N-W.F.P. University of Engineering and Technology Peshawar Steel Structures CE-409

By: Prof Dr. Akhtar Naeem Khan [email protected]

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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 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 2

Course Content

 Design of Column Bases  Design of Beams  Design of Composite Beams  Design of Plate Girders CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 3

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

Lecture 01: Design Philosophies

By: Prof Dr. Akhtar Naeem Khan [email protected]

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Topics to be covered

 Design philosophies  Limit States  Design Considerations  Allowable Stress Design (ASD)  Load and Resistance Factor Design (LRFD)  Design process CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 5

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 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 6

Design Philosophies

 Resistance 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.

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 7

Limit States

 When 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.” CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 8

Limit States

 There are three broad classification of limit states: 1.

Strength limit states 2.

Serviceability limit states 3.

Special limit states Prof. Dr Akhtar Naeem Khan 9 CE-411: Lecture 01

Limit States

Strength Limit States: • Flexure • Torsion • Shear • Fatigue • Settlement • Bearing CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 10

Limit States

Serviceability Limit States: • Cracking • Excessive Deflection • Buckling • Stability Prof. Dr Akhtar Naeem Khan 11 CE-411: Lecture 01

Limit States

Special Limit States:  Damage or collapse in extreme earthquakes.  Structural effects of fire, explosions, or vehicular collisions.

Prof. Dr Akhtar Naeem Khan 12 CE-411: Lecture 01

Limit States

 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.

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

Design Considerations

• 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 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 14

Design Considerations

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

Design Considerations

 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.

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

Design Considerations

 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.

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

Design Considerations

 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.

CE-411: Lecture 01 Prof. Dr. Akhtar Naeem Khan 18

Design Considerations

 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).

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

Allowable Stress Design (ASD)

 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 f y , say one half. Prof. Dr. Akhtar Naeem Khan 20 CE-411: Lecture 01

Allowable Stress Design (ASD)

• This is equivalent to:

FOS = Resistance, R/ Effect of load, Q

= fy/0.5fy

= 2 Prof. Dr. Akhtar Naeem Khan 21 CE-411: Lecture 01

Allowable Stress Design (ASD)

 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.) CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 22

Allowable Stress Design (ASD)

Mathematical Description of A S D  

R n

 

Q i

R n = Resistance or Strength of the component being designed 

Φ =

Resistance Factor or Strength Reduction Factor = Overload or Load Factors   = Factor of Safety FS Q i = Effect of applied loads CE-411:Lecture No . 1 Prof. Dr. Akhtar Naeem Khan 23

Allowable Stress Design (ASD)

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: M n = Nominal Flexural Strength of a Beam M = Moment resulting from applied unfactored loads FS = Factor of Safety

M n FS



M f b

  

F b



F y FS or F y FS I

/

c I

/

c



F b



F cr FS

 

M I

/

c

CE-411:Lecture No . 1 Prof. Dr. Akhtar Naeem Khan 24

S ection Modulus



Section Modulus

: S ≥ effect of load/Allowable stress = M/f b ------(ii) Prof. Dr. Akhtar Naeem Khan 25 CE-411: Lecture 01

ASD Drawbacks

 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.

Prof. Dr. Akhtar Naeem Khan 26 CE-411: Lecture 01

Variation of Residual Stress with Geometry

CE-411: Lecture 01 Material A has more Residual Stresses due to: 1. Non uniform cooling 2. Cutting a plate into smaller pieces reveals the stresses Prof. Dr. Akhtar Naeem Khan 27

ASD Drawbacks

• 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).

CE-411: Lecture 01 Prof. Dr. Akhtar Naeem Khan 28

Load and Resistance Factor Design (LRFD)

 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.

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



Load and Resistance Factor Design (LRFD)

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 

R n



n

 

Q i

R n Q i = Resistance or Strength of the component being designed = Effect of Applied Loads  n = Takes into account ductility, redundancy and operational imp.

Φ =

Resistance Factor or Strength Reduction Factor = Overload or Load Factors   = Factor of Safety CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 30

The role of ‘n’

Ductility:

It implies a large capacity for inelastic deformation without rupture  Ductility will ensure redistribution of load through inelastic deformation.

Prof. Dr. Akhtar Naeem Khan 31 CE-411: Lecture 01

The role of ‘n’

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.

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

Redundancy

Yielding will initiate at mid span due to maximum moment at mid span with no Redistribution of load 33 CE-411: Lecture 01 Prof. Dr. Akhtar Naeem Khan

Redundancy

Yielding will initiate at supports due to maximum moment at supports 34 CE-411: Lecture 01 Prof. Dr. Akhtar Naeem Khan

Redundancy

Redistribution of load to mid span after yielding of section at supports 35 CE-411: Lecture 01 Prof. Dr. Akhtar Naeem Khan

The role of ‘n’

Operational Importance:

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

Prof. Dr. Akhtar Naeem Khan 36 CE-411: Lecture 01

→ hospital CE-411: Lecture 01 → park Prof. Dr. Akhtar Naeem Khan 37

LRFD Advantages

 LRFD accounts for both variability in resistance and load.

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

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 38

LRFD Disadvantages

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

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 39

Comparison of ASD and LRFD Design Approaches

 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 CE-411: Lecture 01 Prof. Dr. Akhtar Naeem Khan 40

Comparison of ASD and LRFD Design Approaches

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

Comparison of ASD and LRFD Design Approaches

 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 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 42

Comparison of ASD and LRFD Design Approaches

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

Comparison of ASD and LRFD Design Approaches

In LRFD For Tension Members: 1.2D + 1.6 L = 0.90 R n  1.33D + 1.78 L = R n (LRFD) In ASD Factor of Safety FS = 1.67, Therefore: 1.0D + 1.0 L = R n / 1.67  1.67D + 1.67D L = Rn (ASD)

LRFD ASD

 1 .

33

D

1 .

67

D

 1 .

78

L

 1 .

67

L

 0 .

8  1 .

07 (

L

1  (

L

/

D

) /

D

)

…. (A)

In LRFD For Dead Load Case: 1.4D = 0.90 R n  1.56D = R n (LRFD)

LRFD ASD

 1 .

1 .

67

D

56 

D

1 .

67

L

 0 .

93 1  (

L

/

D

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

…. (B)

44

Comparison of ASD and LRFD Design Approaches

3% 1.0

0.93

0.9

0.8

0.83

1.4D

0.7

1.2D + 1.6L

0.12

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Live Load Dead Load

4 5 Prof. Dr. Akhtar Naeem Khan 6 45

AREA Code for Design of Railway Structures

 AREA Stands for A merican R ailway E ngineers A ssociation ( 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 46 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan

AASHTO Code for Design of Highway Bridges

• AASHTO Stands for A ssociation of A merican S tate and H ighway T ransportation O fficials ( AASHTO ) • Highway Bridges are usually designed using provisions of the AASHTO Code • AASHTO Code uses both ASD and LRFD Design Methods 47 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan

The role of various Codes

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

The role of various Codes

 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.

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

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 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 50

Overview of LRFD Manual

 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 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 51

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.

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 52

Overview of LRFD Manual

 Part 15: Design of Hanger connections, Bracket plates, and Crane-rail connections  ANSI/LRFD Specifications for structural steel Buildings.

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 53

Design Process

1. Functional planning

• Development of a plan that will enable the structure to fulfill effectively the purpose for which it is to be built CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 54

Design Process

The involvement of Structural engineer in the functional planning is very imp because an Architect can suggest a plane which is practically not possible.

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

2.

Design Process

Structural scheme

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 56

2.

Design Process

Structural scheme (Contd.)

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 57

Design Process

3.

Preliminary Member Sizing of Beams

 Deflection Considerations  ASD Commentary L3.1 suggests following Limits:

L

 800 For fully stressed Beams & Girders

D F y

(

Ksi

)

L D L D

  20 800

F y

(

Ksi

) For Beams & Girders subject to vibrations For Roof Purlins CE-411:Lecture No . 1 Prof. Dr. Akhtar Naeem Khan 58

3.

Design Process

Preliminary Member Sizing of Beams

 Strength/Capacity Considerations Beam CE-411:Lecture No . 1 Prof. Dr. Akhtar Naeem Khan

Unbraced Length

59

3.

Design Process

Preliminary Member Sizing of Columns

 Strength/Capacity Considerations

Tributary Area

• Use of Tributary Areas and Column Tables CE-411:Lecture No . 1 Prof. Dr. Akhtar Naeem Khan 60

Tributary Area

CE-411: Lecture 01 Prof. Dr. Akhtar Naeem Khan 61

Design Process

4.

Structural Analysis - Modeling

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 62

Design Process

4.

Structural Analysis - Analysis

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 63

Design Process

5.

Design Review/ Member Modification

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

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 64

Design Process

6.

Cost Estimation

• 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 CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 65

Design Process

7. Preparation of Structural Drawings & Specifications

CE-411: Lecture 01 Prof. Dr Akhtar Naeem Khan 66

CE-411: Lecture 01

Thanks

Prof. Dr. Akhtar Naeem Khan 67