Generally, for the purpose of applying EN 1990, snow loads are classified as variable, fixed, and static actions.

Snow load may be treated as accidental in two cases: – In particular situation of a snow fall which has an exceptionally

infrequent likelihood of occurring

– In particular situation of a snow deposition pattern which has an exceptionally infrequent likelihood of occurring

Snow action is modelled as a gravity (vertical) load applied on roofs of buildings, acting per unit area of horizontal projection

Snow load on the ground

The characteristic value of snow load on the ground (sk) is based upon the probability of 0.02 being exceeded for a reference period of one year. This is equivalent to a mean return period of 50 years.

CR 1-1-3-2013 gives ground snow load map of Romania, representing characteristic values of snow load on ground, for altitudes below 1000 m

For higher altitudes, the following relations can be used to obtain characteristic values of snow load on ground:

15

Snow load on the ground

Snow load: nature of load

Snow can be deposited on a roof in many different patterns. Properties of a roof or other factors causing different patterns can include:– the shape of the roof;

– its thermal properties;

– the roughness of its surface;

– the amount of heat generated under the roof;

– the proximity of nearby buildings;

– the surrounding terrain;

– the local meteorological climate, in particular its windiness, temperature

– variations, and likelihood of precipitation (either as rain or as snow).

16

Snow load

Two primary load arrangements should be considered when modelling snow action:– undrifted snow load on the roof:

load arrangement which describes the uniformly distributed snow load on the roof, affected only by the shape of the roof, before any redistribution of snow due to other climatic actions.

– drifted snow load on the roof: load arrangement which describes the snow load distribution resulting from snow having been moved from one location to another location on a roof, e.g. by the action of the wind.

undriftedsnow

driftedsnow

Snow load: code procedure

Snow load on the roof in the persistent/transient design situation is determined as follows:s = Is i Ce Ct sk

Is is the importance – exposure factor for snow load

i is the snow load shape coefficient, depending on the shape of the roof

sk is the characteristic value of snow load on the ground, depending on geographic location of the building and on altitude

Ce is the exposure coefficient, accounting for the degree in which wind sweeps the snow from the roof

Ct is the thermal coefficient, defining the reduction of snow load on roofs as a function of the heat flux through the roof, causing snow melting

17


s = Is i Ce Ct sk Is is the importance – exposure factor for snow load


s = Is i Ce Ct sk sk is the characteristic value of snow load on the ground,

depending on geographic location of the building and on altitude

18


s = Is i Ce Ct sk

The thermal coefficient Ct is used to account for the reduction of snow loads on roofs with high thermal transmittance (> 1 W/m2K), in particular for some glass covered roofs, because of melting caused by heat loss

For most building structures, the roofs do not fit the above condition, having a lower thermal transmittance, and, therefore, Ct = 1.0


s = Is i Ce Ct sk Ce is the exposure coefficient, accounting for the degree

in which wind sweeps the snow from the roof, and depends on the topography at the building site

19


s = Is i Ce Ct sk i is the snow load shape coefficient,

depending on the shape of the roof

Roof shape coefficients are available for undrifted and drifted snow

Example: monopitch roofs– Values for roof shape coefficients apply

when the snow is not prevented from sliding off the roof.

– Where snow fences or other obstructions exist or where the lower edge of the roof is terminated with a parapet, then the snow load shape coefficient should not be reduced below 0.8


Example: pitched roofs– case (i): undrifted snow

– case (ii): drifted snow

– case (iii): drifted snow

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Example: multi-span roofs– case (i): undrifted snow

– case (ii): drifted snow


Further guidance is available in codes for roof shape coefficients for:

– Cylindrical roofs

– Roof abutting and close to taller construction works

• s – snow load shape coefficient due to sliding of snow from the upper roof

• w – the snow load shape coefficient due to wind

21


Roof shape coefficients are also specified for local effects: – drifting at projections and obstructions;

– the edge of the roof;

– snow fences

Drifting at projections and obstructions: – in windy conditions drifting of snow can occur on any roof which

has obstructions as these cause areas of aerodynamic shade in which snow accumulates

– accumulation of snow due to parapets at roof edges can be modeled using this procedure


Snow overhanging the edge of a roof: the design of those parts of a roof cantilevered out beyond the walls should take account of snow overhanging the edge of the roof, in addition to the load on that part of the roof

Snow loads on snowguards and other obstacles: under certain conditions snow may slide down a pitched or curved roof. The sliding mass of snow need to be considered for the design of the obstacles preventing this movement.

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Course 10

Actions on structures: Wind loads

Other loads


Wind loading: normative references

Normative references– EN 1991-1-4: Eurocode 1: Actions on structures - Part 1-4:

General actions - Wind actions

– CR 1-1-4/2012: Cod de proiectare. Evaluarea acţiunii vântului asupra construcţiilor.

Wind action is classified as variable fixed actions according to EN 1990

2

Nature of wind loading

Wind represents masses of air moving mainly horizontally (parallel to the ground) from areas of high pressure to ones of low pressure

Wind generates pressures on external (and also internal) surfaces of structures

The main effect of wind is a horizontal loading of buildings (especially high-rise)

The effect of the wind on the structure (i.e. the response of the structure), depends on the size, shape and dynamic properties of the structure.

Basic value of mean wind velocity

The reference value of the wind velocity, vb, is the characteristic 10 minutes mean wind velocity, irrespective of wind direction and time of year, at 10 m above ground level in open country terrain with low vegetation such as grass and isolated obstacles with separations of at least 20 obstacle heights.

Reference values of wind velocity are determined for annual probabilities of exceedence of 0.02, which is equivalent to a mean return period of 50 years.

For design purposes, basic values of wind velocity are obtained from maps and tables given in codes (CR 1-1-4/2012).

3

Reference wind pressure

Reference wind pressure qb is the wind pressure corresponding to the reference value of the wind velocity vb

where: is the air density, which depends on altitude, temperature, latitude and season. The recommended value for design is 1.25 kg/m3

For design purposes, reference wind pressure are obtained from maps and tables given in codes (CR 1-1-4 / 2012).

21

2b bq v

Reference wind pressure

4

Mean wind velocity: gradient height

The mean wind velocity at great heights above the ground is constant and it is called the gradient wind speed.

Near the ground the mean wind velocity is decreasing much due to frictional forces caused by the terrain, being equal with zero at the ground level.

There is a boundary layer within which the wind speed varies from zero to the gradient wind speed (mean wind velocity increases with height).

Mean wind velocity: gradient height

The thickness of the boundary layer (gradient height) depends on the ground roughness. Larger the roughness, larger the gradient height.

5

Mean wind velocity: terrain categories


6


Terrain roughness is described aerodynamically by the roughness length, z0, expressed in meters. It represents a measure of the dimensions of eddies of turbulent wind at the ground surface.

Mean wind velocity: variation with height

The mean wind velocity profile within the atmospheric boundary layer can be described by a logarithmic law:

where:cr(z) is a roughness factorz - height above groundz0 – roughness length

m r bv z c z v

0 min max0

min

min

lnr

r

r

zk z for z z z

zc zz z

c z z

7

Mean wind velocity: variation with height

The terrain factor kr(z0) is given by the relationship:

0,07

00 0,189

0,05r

zk z

Mean wind pressure: variation with height

The roughness factor cr(z) is used to describe the variation of wind pressure with height

2m r bq z c z q

8

Wind turbulence

Wind velocity varies with time as shown in the figure below. This variation with respect to the mean wind velocity is called turbulence and is generated by the eddies caused by the wind blowing over obstacles

Wind turbulence

The turbulence intensity I(z) at height z is defined as the standard deviation of the turbulence divided by the mean wind velocity.

The turbulence intensity I(z) at height z can be expressed as:

v

v

m

I zv z

min max

0

minmin

200

2.5lnv

v

for z z z mz

I zz

for z zI z z

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Wind turbulence

Wind turbulence decreases with height above ground

Wind turbulence: gust factor

The gust factor cpq(z) is the ratio between the peak pressure (due to wind turbulence) and mean pressure (due to mean wind velocity)

The gust factor cpq(z) can be determined as:

where:g = 3.5 is the amplitude factorIv(z) is the turbulence intensity at height z

1 2 1 7pq v vc z g I z I z

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Wind turbulence: gust factor

Wind pressure at height z

Wind pressure at height z above ground can be obtained by considering the effects of mean wind velocity, wind turbulence, and topography on the reference pressure qb

(at the ground level)– Mean wind velocity increases with height above ground. The

effect of mean wind velocity on wind pressure profile is accounted through the roughness factor cr(z)

– Wind turbulence decreases with height above ground. The effect of wind turbulence on wind pressure at height z is accounted through the gust factor cpq(z)

– Isolated hills and other local topographical accidents can affect the mean wind velocity. In design this effect is accounted through the orography factor co. It need not be considered when the slope is less than 5% (co=1.0).

11


Effect of topography

Wind pressure at height z can be obtained as:

The product between the gust factor, the roughness factor and the topographical factor is called the exposure factor, and is denoted by ce(z):

p e bq z c z q

2 2e o r pqc z c c z c z


12

2 2e o r pqc z c c z c z


Nature of wind loading

Wind actions act directly as pressures on the external surfaces of enclosed structures and, because of porosity of the external surface, also act indirectly on the internal surfaces.

They may also act directly on the internal surface of open structures. Pressures act on areas of the surface resulting in forces normal to the surface of the structure or of individual cladding components.

Additionally, when large areas of structures are swept by the wind, friction forces acting tangentially to the surface may be significant.

The wind action is represented by a simplified set of pressures or forces whose effects are equivalent to the extreme effects of the turbulent wind.

13

Wind effects on structures

Wind effects on structures can be classified as follows:– static or quasistatic response

– turbulence induced vibrations

– vortex induced vibrations

– galloping

– flutter

– response due to interference of nearby structures

Wind effects on structures

Most buildings are not streamlined, and are called bluff bodies in aerodynamics. – drag force, in the direction of the flow

FD = CD q

– lift force, perpendicular to flow direction

– torsion moment

For bluff bodies, wind flow separates and causes the formation of the so-called "wake"– pressure on the windward side

– suction on the leeward side

– suction/pressure on lateral surfaces

14

Wind pressure on surfaces

Wind pressure w(z) on rigid exterior and interior surfaces of the structure at height z above ground are obtained as:

where:Iw – the importance factorqp(ze) – peak wind pressure at level ze

ze – reference height for external pressure.cp – aerodynamic pressure coefficient (cpe for exterior surfaces; cpi for internal surfaces)

Pressures are considered positive (+)

Suction is considered negative (-)

The total pressure on a structural element is obtained as the algebraic sum of pressures on one side and suction on the other side

e Iw pe p ew c q z i Iw pi p iw c q z

Wind pressure on surfaces

Wind pressure w(z) on rigid exterior and interior surfaces of the structure at height z above ground are obtained as:

e Iw pe p ew c q z i Iw pi p iw c q z

15

Aerodynamic pressure coefficients

Aerodynamic pressure coefficients depend on:– geometry of the structure/element

– size of the structure/element

– terrain roughness

– wind direction with respect to the structure

– Reynolds number

– etc.

Pressure coefficients: loaded area

Aerodynamic pressure coefficients cpe for buildings and parts of buildings depend on the size of the loaded area A, which is the area of the structure, that produces the wind action in the section to be calculated – Values for cpe,1 are intended for the design of small elements and

fixings with an area per element of 1 m2 or less such as cladding elements and roofing elements. Values for cpe,10 may be used for the design of the overall load bearing structure of buildings.

– Due to non-uniform action of wind, peak pressure on a small area is higher than the peak overall pressure on a large area (for which some portions are loaded less)

16

Press. coeff.: vertical walls of rect. plan buildings

The reference heights, ze, for rectangular plan buildings depend on the aspect ratio h/b and are always the upper heights of the different parts of the walls

Reference heights are used to compute the exposure factor ce(z)

Three cases:– A building, whose height h is less than b should be considered to

be one part.


– A building, whose height h is greater than b, but less than 2b, may be considered to be two parts, comprising: a lower part extending upwards from the ground by a height equal to b and an upper part consisting of the remainder.

17


– A building, whose height h is greater than 2b may be considered to be in multiple parts, comprising: a lower part extending upwards from the ground by a height equal to b; an upper part extending downwards from the top by a height equal to b and a middle region, between the upper and lower parts, which may be divided into horizontal strips with a height hstrip (max hstrip = b)


Depending on geometry and position with respect to wind direction, different regions of vertical walls are assigned different names, with corresponding values of pressure coefficients cp

18


Depending on geometry and position with respect to wind direction, different regions of vertical walls are assigned different names, with corresponding values of pressure coefficients cp

Pressure coefficients

Similar procedure are specified in the code for roofs of buildings (of different geometry), canopies, isolated vertical walls, fences etc.

19

Wind forces method

For structures like signboards, lattice structures and scaffoldings, flags, etc. wind actions is modelled as a resultant force

where:Iw – the importance factorqp(ze) – peak wind pressure at level ze

ze – reference height for external pressure.cf - wind force coefficientcd - dynamic response coefficientAref - reference area perpendicular on wind direction

w Iw d f p e refF c c q z A

Other loads: traffic loads on bridges

– In practice a highway bridge is loaded in a very complex way by vehicles of varying sizes and groupings.

– In order to simplify the design process this real loading is typically simulated by two basic imposed loads - a uniformly distributed load and a knife edge load -representing an extreme condition of normal usage.

– The design is then checked for a further load arrangement representing the passage of an abnormal load.

– The magnitudes of all these loads are generally related to the road classification, the highway authority's requirements and the loaded length of the bridge.

20

Other loads: traffic loads on bridges

– Railway bridge design must take account of static loading and forces associated with the movement of vehicles.

– As for highway bridges, two models of loading are specified for consideration as separate load cases. They represent ordinary traffic on mainline railways and, where appropriate, abnormal heavy loads. They are expressed as static loads due to stationary vehicles and are factored to allow for dynamic effects associated with train speeds up to 300km/h.

– Eurocode 1 also gives guidance on the distribution of loads and their effects and specifies horizontal forces due to vehicle motion. Centrifugal forces associated with the movement around curves, lateral forces due to oscillation of vehicles (nosing) and longitudinal forces due to traction and braking are included.

– Other aspects of bridge loading which need to be considered include accidental loads and the possibility of premature failure due to fatigue under traffic loading.

Other loads: crane loads

– For buildings fitted with travelling overhead cranes, the loads due to the crane itself and the lifted load are considered separately.

– The self weight of the crane installation is generally readily available from the manufacturer, and the load lifted corresponds to the maximum lifting capacity of the crane.

– When a load is lifted from rest, there is an associated acceleration in the vertical direction, which causes an additional force. This force is in addition to the normal force due to gravity, and is generally allowed for by factoring the normal static crane loads.

– Movements of the crane, both along the length and across the width of the building, are also associated with accelerations and retardations, this time in the horizontal plane. The associated horizontal forces must be taken into account in the design of the supporting structure.

21

Other loads: wave loading

– For offshore structures in deep waters, wave loads can be particularly severe. The loads arise due to movement of water associated with wave action. These movements can be described mathematically to relate forces to physical wave characteristics such as height and wavelength.

– The treatment is therefore similar to wind loads in that these physical characteristics are predicted and corresponding forces on the particular structural arrangement then calculated. These calculation procedures are, however, very complicated and must realistically be performed on a computer.

Other loads: temperature effects

Exposed structures such as bridges may be subject to significant temperature variation which must be taken into account in the design.

If it is not provided for in terms of allowing for expansion, significant forces may develop and must be included in the design calculations. In addition, differential temperatures, e.g. between the concrete deck and steel girders of a composite bridge, can induce a stress distribution which must be considered by the designer.

22

Other loads: retained material

Structures for retaining and containing material (granular or liquid) will be subject to a lateral pressure.

For liquids it is simply the hydrostatic pressure. For granular material a similar approach can be adopted, but with a reduction in pressure depending on the ability of the material to maintain a stable slope - this is the Rankine approach.

Ponding of water on flat roofs should be avoided by ensuring adequate falls (1:60 or more) to gutters.

Other loads: seismic loads

Seismic actions on structures are due to strong ground motion.

They are a function of the ground motion itself and of the dynamic characteristics of the structure.

Strong ground motion can be measured by one of its parameters, the peak ground acceleration being the parameter most usually adopted for engineering purposes.

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Other loads: accidental loads

Accidental actions may occur as a result of accidental situations. The situations include fire, impact or explosion. It is very difficult to quantify these effects.

In many cases it may be preferable to avoid the problem, for instance by providing crash barriers to avoid collision from vehicles or roof vents to dissipate pressures from explosions.

Where structures such as crash barriers for vehicles and crowds must be designed for 'impact' the loading is treated as an equivalent static load.

1


Course 11

EN 1990:

Design requirements

The limit state method



EN 1990 establishes principles and requirements for the safety, serviceability and durability of structures, describes the basis for their design and verification and gives guidelines for related aspects of structural reliability

EN 1990 is intended to be used in conjunction with EN 1991 to EN 1999 for the structural design of buildings and civil engineering works, including geotechnical aspects, structural fire design, situations involving earthquakes, execution and temporary structures

EN 1990 is applicable for the design of structures where other materials or other actions outside the scope of EN 1991 to EN 1999 are involved

EN 1990 is applicable for the structural appraisal of existing construction, in developing the design of repairs and alterations or in assessing changes of use

2

Design requirements

A structure shall be designed and executed in such a way that, during its intended life, with appropriate degrees of reliability and in an economical way it will have adequate:– structural resistance [strength] (the structure should be safe for

people and contents, sustaining all actions and influences likely to occur during execution and use)

– serviceability (remain fit for the use for which it is required - the deformations, deflections and vibrations should not be too large)

– durability (a structure should continue to resist environmental influences and be usable after an extended period of time and usage)

– fire resistance (in the case of fire, the structural resistance shall be adequate for the required period of time)

– robustness (a structure should not be damaged by events such as explosion, impact, and the consequences of human errors, to an extent disproportionate to the original cause)

Design requirements: strength and stiffness

Structural resistance and serviceability are usually mutually dependent. In many cases a structure that has sufficient resistance will have also a sufficient stiffness. However, the use of higher strength materials, improved technologies, and advanced analytical techniques, together with more emphasis on economy leads to more slender structures.

For example, a large span structure may have the sufficient strength, but not the required stiffness.

Therefore both strength and serviceability requirements should be checked.

3

Design requirements in the case of fire

Effect of fire on structures:– reduce the strength and stiffness of

materials due to temperature

– impose large deformations

Design requirements in the case of fire

During fire it is necessary to ensure the load-bearing capacity and the structural integrity for a defined period of time in order to:– permit evacuation of occupants

– afford appropriate protection to fire-fighting services

– protect the building and adjoining property from fire spread

The "required period of time" is dependent on the:– use of the building

– height of the building

– size of the building or compartment

4

Design requirements: robustness

A structure shall be designed and executed in such a way that it will not be damaged by events such as: explosion, impact, and the consequences of human errors, to an extent disproportionate to the original cause

Design requirements: robustness

Potential damage shall be avoided or limited by appropriate choice of one or more of the following:– avoiding, eliminating or reducing the

hazards to which the structure can be subjected;

– selecting a structural form which has low sensitivity to the hazards considered;

– selecting a structural form and design that can survive adequately the accidental removal of an individual member or a limited part of the structure, or the occurrence of acceptable localised damage;

– avoiding as far as possible structural systems that can collapse without warning;

– tying the structural members together.

5

Design requirements: reliability

Reliability is the ability of a structure or a structural member to fulfill the specified requirements, during the design working life, for which it has been designed.

Reliability is usually expressed in probabilistic terms.

In a narrow sense, reliability is the probability that the structure will not exceed specified limit states (strength or serviceability requirements) during a specified reference period.

The reliability required for structures by EN 1990 should be achieved:– by design in accordance with EN 1990 to EN 1999 and by

– appropriate execution and quality management measures

Design requirements: the design working life

The design working life is the time for which a structure (or part of it) is to be used for its intended purpose with anticipated maintenance, but without major repair being necessary

6

Design requirements: the design working life

The present state of knowledge is insufficient to enable precise prediction of the life of a structure. The behaviour of materials and structures over extended periods of time can only be estimated. The likely period of maintenance of the structure or time of replacement of various components of a structure can, however, be determined.

The notion of design working life is useful for:– the selection of design actions (snow, wind, etc.) and the

consideration of material property deterioration (corrosion, fatigue or creep)

– estimation of life-cycle cost to evaluate relative economics of different solutions (comparison of different design solutions and choice of materials, with different initial and maintenance costs)

– developing management procedures and strategies for systematic maintenance and renovation of structures

Design requirements: durability

The durability of a structure is its ability to remain fit for use during the design working life given appropriate maintenance. The structure should be designed in such a way that no significant deterioration is likely to occur within the period between successive inspections. The need for critical parts of the structure to be available for inspection, without complicated dismantling, should be part of the design.

"Performance indicator" can be expressed in various units: mechanical (e.g. strength), financial, etc. as a function of time.

7


Initially, upon building completion, – building performance remain constant for a period of time (e.g.

steel structures), or

– building performance can increase with time (e.g. concrete structures in which concrete strength grows)


After a certain period of time, the performance of a building decreases (e.g. corrosion of steel, carbonation of concrete). Some type of damage may occur, like corrosion of reinforcement steel in reinforced concrete elements, wide cracks in reinforced concrete members which remain open, or fatigue cracks in steel members.

If no maintenance is performed at this stage, damage to the structure increases and its condition worsens, progressing to possible structural failure.

If the structure is repaired, a reliability level is ensured above the serviceability level.

In this context, durability requirement from EN 1990 has the following meaning: at the end of its design working life, the reliability level of the structure should not be below the serviceability level.

8

Principles of limit states design: general

Limit states: states beyond which the structure no longer fulfils the relevant design criteria. In other words, limit states are idealisations of undesirable events or phenomena. Each limit state is associated with a certain performance requirement imposed on a structure.

Two categories of limit states need to be considered:– Ultimate limit states (ULS)

– Serviceability limit states (SLS)


Ultimate limit states: states associated with collapse or with other similar forms of structural failure

Serviceability limit states: states that correspond to conditions beyond which specified service requirements (deflections, vibrations, cracks) for a structure or structural member are no longer met

9


According the traditional concept of limit states, a structure is assumed to be fully satisfactory up to a certain value of load effect E0, and beyond this point the structure is assumed to be fully unsatisfactory.

It may be difficult to define precisely a distinct value E0, separating the desirable and undesirable structural conditions. In these cases, a gradual loss of the ability of the structure to perform adequately provides a more realistic concept.


The nature of ULS is essentially different from the nature of SLS: – Infringement of ULS leads always to structural failure, implying

the need to demolish or repair the structure. Infringement of SLS does not usually leads to such fatal consequences for the structure, and the structure may normally be used after the removal of those actions which caused the infringement.

– The criteria of ULS involve parameters of the structure and appropriate actions only. The criteria of SLS are additionally dependent on the requirements of clients and users (sometimes subjective), and on the characteristics of the installed equipment and non-structural elements

10


The difference between the ULS and SLS result in separate formulation of reliability conditions, and different levels of reliability assumed in the verification of both limit states.

In some cases verification of one limit state may be omitted if sufficient information is available to ensure that the requirements of one limit state are met by satisfying the other limit state. For example, in the case of reinforced concrete beams satisfying the ULS condition, the verification of deflection can be omitted provided the span/depth ratio is less than 18 for highly stressed concrete or less than 25 for lightly stressed concrete.


Not all undesirable effects or phenomena can be easily classified into ULS or SLS. For example, – in the case of a railway bridge, some SLS of the deck can be

considered as ULS for the supported track: significant deformation of the track may cause the derailment of a train with loss of human life;

– vibration of a building floor or a footbridge may be very uncomfortable, or even dangerous to human health, without being structurally damaging.

11

Principles of limit states design: design situations

Design situations: sets of physical conditions representing the real conditions occurring during a certain time interval for which the design will demonstrate that relevant limit states are not exceeded:– Persistent design situations

– Transient design situations

– Accidental design situations

– Seismic design situations


Persistent design situations, which refer to the conditions of normal use, which can include extreme conditions from wind, snow, imposed loads, etc. These are generally related to the design working life of the structure.

Transient design situations, which refer to temporary conditions applicable to the structure, e.g. during execution or repair. For example, for the maintenance of a bridge, a lane of the carriageway may be temporarily closed for normal traffic, modifying the conditions of use of the buildings.

12


Accidental design situations, which refer to exceptional conditions applicable to the structure or to its exposure, e.g. to fire, explosion, impact or the consequences of localised failure.

Seismic design situations, which refer to conditions applicable to the structure when subjected to seismic events.


The variation of actions, environmental influences and structural properties which will occur throughout the design working life of a structure should be considered by selecting distinct situations representing a certain time interval with associated hazards or conditions.

The selected design situations shall be sufficiently severe and varied so as to encompass all conditions that can reasonably be foreseen to occur during the execution and use of the structure.

Different levels of reliability may be applicable for different design situations.

13

Principles of limit states design: ULS

The limit states that concern the safety of people, and/or the safety of the structure are classified as ultimate limit states. In some circumstances, the limit states that concern the protection of the contents are classified as ultimate limit states (examples: chemicals, nuclear materials or masterpieces in a museum).

Generally, the first occurrence of a ULS is equivalent to failure.

In some cases (e.g. when excessive deformations are decisive) due to the difficulty of computing analytically the collapse of a structure or a structural element, ultimate limit states can be defined for simplicity as states prior to collapse.

DISPLACEMENT

FO

RC

E STATE PRIORTO COLLAPSE

COLLAPSE

FORCE

DIS

P.

Principles of limit states design: ULS

The following ultimate limit states shall be verified where they are relevant:– loss of equilibrium of the structure or any part of it, considered as

a rigid body;

– failure by excessive deformation, transformation of the structure or any part of it into a mechanism, rupture, loss of stability of the structure or any part of it, including supports and foundations;

– failure caused by fatigue (relevant for bridges and marine platforms) or other time-dependent effects.

14

Principles of limit states design: SLS

Serviceability limit states concern:– the functioning of the

structure or structural members under normal use,

– the comfort of people,

– the appearance of the construction works.

Taking into account the dependency of load effects, it is useful to distinguish between reversible and irreversiblelimit states


Reversible serviceability limit states: – are not exceeded when the load that caused the infringement are

removed

– examples: temporary deflections, excessive vibrations

– the first infringement does not lead to failure or loss of serviceability

15


Irreversible serviceability limit states: – remain permanently exceeded even when the loads that caused

infringement are removed

– examples: permanent local damage or unacceptable deformations

– the first infringement of the limit state is decisive

– the design criteria are similar to those of ultimate limit states


The verification of serviceability limit states should be based on criteria concerning:– deformations that cause damage to finishes or non-structural

members or affect the appearance, the comfort of users, or the functioning of the structure (including the functioning of machines or services),

– vibrations that cause discomfort to people, or that limit the functional effectiveness of the structure

– damage that is likely to adversely affect the appearance, the durability, or the functioning of the structure.

16

Principles of limit states design

The design procedure using the limit state method consists in setting up structural and load models for the relevant ultimate and serviceability limit states which are considered in various design situations and load cases.

The aim of the limit state design is to verify that no limit state is exceeded when relevant design values for actions, for materials or products, and for geometrical properties are used in appropriate structural and load models.

The verification of structural reliability is done using– the method of partial factors, or

– as an alternative, using probabilistic methods (applied to unusual structures, line nuclear power stations).

Principles of limit states design: structural model

17

Principles of limit states design: structural model

pinned connection

rigid connection

Principles of limit states design: load model

Example: real imposed loads modelled as uniformly distributed

1


Course 12

EN 1990:

Basic variables

The partial factor method


EN 1990: Basic variables

The design procedure using the limit state method consists in setting up structural and load models for the relevant ultimate and serviceability limit states which are considered in various design situations and load cases.

Basic variables needed to set up structural and load models– Actions and environmental influences

– Material and product properties

– Geometrical data

2

Variability of loads

Loads change in time.

What loads to design a structure for?

Most loads are determined today based on measurements taken over time

Example: maximum wind speed measured over 5-min periods in each month between 1884 and 1950 in New York at a height of 140 m.

Histogram

Theoretical distribution

Fractiles

A p-fractile is defined as the x-value of the distribution which includes p*N observations, with 0<p<1 and N being the number of observations.

Example : the 0.1-fractile of the distribution shown below is 14.6, as it includes 10% of all observations (starting from the left)

3


Loads are probabilistic variables (not deterministic)

Basic value of a load can be defined in probabilistic terms only, e.g. "for annual probabilities of exceedence of 0.02, which is equivalent to a mean return period of 50 years".

A larger or smaller probability of the loads being exceeded can be considered in design (corresponding to smaller or larger mean return periods)

Smaller or larger design loads

More or less economical structure


The engineer can never be absolutely sure that the loads he designs his structure for will not be exceeded in its lifetime.

4

Actions and environmental influences: classification

The aim of the classifications is to identify the similar or dissimilar characteristics of various actions and to enable the use of appropriate theoretical action models and reliability elements in structural design.

Actions are classified by their variation in time as follows:– permanent actions (G), e.g. self-weight of structures, fixed

equipment and road surfacing, and indirect actions caused by shrinkage and uneven settlements;

– variable actions (Q), e.g. imposed loads on building floors, beams and roofs, wind actions or snow loads;

– accidental actions (A), e.g. explosions, or impact from vehicles.

Actions can also be classified– by their origin, as direct or indirect,

– by their spatial variation, as fixed or free, or

– by their nature and/or the structural response, as static or dynamic.

Characteristic values of actions

Characteristic value of an action (Fk) is a principal representative value of an action

When Fk can be fixed on statistical bases, it is chosen so as to correspond to a prescribed probability of not being exceeded on the unfavourable side during a "reference period" taking into account the design working life of the structure and the duration of the design situation. Depending on the available data and experience, the characteristic value of an action is specified as a mean, upper or lower value.

In some cases there is a lack in statistical data concerning various actions. Under these circumstances, actions can be evaluated based on a fairly subjective assessment, judgment, or decision, and are assigned nominal values.

5

Permanent actions: characteristic values

Usually there is sufficient statistical data for permanent actions, in particular for self-weight of traditional structural materials

If the variability of a permanent action is small, a single characteristic value can be considered: the mean value (G)

If the variability of a permanent action cannot be considered as small, two values are used: an upper value Gk,sup (representing 0.95 fractile) and a lower valueGk,inf (representing 0.05 fractile)

– the self weight of a bridge deck has a small variability because the execution of the bridge is strictly controlled by competent personnel

– the self-weight of items such as vehicle parapets, waterproofing, coatings, railway ballast, etc., has a large variability

Variable actions: characteristic values

For variable actions, the characteristic value (Qk) corresponds to either:– an upper value with an intended probability of not being exceeded

(the most common case) or a lower value with an intended probability of being achieved, during some specific reference period;

– a nominal value, which may be specified in cases where a statistical distribution is not known

The volume of statistical data for the most the common variable actions allows assessment of their characteristic values by a probabilistic approach

Two elements are used to define the characteristic value of the load:– the reference period during which the extreme is observed

(maximum or minimum), and

– the intended probability with which these extreme values should not exceed the characteristic value

6

Variable actions: reference period

Variable action versus time

The reference period (e.g. 1 year)

Variable actions: reference period

During each reference period the variable action Qreaches a maximum value Qmax (e.g. the annual extreme).

A sequence of values Q1,max, Q2,max, Q3,max, … can be obtained

The distribution of these values of Qmax is indicated in the right figure by a probability density function Qmax(Q)

The characteristic value Qk can then be defined by the requirement that it will be exceeded by Qmax (e.g. annual extremes) only with a limited probability (e.g. p=0.02)

Thus, the characteristic value Qk is the p-fractile of the extremes values Qmax.

7

Variable actions: characteristic values

In general, the characteristic value of environmental loads and imposed loads on building floors for persistent design situations is based on an intended probability of the value not being exceeded of 0.98, and a reference period of 1 year.

The probability p of the characteristic value being exceeded and the reference period are linked by the equation:

where T is the return period (expected period between two subsequent occurrences of the characteristic value).

Example: for a probability of the characteristic value being exceeded p=0.02 and a reference period of 1 year, the return period of the characteristic value is T1/0.02=50 years

ppT

1ln

Other representative values of variable actions

In addition to the characteristic values of actions, other representative values for actions are used:– The combination value 0Qk - takes into account the reduced

probability of the simultaneous occurrence of two (or more) independent actions. It is associated with the combination of actions for ultimate and irreversible serviceability limit states.

– The frequent value 1Qk - is primarily associated with the frequent combination in the reversible serviceability limit states and ultimate limit states involving accidental actions.

– The quasi-permanent value 2Qk - is primarily used for assessment of long-term effects, for example creep effects in prestressed concrete elements. It is also used for the representation of variable actions in accidental and seismic combinations of actions (ultimate limit states) and for verification of frequent and quasi-permanent combinations (long-term effects) of serviceability limit states.

8

Other representative values of variable actions

Material and product properties

A material property is usually determined from standardized tests performed under specified conditions.

It is sometimes necessary to apply a conversion factor to convert the test results into values that can be assumed to represent the behaviour of the structure or the ground.

For traditional materials (e.g. steel and concrete), previous experience and extensive tests are available, and conversion factors are well-established and available in codes.

The properties of new materials should be established from an extensive testing program, including tests on complete structures, revealing the relevant properties and conversion factors.

9

Variability of material properties

The strength of a structure cannot be predicted with absolute confidence

Structural materials, whether natural or man-made, vary in quality and strength

Example: result of compression test on 303 cubes of concrete of supposedly the same strength, made during a certain period of construction.

A symmetrical, bell-shaped distribution of results, which can be expressed mathematically using the "normal" or Gauss rule.

Variability of material properties

The strength of concrete specimens taken from a built structure will be even worse, due to insufficient compaction of concrete for example.

Tests specimens drilled from hardened concrete will show much greater variation in strength then the specimens taken during the mixing process, as well ass a lower average strength.

10


Basic properties of a material or product:– strength: mechanical property of a

material indicating its ability to resist actions, usually given in units of stress

– stiffness: the force necessary to produce a unit displacement, characterizing the elastic response of the material/product

– ductility: capacity to deform into the plastic range, without significant loss of strength

Examples of characteristic values of material/product properties:– strength used to check the resistance of a structure that should

respond in the elastic range: lower value is relevant

– stiffness used to estimate deflections and deformations of the structure: the mean value is relevant

– ductility used to prevent collapse of a structure due to excessive plastic deformations: lower value relevant

STRAIN

ST

RE

SS

STRENGTH

STIFFNESS

DUCTILITY


In design calculations properties of structural materials and products are represented by characteristics values.

Generally characteristic values are obtained from statistical data, corresponding to a prescribed probability of not being infringed.– Usually the lower value of material property is unfavourable, and

the 5% fractile (Xk,inf) is considered as the characteristic value.

– For some properties, the mean value (x) is most appropriate.

When enough statistical data is not available, a nominal value can be used in design.

11

Geometrical data

Geometrical variables describe the shape, size and overall arrangement of structures, structural members and cross-sections.

No structure can be erected without some deviations of form, shape and dimension from the ones assumed in design

Example: steel sections are rolled under very careful control, but some variation in thickness and depth cannot be avoided.

Concrete formwork may be slightly out in dimensions.

Load-bearing walls in a multi-storey building may be out of alignment one above the other.

Variability of geometrical properties

In design, account should be taken of the possible variation of their magnitudes, which depend on the level of workmanship in the manufacture and execution process.

In most cases the geometrical data is represented by characteristic values, – Corresponding to values specified in design specifications, and

which are nominal values.

– Where their statistical distribution is sufficiently known, values of geometrical quantities that correspond to a prescribed fractile of the statistical distribution may be used.

In some cases (e.g. imperfections) geometrical data is represented directly by their design values

12

Structural analysis

Generally, any structural model should be regarded as an idealization of the structural system.

A simplified model should take account of significant factors and neglect the less important ones.

The following is a list of factors that may be important for the structural model:– geometric properties (e.g. structural configuration, spans, cross-

sectional dimensions, deviations, imperfections)

– material properties (e.g. strength, constitutive relations, time and stress state dependence, plasticity, temperature and moisture dependence)

– actions (e.g. direct or indirect, variation in time, spatial variation, and static or dynamic)

The appropriate structural model should be chosen based on previous experience and knowledge of structural behaviour.

Verification by the partial factor method: general

Assessment of the reliability of structures in Eurocodes is based on the concept of limit state design and verification by the partial factor method.

Using this method, a structure is considered to be reliable if no relevant limit state is exceeded for all selected design situations, when using the design values of basic variables (actions, material properties and geometrical data) in the design models.

Critical combinations of actions should be identified and used in order to obtain the design values of action effects. Example: a combination of permanent loading due to self weight, snow load and wind load need to be considered for design of a roof panel of an industrial hall.

13

Verification by the partial factor method: general

Actions that cannot occur simultaneously due to physical reasons, should not be considered together in combinationExample: snow and live loads need not be considered simultaneously on the terrace of a multistorey building, because they cannot act simultaneously with their extreme values (just the maintenance personnel will be on the roof in the event of a heavy snow).

Action effects (E) represent the response of the structure to actions applied on it. Examples:– action effects on structural members: internal force, moment,

stress, strain

– action effects on the whole structure: deflection, rotation

The partial factor method: design values

The design value Fd of an action F can be expressed in general terms as: Fd = f · Frep

with Frep = · Fk

where:Fk is the characteristic value of the action.Frep is the relevant representative value of the action.f is a partial factor for the action which takes account of the possibility of unfavourable deviations of the action values from the representative values.

is either 1,00 or 0, 1 or 2.– the combination value 0Qk

– the frequent value 1Qk

– the quasi-permanent value 2Qk

14


For a specific load case the design values of the effects of actions (Ed) can be expressed in general terms as:Ed = Sd · E {f,i · Frep,i ; ad} i1where:ad is the design values of the geometrical data;Sd is a partial factor taking account of uncertainties:- in modelling the effects of actions;- in some cases, in modelling the actions.E {f,i · Frep,i ; ad} is the effect of action for the design

value of the force Fd and the design geometrical characteristics ad


In most cases, the following simplification can be made:Ed = E {F,i · Frep,i ; ad} i1with: F,i = Sd · f,i

F factor accounts for both uncertainties in action values and uncertainties in modelling of actions

Ed = E {F,i · Frep,i ; ad} i1 The design value of the effect of actions Ed is determined by applying the representative values of actions Frep,i amplified by the partial factors F,i on the model of the structure characterised by the geometry ad.

15


Ed = E {F,i · Frep,i ; ad} i1 The design value of the effect of actions Ed is determined by applying the representative values of actions Frep,i amplified by the partial factors F,i on the model of the structure characterised by the geometry ad.

Example:

Ed (MEd)

F,1Frep,1 (Gd = F,GGrep)

ad (L)

Design values of permanent ( F,GGrep)and imposed (

F,QQrep) actions

Geometrical data ad (beam span L)

Action effects Ed (benfing moment M Ed)

+

=

F,2Frep,2 (Qd =

F,QQrep)


Where a distinction has to be made between favourable and unfavourable effects of permanent actions, two different partial factors shall be used (G,inf and G,sup). Examples:– when estimating the effect of permanent and snow loads on a

roof, both loads have the same direction of action (gravitational), therefore the design value of the permanent load should be considered with the upper value G,sup · Gk

– when estimating the effect of permanent and wind loads on a roof, if wind produces suction, the two loads have opposite direction of action, therefore the design value of the permanent load should be considered with the upper value G,inf · Gk, as it reduces the effect of the wind load

16


The design value Xd of a material or product property can be expressed in general terms as:Xd = (Xk/m)where:Xk is the characteristic value of the material property; is the mean value of the conversion factor taking into account– volume and scale effects,

– effects of moisture and temperature, and

– any other relevant parameters;

m is the partial factor for the material or product property to take account of:– the possibility of an unfavourable deviation of a material or

product property from its characteristic value;

– the random part of the conversion factor .


Design values of geometrical data such as dimensions of members that are used to assess action effects and/or resistances may be represented by nominal values:ad = anom

Where the effects of deviations in geometrical data (e.g. inaccuracy in the load application or location of supports) are significant for the reliability of the structure (e.g. by second order effects) the design values of geometrical data shall be defined by:ad = anom awhere:a takes account of:– the possibility of unfavourable deviations from the characteristic

or nominal values;

– the cumulative effect of a simultaneous occurrence of several geometrical deviations.

17


The design resistance Rd can be expressed in the following form

whereRd is a partial factor covering uncertainty in the resistance model, plus geometric deviations if these are not modelled explicitly;Xd,i is the design value of material property i.


The following simplification may be made:

where : M,i = Rd · m,i

18


For members made of a single material (e.g. steel), the design resistance may be obtained directly from the characteristic value of a material or product resistance Rk, without explicit determination of design values for individual basic variables, using

Principle of checking safety of the structure:Ed Rd

where:Ed is the design value of the effect of actions such as internal force, moment or a vector representing several internal forces or moments;Rd is the design value of the corresponding resistance.

Frequencyof

occurence

Effect of actions (Ed)or Resistance (Rd)

Ed Rd

ResistanceRdEffect of actions

Ed

1


Course 13

EN 1990:

The partial factor method (cont.)


Ultimate limit states

The following ultimate limit states shall be verified as relevant:– EQU: Loss of static equilibrium;

– STR: Internal failure or excessive deformation;

– GEO: Failure or excessive deformation of the ground where the strengths of soil or rock are significant in providing resistance;

– FAT: Fatigue failure of the structure or structural members.

2


EQU: Loss of static equilibrium of the structure or any part of it considered as a rigid body, where:– minor variations in the value or the spatial distribution of actions

from a single source are significant, and

– the strengths of construction materials or ground are generally not governing;

Example: a bridge deck launched with a counterweight where loss of static equilibrium may be possible


STR: Internal failure or excessive deformation of the structure or structural members, including footings, piles, basement walls, etc., where the strength of construction materials of the structure governs;

Example: failure of a beam supporting a floor due to excessive stresses

Mmax

3


GEO: Failure or excessive deformation of the ground where the strengths of soil or rock are significant in providing resistance;Example: resistance of foundations like footings, piles, etc.


FAT: Fatigue failure of the structure or structural members.Examples: Cracks developing in steel bridges due to repetitive loading generated by traffic

4

Verifications of static equilibrium and resistance

When considering a limit state of rupture or excessive deformation of a section, member or connection (STR and/or GEO), it shall be verified thatEd Rd

where:Ed is the design value of the effect of actions such as internal force, moment or a vector representing several internal forces or moments;Rd is the design value of the corresponding resistance.

ULS: Combination of actions

For each critical load case, the design values of the effects of actions (Ed) shall be determined by combining the values of actions that are considered to occur simultaneously

Each combination of actions should include:– a leading variable action, or

– an accidental action.

Where the results of a verification are very sensitive to variations of the magnitude of a permanent action from place to place in the structure, the unfavourable and the favourable parts of this action shall be considered as individual actions

5


Combinations of actions for persistent or transient design situations (fundamental combinations)

The general format of effects of actions

and can be simplified as:

The combination of action in curly braces {} can be expressed as:

where "+" implies "to be combined with" implies "the combined effect of"


Gk,j - characteristic permanent action j

G,j - partial safety factor for permanent load Gk,j

P - prestressing

P - partial safety factor for prestressing action P

Qk,1 - leading variable action

Q,1 - partial safety factor for variable load Qk,1

Qk,i - variable action i

Q,i - partial safety factor for variable load Qk,i

0,i - takes into account the reduced probability of the simultaneous occurrence of two (or more) independent variable actions

6


Combinations of actions for accidental design situations

Ad - design value of the accidental action

Combinations of actions for seismic design situation

AEd - design value of the seismic action– permanent actions are taken with characteristic values

– seismic action is taken with design value

– variable loads are taken with the quasi-permanent value 2Qk


Partial factors for actions and combinations of actions: and factors are obtained from EN 1990 or CR0-2012:– permanent actions: G,sup = 1.35

– permanent actions: G,inf = 0.9

– variable actions: Q = 1.5

– 0,i = 0.7, with the exception of loads in storage facilities, water pressure, etc, when 0,i = 1.0

Example of fundamental load combinations

The partial factors for properties of materials and products should be obtained from EN 1992 to EN 1999

7

Serviceability limit states

At the SLS it shall be verified that:Ed Cd

where:– Cd is the limiting design value of the relevant serviceability

criterion.

– Ed is the design value of the effects of actions specified in the serviceability criterion, determined on the basis of the relevant combination

Serviceability limit states in buildings should take into account criteria related, for example, to floor stiffness, differential floor levels, storey sway or/and building sway and roof stiffness.

Stiffness criteria may be expressed in terms of limits for vertical deflections and for vibrations.

Sway criteria may be expressed in terms of limits for horizontal displacements.


EN 1990: "The serviceability criteria should be specified for each project and agreed with the client".

Schematic representation of vertical deflections:– wc - Precamber in the unloaded structural member

– w1 - Initial part of the deflection under permanent loads of the relevant combination of actions

– w2 - Long-term part of the deflection under permanent loads

– w3 - Additional part of the deflection due to the variable actions of the relevant combination of actions

– wtot - Total deflection as sum of w1, w2, w3

– wmax - Remaining total deflection taking into account the precamber

8


Horizontal displacements can be represented schematically:– u - Overall horizontal displacement

over the building height H

– ui - Horizontal displacement over a storey height Hi

SLS: Combination of actions

Three categories of combinations of actions are proposed in EN: – characteristic (normally used for irreversible limit states, e.g. for

exceeding of some cracking limits in concrete)

– frequent (is normally used for reversible limit states) and

– quasi-permanent (is normally used for assessment of long-term effects)

The appropriate combinations of actions should be selected depending on serviceability requirements and performance criteria imposed for the particular project, the client or the relevant national authority

9

SLS: Combination of actions

Characteristic combination

Frequent combination

Quasi-permanent combination

For serviceability limit states the partial factors M for the properties of materials should be taken as 1.0 except if differently specified in EN 1992 to EN 1999.

Examples of limiting values for vertical deflections

10

Examples of limiting values for horizontal deflections

Example: multistorey frame

Objective: design using the partial factor method a steel multistorey frame

For the design of the structure, the STR category of limit states is relevant

11


The following actions can be identified:– Permanent loads Gk

– Imposed loads Qk

– Snow load Sk

– Wind load Wk

– Seismic action Aed

Self-weight (Gk,1

) Dead load on floors (Gk,2) Exterior cladding (Gk,3

)

Snow load (Sk) Wind load (Wk)

Imposed load (Qk,1

)Imposed load -

chessboard (Qk,2

)Seismic load (Aed)


Of the four possible design situations,– Persistent design situations,

– Transient design situations,

– Accidental design situations,

– Seismic design situations.

Two categories of limit states need to be considered:– Ultimate limit states (ULS)


Seismic designsituation

ULS SLS

Persistent designsituation

ULS SLS

most relevant

12


Load cases (combinations of actions)

Persistent design situation– Ultimate limit states (ULS)


Seismic design situation– Ultimate limit states (ULS)

– Serviceability limit states (SLS) see EN 1998-1



Persistent design situation– Ultimate limit states (ULS)

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Qk,1

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Qk,2

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Sk,1

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Wk

• 0.9(Gk,1 + Gk,2 + Gk,3) + 1.5Wk

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Qk,1 + 1.05Sk

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Sk + 1.05Qk,1

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Qk,1 + 1.05Sk + 1.05Wk

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Sk + 1.05Qk,1 + 1.05Wk

• 1.35(Gk,1 + Gk,2 + Gk,3) + 1.5Wk + 1.05Qk,1 + 1.05Sk

check strength and stability of members and connections

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Persistent design situation– Serviceability limit states (SLS)

• (Gk,1 + Gk,2 + Gk,3) + Qk,1

• (Gk,1 + Gk,2 + Gk,3) + Qk,2

• (Gk,1 + Gk,2 + Gk,3) + Sk,1

• (Gk,1 + Gk,2 + Gk,3) + Wk

• (Gk,1 + Gk,2 + Gk,3) + Wk + 0.7Qk,1 + 0.7Sk

Seismic design situation– Ultimate limit states (ULS)

• (Gk,1 + Gk,2 + Gk,3) + 0.4(Qk,1 + Sk) + Aed

– Serviceability limit states (SLS) check lateral storey displacements determined according to specific requirements of EN 1998-1

check beam deflections

check lateral storey deformations

check strength, stability and ductility of members and connections

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Course 14

Building design, construction and maintenance


Principles

A building is an assemblage that is firmly attached to the ground and that provides total or nearly total shelter for machines, processing equipment, performance of human activities, storage of human possessions, or any combination of these

Building design is the process of providing all information necessary for construction of a building that will meet its owner’s requirements and also satisfy public health, welfare, and safety requirements

Architecture is the art and science of building design. It is primarily driven by the creative manipulation of mass, space, volume, texture, light, shadow, materials, program, and pragmatic elements such as cost, construction and technology, in order to achieve an end which is aesthetic, functional and often artistic.

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Principles

Engineering design is driven primarily by the creative manipulation of materials and forms using mathematical and scientific principles

Building construction is the process of assembling materials to form a building

Building design may be legally executed only by persons deemed competent to do so by the state in which the building is to be constructed. Competency is determined on the basis of education, experience, and ability to pass a written test of design skills– Architects: persons legally permitted to practice architecture

– Engineers: experts in specific scientific disciplines and are legally permitted to design parts of buildings; in some cases, complete buildings (structural, mechanical, electrical engineers)

Principles

Building construction is generally performed by labourers and craftspeople engaged for the purpose by an individual or organization, called a contractor.

The contractor signs an agreement, or contract, with the building owner under which the contractor agrees to construct a specific building on a specified site and the owner agrees to pay for the materials and services provided.

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Principles of building design

1. The building should be constructed to serve purposesspecified by the client.

2. The design should be constructable by known techniques and with available labour and equipment, within an acceptable time.

3. The building should be capable of withstanding the actions due to environment and normal usage for a period of time specified by the client.

4. Both inside and outside, the building should be visually pleasing.

5. No part of the building should pose a hazard to the safety or health of its occupants under normal usage, and the building should provide for safe evacuation or refuge in emergencies.


6. The building should provide the degree of shelter from the environmental actions and of control of the interior environment - air, temperature, humidity, light, and acoustics - specified by the client and not less than the minimums required for safety and health of the occupants.

7. The building should be constructed to minimize adverse impact on the environment.

8. Operation of the building should consume a minimum of energy while permitting the structure to serve its purposes.

9. The sum of costs of construction, operation, maintenance, repair, and anticipated future alterations should be kept within the limit specified by the client.

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The ultimate objective of design is to provide all the information necessary for the construction of a building.

This objective is achieved by the production of drawings, or plans, showing what is to be constructed, specifications stating what materials and equipment are to be incorporated in the building, and a construction contract between the client and a contractor.

Designers also should observe construction of the building while it is in process. This should be done not only to assist the client in ensuring that the building is being constructed in accordance with plans and specifications but also to obtain information that will be useful in design of future buildings.

Design procedure

Design usually starts when a client recognizes the need for and economic feasibility of a building and engages an architect, a professional with a broad background in building design.

The architect, in turn, engages consulting engineers and other consultants– structural engineer: a specialist having the knowledge to design

load-bearing walls, floors, roofs, foundations, and skeleton framing needed for the support of buildings and building components

– mechanical engineer: a specialist having the knowledge to design plumbing, elevators, escalators, horizontal walkways, conveyors, installed machinery, and heating, ventilation, and air conditioning

– electrical engineer: a specialist having the knowledge to design electric circuits, electric controls and safety devices, electric motors and generators, electric lighting, and other electric equipment

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Design procedure

– for buildings on a large site, the architect may engage a landscape architect as a consultant

– for a concert hall, an acoustics consultant may be engaged; for a hospital, a hospital specialist; for a school, a school specialist

The architect does the overall planning of the building and incorporates the output of the consultants into the contract documents

The architect determines what internal and external spaces the client needs, the sizes of these spaces, their relative locations, and their interconnections shown in floor plans

Major responsibilities of the architect are enhancement of the appearance inside and outside of the building and keeping adverse environmental impact of the structure to a minimum. The exterior of the building is shown in drawings, called elevations.

Design procedure

The location and orientation of the building is shown in a site plan.

The architect also prepares the specifications for the building. These describe in detail the materials and equipment to be installed in the structure.

In addition, the architect, usually with the aid of a lawyer engaged by the client, prepares the construction contract.

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Stages of building design

1. Preliminary stage: development of a program (a list client's requirements) by the architect and client. Getting a city-planning certificate from local authorities.

2. Conceptual stage: the architect translates requirements into spaces, relates the spaces and makes sketches, called schematics, to illustrate the concepts. When sufficient information is obtained on the size and general construction of the building, a rough estimate is made of construction cost.

3. Design development: the architect and consultants work out more details and show the results in preliminary construction drawings and outline specifications. A preliminary cost estimate utilizing the greater amount of information on the building now available is then prepared.

Stages of building design

4. Contract documents phase: production of construction drawings and specifications, which are incorporated in the contract between the client and a builder and therefore become legal documents. The design may need to be revised for the final cost estimates to fit into the budget. A building permit need to be obtained from local authorities.

5. Construction stage: after the client has approved the contract documents, the architect helps the owner in obtaining bids from contractors or in negotiating a construction price with a qualified contractor– For private work, the owner generally awards the construction

contract to a general contractor. This contractor may perform some, all, or none of the work. Usually, much of the work is let out to specialists, called subcontractors.

– For public work, there may be a legal requirement that bids be taken and the contract awarded to the lowest responsible bidder.

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Construction procedure

Construction is performed by contractors– they need to satisfy the requirements of the client and the

building designers

– however, their primary objective is to make profit contractors must prepare bid prices based on accurate estimates of construction costs

– after the contract has been awarded, contractors must furnish and pay for all materials, equipment, power, labour, and supervision required for construction

The client compensates the contractors for construction costs and services

A general contractor assumes overall responsibility for construction of a building, and may engage subcontractors


Subcontractors have contracts with the general contractor, and they are paid by the general contractor– a plumbing contractor installs the plumbing

– an electrical contractor installs the electrical system

– a steel erector structural steel, and an elevator contractor installs elevators

Sometimes, in addition to a general contractor, the owners contracts separately with specialty contractors, called prime contractors. Their work is scheduled and coordinated by the general contractor, but they are paid directly by the owner.

Sometimes also, the owner may award a contract to an organization for both the design and construction of a building. Such organizations are called design-build contractors.

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Administration of the construction procedure often is difficult some owners seek assistance from an expert, called a professional construction manager, with extensive construction experience, who receives a fee.

The construction manager negotiates with general contractors and helps select one to construct the building. Managers usually also supervise selection of subcontractors.

During construction, they help control costs, expedite equipment and material deliveries, and keep the work on schedule.


Owners also engage an inspector who has the responsibility of ensuring that construction meets the requirements of the contract documents and is performed under safe conditions

Inspections also are made by representatives of one or more governmental agencies. They have the responsibility of ensuring that construction meets legal requirements and have little or no concern with detailed conformance with the contract documents. Such legal inspections are made periodically or at the end of certain stages of construction.

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Building costs

Construction cost of a building usually is a dominant design concern

If construction cost exceeds the owner’s budget, the owner may cancel the project

Costs that occur after completion of the building (such as property taxes and insurance) often are proportional to the initial cost

Building costs

Before construction of a building starts, the owner generally has to make a sizable investment in the project. – The major portion of this expenditure usually goes for purchase

of the site and building design.

– Remaining preconstruction costs include those for feasibility studies, site selection and evaluation, surveys, and program definition.

The major portion of the construction cost is the sum of the payments to the general and prime contractors.

Remaining construction costs usually consist of interest on the construction loan, permit fees, and costs of materials, equipment, and labour not covered by the construction contracts.

The occupancy costs covers costs of moving possessions into the building and start-up of utility services, such as water, gas, electricity, and telephone.

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Building costs

The initial cost to the owner is the sum of preconstruction, construction, and occupancy costs.

After the building is occupied, the owner incurs costs for operation and maintenance of the buildings. Such costs are a consequence of decisions made during building design.

Life-cycle cost is the sum of initial, operating, and maintenance costs.

Often, post-construction costs are permitted to be high so that initial costs can be kept low– the design makes artificial lighting necessary when daylight could

have been made available

– the extra heating and air conditioning are necessary because of inadequate insulation of walls and roof

– frequent repairs may be needed because of poor choice of materials during design

Building costs

Generally, it is life-cycle cost that should be minimized in building design rather than construction cost, enabling the owner to receive the greatest return on the investment in the building.

Nevertheless, construction cost frequently is a dominant concern in design, because the client establishes a construction budget independent of life-cycle cost. Reasons:– the client does not have adequate capital for an optimum building

and places too low a limit on construction cost. The client hopes to have sufficient capital later to pay for the higher operating and maintenance costs or for replacement of undesirable building materials and installed equipment.

– The client’s goal is a quick profit on early sale of the building, in which case the client has little or no concern with future high operating and maintenance costs for the building.

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Building legislation

Many of the restrictions encountered in building design are imposed by building regulations: – Dealing with administration and enforcement. They cover

licenses, permits, fees, certificates of occupancy, safety, projections beyond street lines, alterations, maintenance, applications, approval of drawings, stop-work orders.

– Specifying requirements for design and construction in detail. They give requirements for structural components, lighting, HVAC, plumbing, gas piping and fixtures, elevators and escalators, electrical distribution, stairs, corridors, walls, doors, and windows.

– Requirements for fire-protection purposes. Objective - to delay or prevent spread of fire over large portions of the building

• set allowable floor areas

• restrict building height and number of stories

• limit the occupant load, or number of persons allowed in a building or room to permit rapid egress


Zoning regulations are established by local authorities to protect the health, welfare, and safety of the public

Zoning primarily regulates land use by controlling types of occupancy of buildings, building height, and density and activity of population in specific parts of a jurisdiction

Zoning regulations are usually developed by a planning commission and administered by the commission or a building department. Land-use controls adopted by the local planning commission for current application are indicated on a zoning map.

Zoning map divides the jurisdiction into districts, shows the type of occupancy, such as commercial, industrial, or residential, permitted in each district, and notes limitations on building height and bulk and on population density in each district.

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The planning commission usually also prepares a master plan as a guide to the growth of the jurisdiction. A future land-use plan is an important part of the master plan. The commission’s objective is to steer changes in the zoning map in the direction of the future land-use plan.

In the vicinity of airports, zoning may be applied to maintain obstruction-free approach zones for aircraft and to provide noise-attenuating distances around the airports. Airport zoning limits building heights in accordance with distance from the airport.


Control of Building Height. Zoning places limitations on building dimensions to limit population density and to protect the rights of occupants of existing buildings to light, air, and aesthetic surroundings.– establishment of a specific maximum height or number of stories,

– limitation of height in accordance with street width,

– setting minimums for distances of buildings from lot lines, or

– relating total floor area in a building to the lot area or to the area of the lot occupied by a building

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In addition to building and zoning codes, building design and construction must comply with many other regulations. These include those of the local or state health, labour, and fire departments; local utility companies; and local departments of highways, streets, sewers, and water. These agencies may require that drawings for the building be submitted for review and that a permit be granted before construction starts.

Building design and construction regulations RO

source: http://www.beniks-constructii.ro/informatii-utile.php

1. Stabilirea terenului pentru realizarea unei constructiiPentru a putea fi folosit in scopul construirii, terenul trebuie sa indeplineasca urmatoarele conditii:– Sa fie in intravilanul localitatii (atunci când nu este introducerea in

intravilan se face prin P.U.Z.)

– Sa nu se afle in circuitul agricol (atunci când este in circuitul agricol, scoaterea se face dupa trecerea in intravilan)

– Sa aiba o cale de acces auto si pietonala

– Sa se incadreze intr-o zona care sa permita functiunea ce se da constructiei

– Sa aiba asigurate utilitatile (energie electrica, alimentare cu apa, gaze)

– Sa aiba forma si dimensiunile minime conform reglementarilor urbanistice locale

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2. Obtinerea certificatului de urbanism (CU)Certificatul de Urbanism in vederea construirii (CU) se obtine de la primarie, directia de urbanism. CU contine "regulile" pe care trebuie sa le urmeze proiectantul pentru realizarea unei constructii pe terenul in cauza. Mai exact, CU stabileste anumiti parametri tehnici ai terenului (inaltimea maxima a cladirii, procentul de teren ce poate fi ocupat de cladire, pozitia pe teren, etc) destinatii admise ale cladirii (locuinta, birouri, comert, etc) precum si avizele necesare pentru Autorizatia de Construire (apa, canalizare, gaze, pompieri, mediu, etc). Avizele necesare difera de la o zona la alta si de la un teren la altul.


3. Elaborarea proiectuluiLa intocmirea unui proiect sunt stabilite mai multe faze de elaborare a acestuia:– Studiu de (pre)fezabilitate

– Proiect pentru Autorizatia de Construire (P.A.C.)

– Proiect tehnic (P.T.)

– Detalii de executie (D.D.E.)

Proiectul Tehnic (PT) este piesa principala din dosarul depus pentru obtinerea Autorizatiei de Construire si se realizeaza urmand specificatiile din Certificatul de Urbanism. Proiectul tehnic se realizeaza de catre un proiectant si trebuie sa fie aprobat si stampilat de "verificatori" stabiliti prin lege: verificatorul de arhitectura (arhitect), verificatorul de rezistenta (inginer constructor) si verificatorul de pompieri. Proiectul trebuie sa obtina de asemenea un numar de avize, mentionate prin Certificatul de Urbanism (apa, canalizare, mediu, pompieri, etc).

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4. Obtinerea Autorizatiei de Construire (AC) de la Primarie pe baza Certificatului de Urbanism, a proiectului si a avizelor de construireAutorizatia de Construire este actul absolut necesar pentru inceperea oricarei lucrari de constructii. Autorizatia de Construire se obtine de la primarie, directia de urbanism pe baza proiectului tehnic si a avizelor de construire. Dosarul (documentatia) care se depune in vederea emiterii autorizatiei de construire cuprinde urmatoarele documente:– cerere pentru emiterea autorizatiei de construire completata cu

elementele de identificare si datele tehnice conform P.A.C.

– actul doveditor al titlului asupra imobilului, care sa ii confere solicitantului dreptul de executie a lucrarilor de constructii (in copie legalizata)


– certificatul de urbanism (in copie)

– proiectul pentru autorizarea executarii lucrarilor de construire P.A.C., inclusiv referatele de verificare si, dupa caz, raportul de expertiza tehnica semnate si stampilate in original

– fisele tehnice pentru obtinerea avizelor cerute prin certificatul de urbanism, necesare emiterii acordului unic, precum si, dupa caz, documentatiile tehnice necesare emiterii avizelor si acordurilor care sunt in competenta de obtinere a emitentului

– avizele si acordurile obtinute de solicitant, altele decât cele din competenta de obtinere a emitentului, stabilite prin certificatul de urbanism (in copie)

– declaratie pe propria raspundere privind inexistenta unor litigii asupra imobilului

– documentul de plata a taxei de emitere a autorizatiei de construire

– documentele de plata a taxelor legale pentru avizele si acordurile necesare emiterii acordului unic, conform listei avizelor si acordurilor necesare, comunicate o data cu certificatul de urbanism (in copie)

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5. Anuntarea inceperii lucrarilor la Primarie si la Inspectia de Stat in Constructii si inceperea constructieiLucrarile de construire pot incepe numai dupa ce a fost obtinuta Autorizatia de construire (A.C.) si au fost anuntate (cu minim 5 zile inainte de inceperea lucrarilor) Primaria locala si Inspectoratul de Stat in Constructii (I.S.C.).– La inceperea lucrarilor, la locul constructiei trebuie sa existe un

panou de identificare a santierului de constructii pe care sunt inscrise date despre lucrare (denumire si adresa obiectiv, beneficiar, proiectant, constructor, nr. autorizatie, termen de executie, data inceperii si finalizarii constructiei).

– Formularele necesare, inclusiv formularul "model panou", se elibereaza de catre Primaria locala, o data cu eliberarea autorizatiei de construire.


6. Executia constructiei– In perioada executiei constructiei proiectantului ii revine obligatia

de a urmari realizarea pe santier a fazelor determinante specificate in documentatia de autorizare.

– Realizarea constructiei in regie proprie conduce la obtinerea unor costuri mai scazute dar implica riscul unor erori grave, generate de necunoastere, folosirea de personal slab sau deloc calificat, aprecieri eronate ale calitatii si cantitatilor de materiale

– Realizarea constructiei cu o firma specializata poate asigura calitatea lucrarilor de constructie si garantii din partea constructorului privind calitatea lucrarilor executate si incadrarea in termenele de executie planificate

– Lucrarile de constructie trebuie supravegheate de catre un diriginte de santier independent.

– Eventualele modificari fata de proiectul aprobat de primarie se pot face prin dispozitii de santier intocmite de proiectant si emiterea unei noi Autorizatii de Construire pentru modificare de tema de proiectare.

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7. Intocmirea cartii tehnice a constructieiSe realizeaza pe parcursul executarii lucrarilor si contine toate actele si documentatiile privind constructia inclusiv eventualele dispozitii de santier.

8. Receptia constructiei

Receptia constructiilor se face pe baza proiectului tehnic (P.T.) prin proces verbal de receptie, intocmit de catre o comisie formata din:– reprezentantul Inspectoratului de Stat in Constructii (I.S.C.)

– reprezentantul autorizatorului (primariei) printr-un membru al compartimentului Disciplina in Constructii

– reprezentantul proiectantului, prin arhitect si inginerul proiectant al structurii de rezistenta

– reprezentantul beneficiarului prin dirigintele de santier. Cu aceasta ocazie proiectantul da o declaratie de conformitate prin care se atesta ca imobilul supus receptiei este conform proiectului

basics of structural design

Documents

structural action course

shell structures

oradeasome structures

inhabitants structures

oradea4some structures

oradea3some structures

cable structures

multistorey structures