1. By: Prof Dr. Akhtar Naeem Khan chairciv@nwfpuet.edu.pk
Steel Structures CE-409
By: Prof Dr. Akhtar Naeem Khan

2. Course Content Design philosophies Introduction to Steel Structures
Design of Welded connections
Design of Bolted connections
Design of Tension Members
Design of Compression Members

3. Course Content Design of Column Bases Design of Beams
Design of Composite Beams
Design of Plate Girders

4. By: Prof Dr. Akhtar Naeem Khan chairciv@nwfpuet.edu.pk
Lecture 01: Design Philosophies
By: Prof Dr. Akhtar Naeem Khan

5. Topics to be covered Design philosophies Limit States
Design Considerations
Allowable Stress Design (ASD)
Load and Resistance Factor Design (LRFD)
Design process

6. 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

7. 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.

8. 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.”

9. Limit States There are three broad classification of limit states:
Strength limit states
Serviceability limit states
Special limit states

10. Limit States Strength Limit States: Flexure Torsion Settlement Shear
Fatigue
Settlement
Bearing

11. Limit States Serviceability Limit States: Cracking
Excessive Deflection
Buckling
Stability

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

13. 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.

14. 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

15. 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

16. 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.

17. 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.

18. 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.

19. 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).

20. 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 fy, say one half.

21. Allowable Stress Design (ASD)
This is equivalent to:
FOS = Resistance, R/ Effect of load, Q
= fy/0.5fy
= 2

22. 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.)

23. Allowable Stress Design (ASD)
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

24. 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:
Mn = Nominal Flexural Strength of a Beam
M = Moment resulting from applied unfactored loads
FS = Factor of Safety
CE-411:Lecture No. 1
Prof. Dr. Akhtar Naeem Khan

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

26. 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.

27. Variation of Residual Stress with Geometry
Material A has more Residual Stresses due to:
1. Non uniform cooling
2. Cutting a plate into smaller
pieces reveals the stresses

28. 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).

29. 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.

30. 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
Rn = Resistance or Strength of the component being designed
Qi = 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

31. The role of ‘n’
Ductility: It implies a large capacity for inelastic deformation without rupture
Ductility will ensure
redistribution of load through
inelastic deformation.

32. The role of ‘n’ Redundancy:
A simply supported beam is a determinate structure so it has no redundant actions.
A fixed beam is indeterminate by 2 degrees so it has two redundant actions.

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

34. Yielding will initiate at supports due to maximum moment at supports
Redundancy
Yielding will initiate at supports due to maximum moment at supports

35. Redundancy
Redistribution of load to mid span after yielding of section at supports

36. The role of ‘n’ Operational Importance:
A hospital and a school require more conservative design than an ordinary residential building.

37. Operational Importance
→ hospital
Operational Importance
→ park

38. LRFD Advantages
LRFD accounts for both variability in resistance and load.
It achieves fairly uniform levels of safety for different limit states.

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

40. 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

41. 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?

42. 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

43. 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

44. Comparison of ASD and LRFD Design Approaches
In LRFD For Tension Members:
1.2D L = 0.90 Rn  D L = Rn (LRFD)
In ASD Factor of Safety FS = 1.67, Therefore:
1.0D L = Rn /  D D L = Rn (ASD)
…. (A)
In LRFD For Dead Load Case:
1.4D = 0.90 Rn  D = Rn (LRFD)
…. (B)

45. Comparison of ASD and LRFD Design Approaches
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
LRFD
ASD
Live Load
Dead Load 3%

46. AREA Code for Design of Railway Structures
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

47. AASHTO Code for Design of Highway Bridges
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

48. 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

49. 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.

50. 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

51. 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

52. 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.

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

54. 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

55. 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.

56. Design Process
2. Structural scheme

57. Design Process
2. Structural scheme (Contd.)

58. Design Process 3. Preliminary Member Sizing of Beams
Deflection Considerations
ASD Commentary L3.1 suggests following Limits:
For fully stressed Beams & Girders
For Beams & Girders subject to
vibrations
For Roof Purlins
CE-411:Lecture No. 1
Prof. Dr. Akhtar Naeem Khan

59. Design Process 3. Preliminary Member Sizing of Beams
Strength/Capacity Considerations
Unbraced Length
Design Moment
Tributary Area
Beam
CE-411:Lecture No. 1
Prof. Dr. Akhtar Naeem Khan

60. Design Process 3. 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

61. Tributary Area

62. Design Process
4. Structural Analysis - Modeling

63. Design Process
4. Structural Analysis - Analysis

64. 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.

65. 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

66. Design Process
7. Preparation of Structural Drawings & Specifications

67. Thanks