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Design of Steel StructuresCE-409

Prof.Dr. A.N. KhanAsst.Prof.Dr. A. AhmedAsst.Prof.Dr. M. Fahad

Department of Civil EngineeringUniversity of Engineering & Technology Peshawar

Course outline

1 Design philosophies

2 Introduction to steel structures

3 Design of welded connections

4 Design of bolted connections

5 Design of tension members

6 Design of compression members

7 Design of column bases

8 Design of beams

9 Design of composite beams

10 Design of plate girders

Asst.Prof.Dr. A.Ahmed (UETP) Design of steel structures-CE409 2

Design philosophies

By Asst.Prof.Dr. A. Ahmed([email protected])

Department of Civil EngineeringUniversity of Engineering & Technology Peshawar

Topics to be covered

Design philosophies

Limit States

Design Considerations

Allowable Stress Design (ASD)

Load and Resistance Factor Design (LRFD)

Design process


Design philosophies

A general statement assuming safety in engineering design is

Resistance ≥ Effect of applied loads (1)

In equation (1) it is essential that both sides are evaluated forsame conditions and units e.g

⇒ Compressive Stress on Column should be compared withCompressive Strength of Column Material (Steel, Concrete,Wood etc)


Design philosophies contd;

Resistance of structures is composed of its members whichcomes from

⇒ Materials and⇒ X-section

Resistance, Capacity, and Strength are somewhat synonymterms

Terms like Demand, Stresses, and Loads are used to expressEffect of applied loads.


Limit states

When particular loading reaches its limit, failure is theassumed result, i.e. the loading condition become failuremodes, such a condition is referred to as limit state and it canbe defined as

⇒ A limit state is a condition beyond which a structural systemor a structural component ceases to fulfill the function forwhich it is designed


Limit states

There are three broad classification of limit states:

⇒ Strength limit states⇒ Serviceability limit states⇒ Special limit states


Limit states



Strength limit statesflexure, torsion, tension, compression, shear, fatigue,settlement, bearing


Limit states



Serviceability limit statesCracking, Excessive deflection, Buckling, Stability


Limit states



Special limit statesDamage or collapse in extreme earthquakes, Structural effectsof fire, explosions, or vehicular collisions, radiation


Limit states contd;

Design Approach used must ensure that the probability of aLimit State being reached in the Design/Service Life of astructure is within acceptable limits

However, complete elimination of probability of a Limit Statebeing achieved in the service life of a structure is impracticalas it would result in uneconomical designs


Limit states contd;

Design Approach used must ensure that the probability of aLimit State being reached in the Design/Service Life of astructure is within acceptable limits

However, complete elimination of probability of a Limit Statebeing achieved in the service life of a structure is impracticalas it would result in uneconomical designs

For a tower

if we have 500m/hr wind..chances once in 50 years200m/hr wind..chances three times in 50 years

⇒ we will prefer 200m/hr only we will do some steps to safe it


Design considerations

Structure and Structural Members should have adequatestrength, stiffness and toughness to ensure proper functioningduring service life

Reserve Strength should be available to cater for:

Occasional overloads and underestimation of loads

Variability of strength of materials from those specified

Variation in strength arising from quality of workmanship andconstruction practices


Design considerations contd;

Structural Design must provide adequate margin of safetyirrespective of Design Method

Design Approach should take into account the probability ofoccurrence of failure in the design process

An important goal in design is to prevent limit state frombeing reached

It is not economical to design a structure so that none of itsmembers or components could ever fail. Thus, it is necessaryto establish an acceptable level of risk or probability of failure



Brittle behavior is to be avoided as it will imply a sudden lossof load carrying capacity when elastic limit is exceeded

Reinforced concrete can be made ductile by limiting the steelreinforcement

To determine the acceptable margin of safety, opinion shouldbe sought from experience and qualified group of engineers

⇒ In steel design AISC manuals for ASD & LRFD guidelines canbe accepted as reflection of such opinions



Any design procedure require the confidence of Engineer onthe analysis of load effects and strength of the materials

The two distinct procedures employed by designers are

⇒ Allowable Stress Design (ASD) and⇒ Load & Resistance Factor Design (LRFD)



Safety in the design is obtained by specifying, that the effectof the loads should produce stresses that is a fraction of theyield stress Fy, say one half

⇒ This is equivalent to:

FOS =Resistance, R

Effect of load, Q

FOS =Fy (material strength)

0.5fy(imposed stresses)= 2



Since the specifications set limit on the stresses, it becameallowable stress design (ASD)

It is mostly reasonable where stresses are uniformly distributedover X-section (such on determinate trusses, arches, cablesetc.)



In ASD we check the adequacy of a design in terms of stressestherefore design checks are cast in terms of stresses forexample if:

Mn

FS≥ M

Fy

FS

I/c

I/c≥

M

I/c

fb ≤

[

Fb =Fy

FSor Fb =

Fcr

FS

]

Mn = Nominal Flexural Strength of a BeamM = Moment resulting from applied unfactored loadsFS = Factor of Safety


Section Modulus

Section Modulus

S ≥ effect of load / Allowable stress

= M/Fb

For rectangular sections

S = I/c = bd2/6


ASD drawbacks

Implied in the ASD method is the assumption that the stressin the member is zero before any loads are applied, i.e., noresidual stresses exist from forming the members


ASD drawbacks


Residual stressesDue to difference in shear rate of cooling, some stresses wouldbuilt. These stresses are known as residual stresses.(example.cutting of plate at the centre with flame)


ASD drawbacks



Material A has more Residual Stresses due to:

Non uniform coolingCutting a plate into smaller pieces reveals the stresses


ASD drawbacks



Material A has more Residual Stresses due to:

Non uniform coolingCutting a plate into smaller pieces reveals the stresses

ASD does not give reasonable measure of strength, which ismore fundamental measure of resistance than is allowablestress


ASD drawbacks



Material A has more Residual Stresses due to:Non uniform coolingCutting a plate into smaller pieces reveals the stresses

ASD does not give reasonable measure of strength, which ismore fundamental measure of resistance than is allowablestress

Another drawback in ASD is that safety is applied only tostress level. Loads are considered to be deterministic (withoutvariation)



To overcome the deficiencies of ASD, the LRFD method isbased on: Strength of materials

It consider the variability not only in resistance but also in theeffects of load.

It provides measure of safety related to probability of failure.



Safety in the design is obtained by specifying that the reducedNominal Strength of a designed structure is greater than theeffect of factored loads acting on the structure

φRn ≥ n∑

γQi

Rn = Resistance or Strength of the component being designed

Qi = Effect of Applied Loads

n = Takes into account ductility, redundancy and operationalimp.

φ = Resistance Factor or Strength Reduction Factor

γ = Overload or Load Factors


The role of ”n”

Ductility: It implies a large capacity for inelastic deformationwithout rupture.

Ductility will ensure redistribution of load through inelasticdeformation.


The role of ”n”

Redundancy:

A simply supported beam is a determinate structure so it hasno redundant actions.

A fixed beam is indeterminate by 2 degrees so it has tworedundant actions.


The role of ”n”


The role of ”n”


The role of ”n”


The role of ”n”

Operational Importance:A hospital and a school require more conservative design thanan ordinary residential building


LRFD advantages

LRFD accounts for both variability in resistance and load

It achieves fairly uniform levels of safety for different limitstates.


ASD vs LRFD

ASD combines Dead and Live Loads and treats them in thesame way


ASD vs LRFD


In LRFD different load factors are assigned to Dead Loads andLive Loads which is appealing


ASD vs LRFD



Changes in load factors and resistance factors are much easierto make in LRFD compared to changing the allowable stressin ASD


ASD vs LRFD




LRFD is intrinsically appealing as it requires betterunderstanding of behavior of the structure in its limit states


ASD vs LRFD




LRFD is intrinsically appealing as it requires betterunderstanding of behavior of the structure in its limit states

Design approach similar to LRFD is being followed in Designof concrete structures in form of Ultimate Strength Design –why not use similar approach design of steel structures?


ASD vs LRFD

ASD indirectly incorporates the Factors of Safety by limitingthe stress whereas LRFD aims to specify Factors of Safetydirectly by specifying Resistance Factors and Load Factors


ASD vs LRFD


LRFD is more rational as different Factors of Safety can beassigned to different loadings such as Dead Loads, Live Loads,Earthquake Loads and Impact Loads


ASD vs LRFD



LRFD considers variability not only in resistance but also inthe effects of load which provides measure of safety related toprobability of failure


ASD vs LRFD




It achieves fairly uniform levels of safety for different limitstates


ASD vs LRFD




It achieves fairly uniform levels of safety for different limitstates

ASD still remains as a valid Design Method


ASD vs LRFD

In LRFD for tension members:

1.2D + 1.6L = 0.90Rn −→ 1.33D + 1.78L = Rn

In ASD factor of safety FS = 1.67, Therefore:

1.0D + 1.0L = Rn/1.67 −→ 1.67D + 1.67L = Rn

LRFD

ASD=

1.33D + 1.78L

1.67D + 1.67L=

0.8 + 1.07(L/D)

1 + (L/D)

In LRFD for Dead load case:

1.4D = 0.90Rn

1.56D = Rn

LRFD

ASD=

1.56D

1.67D + 1.67L=

0.93

1 + (L/D)


ASD vs LRFD


Allowable Strength Design (ASD)

Mathematical Description of ASD

φRn

γ≥

∑

Qi

in which

Rn is Resistance or strength of the component being designedφ is Resistance Factor or Strength Reduction Factorγ is Overload or Load FactorsγφFactor of Safety FS

Qi Effect of applied loads


AREA code for railway structures

AREA stands for American Railway Engineers Association

Railway Bridges and Structures are usually designed usingprovisions of the AREA Code

AREA Code uses only the Allowable Stress Design Method.However, the allowable stresses and design requirements maydiffer from AISC/ASD method


AASHTO code for railway structures

AASHTO Stands for Association of American State andHighway Transportation Officials (AASHTO)

Highway Bridges are usually designed using provisions of theAASHTO Code

AASHTO Code uses both ASD and LRFD Design Methods


The role of various codes

It is very difficult to devise a design code that is applicable toall uses and all types of structures such as buildings, highwaybridges, railway bridges and transmission towers

The responsibility of infrastructure on roads, bridges andelectrical transmission towers rests with the organizationresponsible for approving, operating and maintaining thesefacilities

Uses and critical loads may be different in different types ofstructures and no one code can cater to all the differentimportant considerations

For above reasons different codes prevail and will continue todo so

AISC ASD Code and LRFD Code primarily is pertinent toBuilding Structures


Overview of LRFD manual

Part 1: Dimensions and properties

Part 2: General Design considerations

Part 3: Design of flexural members

Part 4: Design of compression members

Part 5: Design of Tension members

Part 6: Design of members subject to combined loading

Part 7: Design considerations for bolts

Part 8: Design considerations for welds

Part 9: Design of connecting elements

Part 10: Design of simple shear connections

Part 11: Design of flexible moment connections


Overview of LRFD manual

Part 12: Design of fully restrained (FR) moment connections

Part 13: Design of Bracing connections and truss connections

Part 14: Design of Beam bearing plates, column base plates,anchor rods, and column splices.

Part 15: Design of Hanger connections, bracket plates, andCrane-rail connections

ANSI/LRFD Specifications for structural steel Buildings.


Design process

Functional planning

Development of a plan that will enable the structure to fulfilleffectively the purpose for which it is to be built


Design process

Structural scheme


Design process

Structural scheme


Design process

Preliminary Member Sizing of Beams

Deflection ConsiderationsASD Commentary L3.1 suggests following Limits: For fullystresses beams and girders

L

D≤

800

Fy(Ksi)

For beams and girders subject to vibrations

L

D≤ 20

For roof purlins

L

D≤

800

Fy(Ksi)


Design process

Preliminary Member Sizing of Beams

Strength/Capacity Considerations


Design process

Preliminary Member Sizing of ColumnsStrength/Capacity ConsiderationsUse of Tributary Areas and Column Tables


Design process

Structural Analysis - Modeling


Design process

Structural Analysis - Analysis


Design process

Design review/member modificationMust be chosen so they will be able to resist, withinappropriate margin of safety, the forces which the structuralanalysis has disclosed.


Design process

Cost estimation

Make a tentative cost estimates for several preliminarystructural layouts

Selection of constructional material based on:

Availability of specific material

Corresponding skilled labor

Relative costs

Wage scales


Design process

Preparation of structural drawings and specifications


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