Lecture -1 Steel Structures Design Philosophies

N-W.F.P. University of Engineering and Technology

Peshawar

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Steel Structures CE-409

By: Prof Dr. Akhtar Naeem [email protected]

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan

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Course Content

Design philosophies

Introduction to Steel Structures

Design of Welded connections

Design of Bolted connections

Design of Tension Members

Design of Compression Members


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Course Content

Design of Column Bases

Design of Beams

Design of Composite Beams

Design of Plate Girders

N-W.F.P. University of Engineering and Technology

Peshawar

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By: Prof Dr. Akhtar Naeem [email protected]

Lecture 01: Design Philosophies


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Topics to be coveredDesign philosophies

Limit States

Design Considerations

Allowable Stress Design (ASD)

Load and Resistance Factor Design (LRFD)

Design process


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Design Philosophies

A general statement assuming safety in

engineering design is:

Resistance ≥ Effect of applied loads ---(1)

In eq(1) it is essential that both sides are

evaluated for same conditions and units e.g.

compressive stress on soil should be

compared with bearing capacity of soil


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Design PhilosophiesResistance of structures is composed

of its members which comes from

materials & X-section

Resistance, Capacity, and Strength are

somewhat synonym terms.

Terms like Demand, Stresses, and

Loads are used to express Effect of

applied loads.


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Limit StatesWhen particular loading reaches its

limit, failure is the assumed result, i.e. the loading condition become failure modes, such a condition is referred to as limit state and it can be defined as

“A limit state is a condition beyond which a structural system or a structural component ceases to fulfill the function for which it is designed.”


There are three broad classification of limit states:

1. Strength limit states

2. Serviceability limit states

3. Special limit states

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Limit States


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Strength Limit States:• Flexure• Torsion• Shear

Limit States

• Fatigue• Settlement• Bearing


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Serviceability Limit States:• Cracking• Excessive Deflection• Buckling• Stability

Limit States


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Limit StatesSpecial Limit States:

Damage or collapse in extreme

earthquakes.

Structural effects of fire, explosions, or

vehicular collisions.


Design Approach used must ensure that the probability of a Limit State being reached in the Design/Service Life of a structure is within acceptable limits;

However, complete elimination of probability of a Limit State being achieved in the service life of a structure is impractical as it would result in uneconomical designs.

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Limit States



• Structure and Structural Members should have adequate strength, stiffness and toughness to ensure proper functioning during 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 and construction practices

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CE-411: Lecture 01 Prof. Dr. Akhtar Naeem Khan

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Structural Design must provide adequate margin of safety irrespective of Design Method

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



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An important goal in design is to prevent limit state from being reached.

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



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

Reinforced concrete can be made ductile by limiting the steel reinforcement.

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

In steel design AISC manuals for ASD & LRFD guidelines can be accepted as reflection of such opinions.



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

The two distinct procedures employed by designers are Allowable Stress Design (ASD) & Load & Resistance Factor Design (LRFD).



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Safety in the design is obtained by specifying, that the effect of the loads should produce stresses that is a fraction of the yield stress fy, say one half.



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• This is equivalent to:

FOS = Resistance, R/ Effect of load, Q

= fy/0.5fy

= 2



Since the specifications set limit on the stresses, it became allowable stress design (ASD).

It is mostly reasonable where stresses are uniformly distributed over X-section (such on determinate trusses, arches, cables etc.)

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in Q

R

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Mathematical Description of A S D

Rn = Resistance or Strength of the component being designed

Φ = Resistance Factor or Strength Reduction Factor

= Overload or Load Factors

= Factor of Safety FS

Qi = Effect of applied loads

CE-411:Lecture No. 1


Prof. Dr. Akhtar Naeem Khan

FS

FFor

FS

FFf cr

by

bb

MFS

M n cI

M

cI

cI

FS

Fy

//

/

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Mathematical Description of Allowable Stress Design

In ASD we check the adequacy of a design in terms of stresses therefore design checks are cast in terms of stresses for example if:Mn = Nominal Flexural Strength of a BeamM = Moment resulting from applied unfactored loadsFS = Factor of Safety


Prof. Dr. Akhtar Naeem KhanCE-411:Lecture No. 1


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Section Modulus: S ≥ effect of load/Allowable stress = M/fb ------(ii)

Section Modulus


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Implied in the ASD method is the assumption that the stress in the member is zero before any loads are applied, i.e., no residual stresses exist from forming the members.

ASD Drawbacks


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Material A has more Residual Stresses due to: 1. Non uniform cooling

2. Cutting a plate into smaller pieces reveals the stresses

Variation of Residual Stress with Geometry


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• ASD does not give reasonable measure of strength, which is more fundamental measure of resistance than is allowable stress.

• Another drawback in ASD is that safety is applied only to stress level. Loads are considered to be deterministic (without variation).

ASD Drawbacks


To overcome the deficiencies of ASD, the LRFD method is based on:

Strength of Materials

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

It provides measure of safety related to probability of failure.

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Safety in the design is obtained by specifying that the reduced Nominal Strength of a designed structure is less than the effect of factored loads acting on the structure

in QnR

Rn = Resistance or Strength of the component being designedQi = Effect of Applied Loadsn = Takes into account ductility, redundancy and operational imp. Φ = Resistance Factor or Strength Reduction Factor = Overload or Load Factors

= Factor of Safety



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Ductility: It implies a large capacity for inelastic deformation without rupture

Ductility will ensure

redistribution of load through

inelastic deformation.

The role of ‘n’


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Redundancy:

1. A simply supported beam is a determinate structure so it has no redundant actions.

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

The role of ‘n’


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Yielding will initiate at mid span due to maximum moment at mid span with no Redistribution of load

Redundancy


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Yielding will initiate at supports due to maximum moment at supports

Redundancy


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Redistribution of load to mid span after yielding of section at supports

Redundancy


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Operational Importance:

A hospital and a school require more conservative design than an ordinary residential building.

The role of ‘n’


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→ hospital

→ park

Op

era

tion

al Im

port

an

ce


LRFD accounts for both variability in resistance and load.

It achieves fairly uniform levels of safety for different limit states.

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LRFD Advantages


It’s disadvantage is change in design philosophy from previous method.

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LRFD Disadvantages


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ASD combines Dead and Live Loads and treats them in the same way

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

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

Comparison of ASD and LRFD Design Approaches


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

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



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

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

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LRFD considers variability not only in resistance but also in the effects of load which provides measure of safety related to probability of failure

It achieves fairly uniform levels of safety for different limit states.

ASD still remains as a valid Design Method



)/(1

)/(07.18.0

67.167.1

78.133.1

DL

DL

LD

LD

ASD

LRFD

)/(1

93.0

67.167.1

56.1

DLLD

D

ASD

LRFD

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In LRFD For Tension Members:1.2D + 1.6 L = 0.90 Rn 1.33D + 1.78 L = Rn (LRFD)

In ASD Factor of Safety FS = 1.67, Therefore:1.0D + 1.0 L = Rn / 1.67 1.67D + 1.67D L = Rn (ASD)

In LRFD For Dead Load Case:1.4D = 0.90 Rn 1.56D = Rn (LRFD)

…. (A)

…. (B)



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0.7

0.8

0.9

1.0

1 2 3 4 5 6

0.93

0.12

0.83

1.4D

1.2D + 1.6L

LR

FD

AS

D

Live Load Dead Load

3%



AREA Stands for American Railway Engineers Association (AREA)

Railway Bridges and Structures are usually designed using provisions of the AREA Code

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

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AREA Code for Design of Railway Structures


• AASHTO Stands for Association of American State and Highway Transportation Officials (AASHTO)

• Highway Bridges are usually designed using provisions of the AASHTO Code

• AASHTO Code uses both ASD and LRFD Design Methods

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AASHTO Code for Design of Highway Bridges


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

The responsibility of infrastructure on roads, bridges and electrical transmission towers rests with the organization responsible for approving, operating and maintaining these facilities

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The role of various Codes


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Uses and critical loads may be different in different types of structures and no one code can cater to all the different important considerations

For above reasons different codes prevail and will continue to do so

AISC ASD Code and LRFD Code primarily is pertinent to Building Structures.

The role of various Codes


Overview of LRFD ManualPart 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

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Overview of LRFD ManualPart 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

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Overview of LRFD ManualPart 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.

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Overview of LRFD ManualPart 15: Design of Hanger connections,

Bracket plates, and Crane-rail connections

ANSI/LRFD Specifications for structural steel Buildings.

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1. Functional planning• Development of a plan that will enable the structure to fulfill effectively the purpose for which it is to be built

Design Process


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The involvement of Structural engineer in the functional planning is very imp

because an Architect can suggest a plane which is practically not possible.

Design Process


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2. Structural scheme

Design Process


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2. Structural scheme (Contd.)

Design Process

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Deflection Considerations

ASD Commentary L3.1 suggests following Limits:

3. Preliminary Member Sizing of Beams

For fully stressed Beams & Girders)(

800

KsiFD

L

y

20D

L

)(

800

KsiFD

L

y

For Beams & Girders subject to vibrations

For Roof Purlins

Design Process

CE-411:Lecture No. 1 Prof. Dr. Akhtar Naeem Khan

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Strength/Capacity Considerations

3. Preliminary Member Sizing of Beams

Tributary Area

BeamUnbraced Length

Des

ign

Mom

ent

Design Process


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Strength/Capacity Considerations

3. Preliminary Member Sizing of Columns

Tributary Area

• Use of Tributary Areas and Column Tables

Design Process



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Tributary Area


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Design Process 4. Structural Analysis - Modeling


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Design Process 4. Structural Analysis - Analysis


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Design Process

• Must be chosen so that they will be able to resist, within appropriate margin of safety, the forces which the structural analysis has disclosed.

5. Design Review/ Member Modification


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Design Process

• Make a tentative cost estimates for several preliminary structural layouts.

• Selection of constructional material based on: • Availability of specific material• Corresponding skilled labor• Relative costs• Wage scales

6. Cost Estimation


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Design Process

7. Preparation of Structural Drawings & Specifications


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Lecture -1 Steel Structures Design Philosophies

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