Steel Structures CE-409 By: Prof Dr. Akhtar Naeem Khan email@example.com
Course Content • Design philosophies • Introduction to Steel Structures • Design of Welded connections • Design of Bolted connections • Design of Tension Members • Design of Compression Members
Course Content • Design of Column Bases • Design of Beams • Design of Composite Beams • Design of Plate Girders
Lecture 01: Design Philosophies By: Prof Dr. Akhtar Naeem Khan firstname.lastname@example.org
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 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
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.
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.”
Limit States • There are three broad classification of limit states: • Strength limit states • Serviceability limit states • Special limit states
Limit States • Strength Limit States: • Flexure • Torsion • Shear • Fatigue • Settlement • Bearing
Limit States • Serviceability Limit States: • Cracking • Excessive Deflection • Buckling • Stability
Limit States Special Limit States: • Damage or collapse in extreme earthquakes. • Structural effects of fire, explosions, or vehicular collisions.
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.
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
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 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. Design Considerations
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.
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. 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). Design Considerations
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. Allowable Stress Design (ASD)
This is equivalent to: FOS = Resistance, R/ Effect of load, Q = fy/0.5fy = 2 Allowable Stress Design (ASD)
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.)
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 Prof. Dr. Akhtar Naeem Khan
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 Prof. Dr. Akhtar Naeem Khan
Section Modulus • SectionModulus: S ≥ effect of load/Allowable stress = M/fb ------(ii)
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
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
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
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.
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
The role of ‘n’ Ductility: It implies a large capacity for inelastic deformation without rupture • Ductility will ensure • redistribution of load through • inelastic deformation.
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. The role of ‘n’
Redundancy Yielding will initiate at mid span due to maximum moment at mid span with no Redistribution of load
Redundancy Yielding will initiate at supports due to maximum moment at supports
Redundancy Redistribution of load to mid span after yielding of section at supports
OperationalImportance: A hospital and a school require more conservative design than an ordinary residential building. The role of ‘n’
→ hospital Operational Importance → park
LRFD Advantages • LRFD accounts for both variability in resistance and load. • It achieves fairly uniform levels of safety for different limit states.
LRFD Disadvantages • It’s disadvantage is change in design philosophy from previous method.
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
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? Comparison of ASD and LRFD Design Approaches
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
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 Comparison of ASD and LRFD Design Approaches
Comparison of ASD and LRFD Design Approaches 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) …. (A) In LRFD For Dead Load Case: 1.4D = 0.90 Rn 1.56D = Rn (LRFD) …. (B)
3% 1.0 0.93 1.2D + 1.6L 0.9 LRFD ASD 0.83 0.8 1.4D 0.7 Live Load Dead Load 4 5 3 6 2 1 0.12 Comparison of ASD and LRFD Design Approaches
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
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
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
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 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