General Introduction

Structural design is a systematic & iterative process that involves:

Identification of intended use & occupancy of a structure – by owner

Development of architectural plans & layout – by architect

Identification of structural framework – by engineer

Estimation of structural loads depending on use & occupancy

General Introduction

Analysis of the structure to determine member & connection design forces

Design of structural members & connections

Verification of design

Fabrication & Erection – by steel fabricator & contractor

Inspection & Approval – by state building official

The primary responsibilities are:

Owner - primary responsibility is deciding the use &occupancy, & approving the arch. plans of thebuilding.

Architect - primary responsibility is ensuring that thearchitectural plan of the building interior isappropriate for the intended use & the overallbuilding is aesthetically pleasing.

Engineer – primary responsibility is ensuring thesafety & serviceability of the structure, i.e., designingthe building to carry the loads safely.

Fabricator – primary responsibility is ensuring thatthe designed members & connections are fabricatedeconomically in the shop or field as required.

Contractor/Erector - primary responsibility isensuring that the members & connections areeconomically assembled in the field to build thestructure.

Building Official – primary responsibility is ensuringthat the built structure satisfies the appropriatebuilding codes accepted by the Govt.

Conceptually, from an engineering standpoint, theparameters that can be varied (somewhat) are: The material of construction The structural framing plan.

The choices for material include: Steel Reinforced concrete Steel-concrete composite construction.

The choices for structural framing plan include: Moment resisting frames. Braced frames. Dual frames Shear wall frames, and so on.

The engineer can also innovate a new structural framingplan for a particular structure if required.

Structural Design

All viable material + framing plan alternativesmust be considered & designed to compare theindividual material + fabrication / erection coststo identify the most efficient & economicaldesign for the structure.

For each material + framing plan alternativeconsidered, designing the structure consists ofdesigning the individual structural components,i.e., the members & the connections, of theframing plan.

Determination of dimensions and selection of crosssections.

The design process is a loop:Assume dimensions, structural conditions and cross sections

Structural Analysis

Selection of cross sections to satisfy structural requirements

Does the design violate the initial assumptions?

Final Design

YES NO

Optimal structural design shall achieve balance between the following requirements:

Strength

Economy

Serviceability

Optimal design

Arrange and proportion the members of the structures,using engineer’s intuition and sound engineeringprinciples, so that they can be practically erected, havesufficient strength (safe), and are economical.

Practicality: Ensure structures can be fabricated and erectedwithout problems

Safety: Ensure structures can safely support the loads.Ensure deflections and vibrations arecontrolled for occupants comfort.

Cost: Minimize costs without sacrifice of strength(consider labor costs in fabrication anderection, not just material costs)

Trusses

Frames ( Beam-Column) Beams Girders Columns

Space trusses/frames

Beams-Frames

Bracing

Columns

Purlins

Industrial/Parking structures “Frames”

Joists/Trusses

High rise buildings

Girder bridges

Truss bridges

Cable stayed & suspended bridges

Structural members are categorized based up on theinternal forces in them. For example:

Tension member –subjected to tensile axial force only

Column or compression member –subjected to compressive axial force only

Tension/Compression member –subjected to tensile/compressive axial forces

Beam member –subjected to flexural loads, i.e., shear force & bending moment only. The axial force in a beam member is negligible.

Beam-column member – member subjected to combined axial force & flexural loads (shear force, & bending moments)

In trusses: All the members are connected using pin/hinge connections.

All external forces are applied at the pins/hinges.

All truss members are subjected to axial forces (tension or

compression) only.

In frames: The horizontal members (beams) are subjected to flexural loads only.

In braced frames: The vertical members (columns) are subjected to compressive axial forces

only.

The diagonal members (braces) are subjected to tension/compression axial

forces only.

In moment frames The vertical members (beam-columns) are subjected to combined axial &

flexural loads.

Members of a structural frame are connected togetherusing connections. Prominent connection types include:

Truss / bracing member connections are used to connect two ormore truss members together. Only the axial forces in themembers have to be transferred through the connection forcontinuity.

Simple shear connections are the pin connections used to connectbeam to column members. Only the shear forces are transferredthrough the connection for continuity. The bending moments arenot transferred through the connection.

Moment connections are fix connections used to connect beam tocolumn members. Both the shear forces & bending moments aretransferred through the connections with very smalldeformations (full restraint).

Truss connection

Simple Shear connection

Moment resisting

connection

The building structure must be designed to carry orresist the loads that are applied to it over its design-life.The building structure will be subjected to loads thathave been categorized as follows:

Dead Loads (D): are permanent loads acting on the structure.These include the self-weight of structural & non-structuralcomponents. They are usually gravity loads.

Live Loads (L): are non-permanent loads acting on the structuredue to its use & occupancy. The magnitude & location of liveloads changes frequently over the design life. Hence, they cannotbe estimated with the same accuracy as dead loads.

Wind Loads (W): are in the form of pressure or suction on theexterior surfaces of the building. They cause horizontal lateralloads (forces) on the structure, which can be critical for tallbuildings. Wind loads also cause uplift of light roof systems.

Snow Loads (S): are vertical gravity loads due tosnow, which are subjected to variability due toseasons & drift.

Roof Live Load (Lr): are live loads on the roof causedduring the design life by planters, people, or byworkers, equipment, & materials duringmaintenance.

Values of structural loads can be computed based onthe design code.

Dead loads consist of the weight of all materials ofconstruction incorporated into the building includingbut not limited to walls, floors, roofs, ceilings, stairways,built-in partitions, finishes, cladding & other similarlyincorporated architectural & structural items, & fixedservice equipment such as plumbing stacks & risers,electrical feeders, & heating, ventilating, & airconditioning systems.

In some cases, the structural dead load can be estimatedsatisfactorily from simple formulas based in the weights& sizes of similar structures. For example, the averageweight of steel framed buildings is 3 - 3.6 kPa, & theaverage weight for reinforced concrete buildings is 5 - 6kPa.

From an engineering standpoint, once the materialsand sizes of the various components of the structureare determined, their weights can be found from tablesthat list their densities. See Tables 1.2 & 1.3, which aretaken from Hibbeler, R.C. (1999), Structural Analysis,4th Edition.

Building floors are usually subjected to uniform live loads orconcentrated live loads. They have to be designed to safely supportthese loads.

Type of occupancy kPa

Offices 2.5 - 5

Corridors 5

Residential 2

Stairs and exit ways 5

Stadiums 5

Sidewalks 12

Design wind loads for buildings can be based on: (a)simplified procedure; (b) analytical procedure; & (c) windtunnel or small-scale procedure.

Refer to ASCE 7-05 for the simplified procedure. Thissimplified procedure is applicable only to buildings withmean roof height less than 18 m or the least dimension of thebuilding.

The wind tunnel procedure consists of developing a small-scale model of the building & testing it in a wind tunnel todetermine the expected wind pressures etc. It is expensive &may be utilized for difficult or special situations.

The analytical procedure is used in most design offices. It isfairly systematic but somewhat complicated to account forthe various situations that can occur:

Wind velocity will cause pressure on any surface in itspath. The wind velocity & hence the velocity pressuredepend on the height from the ground level. Equation1.3 is recommended by ASCE 7-05 for calculating thevelocity pressure (qz) in SI

qz = 0.613 Kz KztKd V2 I (N/m2)

qz – Static wind pressure

V - the wind velocity in m/s

Kd - a directionality factor (= 0.85 see Table 6.4 page 80)

Kzt - a topographic factor (= 1.0)

I - the importance factor (=1.0)

Kz - varies with height z above the ground level (see Table 6.3

page 79)

exposure B structure surrounded by buildings/forests/…

at least 6m height

exposure C open terrain

A significant portion of Palestine has V = 100 km/h. Atthese location

qz = 402 Kz (N/m2)

The velocity pressure qz is used to calculate the design wind pressure (p)

for the building structure conservatively as follows:

p = q GCp (N/m2)

Kz - varies with height z above the ground level

A – large city centers

B – urban/ suburban area

C – open terrain with scattered obstructions

D – Flat unobstructed surface

G - gust effect factor (= 0.85)

Cp - external pressure coefficient from Figure 6-6 page 48-49 inASCE 7-05 or

Cp = 0.8 windward

Cp = -0.5 leeward

Cp = -0.7 sidewalls

Cp = -0.7 slope<0.75(1.5)

• Note that:

• A positive sign indicates pressure acting towards a surface.

• Negative sign indicates pressure away from the surface

Consider the building structure with the structural floor plan &elevation shown below. Estimate the wind loads acting on thestructure when the wind blows in the east-west direction. Thestructure is located in Nablus.

15 m 15 m

15 m

15 m

Plan

6 @

3m

6 @

3m

Velocity pressure (qz) Kd - directionality factor = 0.85

Kzt - topographic factor = 1.0

I - importance factor = 1.0

V = 100 kph in Nablus

qz = 402 Kz (N/m2)

Kz - varies with height z above the ground level

Kz values for Exposure B, Case 2

Wind pressure (p) Gust factor = G = 0.85 for rigid structures

External pressure coefficient = Cp = +0.8 for windward walls

Cp = -0.5 for leeward walls

Cp = -0.7 for side walls

External pressure = q G Cp

External pressure on windward wall = qz GCp = 402 Kz x 0.85 x 0.8 = 273.4 Kz Pa toward surface

External pressure on leeward wall = qh GCp = 402 K18 x 0.85 x (-0.5) = 145.2 Pa away from surface

External pressure on side wall = qh GCp = 402 K18 x 0.85 x (-0.7) = 203.3 Pa away from surface

The external pressures on the structure are shown in the following two figures.

273.4 Kz

203.3

145.2

203.3

3 m

3 m

3 m

3 m

3 m

3 m

155.8

169.5180.4

191.4

207.8

221.5

232.4

145.2

Economical production in large volume not available until mid 19th

century and the introduction of the Bessemer process. Steel becamethe principal metallic structural material by 1890.

Steels consists almost entirely of iron (over 98%) and smallquantities of carbon, silicon, manganese, sulfur, phosphorus, andother elements.

The quantities of carbon affect properties of steel the most.

Increase of carbon content increases hardness and strength

Alloy steel – has additional amounts of alloy elements suchchronium, vanadium, nickel, manganese, copper, or zirconium.

The American Society for Testing of Materials (ASTM) specifies exactmaximum percentages of carbon content and other additions for anumber of structural steels. Consult Manual, Part 2, Table 2-1 to 2-3for availability of steel in structural shapes, plate products, andstructural fasteners.

Carbon steels – A36, A53, A500, A501, A529, A570. Have well-defined yield point. Divided into four categories:

Low-carbon steel (< 0.15%)

Mild steel (0.15 to 0.29%, structural carbon steels)

Medium-carbon steel (0.3 to 0.59%)

High-carbon steel (0.6 to 1.7%)

High-Strength Low-Alloy steels – A242, A572, A588, A606, A607, A618, A709

Well-defined yield point

Higher strengths and other properties

Alloy Steels – A514, A709, A852, A913.

Yield point defined as the stress at 0.2% offset strain

Low-alloy steels quenched and tempered → 550 to 760 MPa yield strengths

Advantages High strength per unit of weight → smaller weight of structures

Uniformity

Elasticity

Long lasting

Ductility

Toughness

Easy connection

Speed of erection

Ability to be rolled into various sizes and shapes

Possible reuse and recyclable

Disadvantages Maintenance costs

Fire protection/Fireproofing costs

Susceptibility to buckling failure

Fatigue

Brittle fracture

Three basic types of steel used for structural steel

Plain Carbon Steel

Low-alloy steel

High-alloy “specialty steel”

The most commonly used is mild steel - ASTM A36

Typical high strength steel:

The higher the steel strength, the higher the carbon content and the less ductile it is.

248 (36 )

400 (58 )

y

u

F MPa ksi

F MPa ksi

290 344 (42 50 )

444 482 (63 70 )

y

u

F MPa ksi

F MPa ksi

ASTM A242

344 (50 )

448 (65 )

y

u

F MPa ksi

F MPa ksi

ASTM A992

Standard Plain Carbon Steel

)(

)(

LengthOriginalL

nDeformatioL

o

)(

)(

AreaA

LoadPf

Yield plateauFy

Strain “”

Stress “f”

Fu

E

Necking & FractureStrain Hardening

Elastic

When a structure or structural element becomes

unfit for its intended purpose it has reached or

exceeded a limit state

Two categories of limit states:

Strength limit states

Serviceability limit states

Strength Limit States

a) Loss of Equilibrium

b) Loss of load bearing capacity

c) Spread of local failure

d) Very large deformations

Serviceability Limit States

a) Excessive deflection

b) Excessive local damage

c) Unwanted vibration

Allowable Stress Design (ASD)

Plastic Design (PD)

Load and Resistance Factor Design (LRFD)

Service loads are calculated as expected during service life.

Linear elastic analysis is performed.

A factor of safety (FOS) of the material strength is assumed (usually 3-4)

Design is satisfactory if (maximum stress < allowable stress)

Limitations Case specific, no guarantee that our design covers all cases

Arbitrary choice of FOS?!

FOS

StrengthMaterialStressAllowable

Service loads are factored by a “load factor”.

The structure is assumed to fail under these loads, thus, plastic hinges will form under these loads “Plastic Analysis”.

The cross section is designed to resist bending moments and shear forces from the plastic analysis.

Members are safe as they are designed to fail under these factored loads while they will only experience service loads.

Limitations No FOS of the material is considered, neglecting the uncertainty in

material strength!

Arbitrary choice of overall FOS?!

LRFD is similar to plastic design in that it performs design with the assumption of failure! - Reliability Based Design

Service loads are multiplied by load factors (g) and linear elastic analysis is performed.

Material strength is reduced by multiplying the nominal material strength by a resistance factor (f)

The design rule is: Load Effect < Resistance

Where Rn is the nominal strength and Q is the load effect for the ith

limit state

niii RQ fg This rule shall be attained

for all limit states!!

Resistance: Shear, Bending, Axial Forces

Advantages of LRFD Non-case specific, statistical calculations guarantee population

behavior.

Uniform factor of safety as both load and material factors are tied by reliability analysis

If we have the probability distribution of the load effect (Q) and the material resistance (R) then:

• The probability of failure can be represented by observing the probability of the function (R-Q)

• The probability of failure PF can be represented as the probability that Q ≥ R:

Probability

of failure

AISC considers the following load combinations in design niii RQ fg

)(5.06.12.12 RorSorLLD r

D4.11

)8.0(5.0)(6.12.13 WorLRorSorLD r

SLED 2.05.00.12.15

6 0.9 (1.6 1.0 )D Wor E

)(5.05.06.12.14 RorSorLLWD r

00.175.0 f

ii Qg

ni Rf

Dead loads (D)

Live loads (LL)

• Occupancy load (L)

• Roof load (Lr)

• Snow load (S)

• Rain loads (R)

• Trucks and pedestrians

Wind Loads (W)

Earthquakes (E)e.g. f for yield is 0.9 and for bolt shear is 0.75