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Design of Steel Structures

Design of Steel Structures. Department of Civil Engineering University of Engineering & Technology, Taxila. Design of Steel Structures. Chapter – 1 Fundamentals of Steel Design. Basic Design Equation

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Design of Steel Structures

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  1. Design of Steel Structures Department of Civil Engineering University of Engineering & Technology, Taxila

  2. Design of Steel Structures Chapter – 1 Fundamentals of Steel Design

  3. Basic Design Equation • In design, the applied forces and moments due to external loads are equated to the maximum resistive forces and moments with a FOS which is always greater than or equal to one. • The concept may be summarized by the following design equation: Load Effects X Factor of safety (F.O.S) = Max. Internal Resistance offered by Material of the Structure

  4. Basic Design Equation • Load effects are defined as the forces, stresses and deformations produced in a structural component by the applied loads. • A simply supported beam of span L subjected to a point load P can be analyzed to get the maximum bending moment of PL/4.

  5. Basic Design Equation • However, this bending moment will only be produced if the material of the beam is strong enough to develop the required strength. • This means that the answer of analysis may be true for bigger steel girder but may not be true for small wooden batten.

  6. Factor of Safety • Factor of safety is required to bring the structure from the state of collapse to a usable state. It additionally covers the following aspects: • Uncertainties in applied forces or loads. • The deflections should be small at service load conditions.

  7. Factor of Safety • To cover uncertainties in material strength. • To cover, in part, poor workmanship. • To cover unexpected behavior in case the theory is not fully developed. • To cover natural disasters. • The stresses produced during fabrication and erection.

  8. Factor of Safety • Presence of residual stresses and stress concentrations. In case of allowable stress design, the factor of safety is applied in the form of safety factor (Ω), while in case of LRFD, it is applied in the form of overload factors and the resistance factor (Φ).

  9. In name only Nominal Strength • Nominal strength (Rn) is defined as the strength of a structure or component to resist load effects determined by using formulas given in the specifications.

  10. Types of Design • Load and Resistance Factor Design (LRFD), Strength Design or Limit State Design • Allowable Stress Design (ASD) • Plastic Design

  11. 1. Load & Resistance Factor Design (LRFD) • Major part of FOS is applied on load actions called overload factor. • Minor part of FOS is taken on RS of design equation called resistance factor or capacity reduction factor (ø). • Resistance factor (ø) is lesser than or equal to 1.0 and is applied on material strength.

  12. 1. Load & Resistance Factor Design (LRFD) • The design equation is checked for each strength and serviceability limit states one-by-one. • Limit state is defined as the limiting stage in the loading after which the structure cannot fulfill its intended function due to strength or serviceability considerations.

  13. 1. Load & Resistance Factor Design (LRFD) • Analysis of structures for loads is performed considering the structure to be within elastic range. • However, inelastic behavior, ultimate failure modes and redistribution of forces after elastic range are considered in this method. • This is more realistic design as compared with the old Allowable Stress Design.

  14. 1. Load & Resistance Factor Design (LRFD) • Nominal strength (Rn) is defined as the strength of the structure or its component determined by using formulas given in specifications. • Any particular load effect increased by the load factors is called the Required Strength (Ru).

  15. 1. Load & Resistance Factor Design (LRFD) • The nominal strength reduced by the resistance factor (ΦRn) is called the Design Strength. • The design equation in case of LRFD becomes: Ru≤ (ø)Rn

  16. Advantages of Using LRFD • LRFD is another tool for steel design, which provides a flexibility of options to the designer in selecting the design methodology. • Economical in case dead loads are larger, compared with live loads. • Every type of load may be given a different FOS depending upon its probability of overload, number of severe occurrences and changes in point of application.

  17. Advantages of Using LRFD • Behavior at collapse including ductility, warning before failure and strain hardening etc. This is not directly possible in ASD because here the structure is considered at service stage and not approaching close to collapse. • More safe structures result due to better awareness of behavior near collapse.

  18. Advantages of Using LRFD • Plastic design concepts may conveniently be employed in LRFD Method.

  19. Disadvantages of Using LRFD • Elastic behavior considered for load analysis and ultimate plastic behavior taken for material strengths are not compatible, however, percentage difference is less. • Engineers experienced in ASD have to become familiar with this technique. • Old books and design aids become ineffective.

  20. Disadvantages of Using LRFD • Validity of previous designs is still to be checked according to ASD.

  21. 2. Allowable Stress Design (ASD) • F.O.S is taken on right side of the basic design equation. This is denoted by Ω. • Allowable strength (Rn/Ω) is defined as the nominal strength divided by the safety factor. Material Resistive Forces FOS Loads Effects =

  22. 2. Allowable Stress Design (ASD) • Required ASD Strength (Ra) is the load effect obtained from the service loads without any additional factor. • The design equation for ASD becomes: • This method is now gradually replaced by LRFD for the structures, where behavior near collapse is fully understood. Ra≤ Rn/Ω

  23. 2. Allowable Stress Design (ASD) • It is still preferred by some engineers for important structures like atomic reactors and pre-stressed concrete. • It is included in the specifications as an alternate method of design.

  24. Advantages of ASD • Elastic analysis for loads and elastic material behavior compatible for the design. • Senior engineers are used to this method. • Old famous books are according to this method. • Was the only design method in past. • Is included as alternate design method in AISC-05 Specifications.

  25. Disadvantage of ASD • Latest research and literature is very much limited. • Same factor of safety is used for different loads. • The failure mode is not directly predicted. • With some overloading, the material stresses increases but do not go to collapse. (The failure mode cannot be observed).

  26. Disadvantage of ASD • The ductility and warning before failure cannot be studied precisely. • Results cannot be compared with experimental tests up to collapse.

  27. 3. Plastic Design • It is somewhat similar to the LRFD but here the analysis for loads is performed considering the collapse mechanism of the structure. • Full reserve strength due to indeterminacy of the structure and inner elastic portion of the structure is utilized.

  28. 3. Plastic Design • Inelastic material behavior is considered in the analysis and design. • Deflections and other serviceability conditions become more important along with the strength requirements.

  29. DESIGN STRENGTH • In LRFD, design strength of all elements is obtained as resistance factor multiplied with maximum stress that can be developed multiplied with sectional area or section modulus. • The design strength is also called the load capacity, or sometimes only capacity, of a member.

  30. DESIGN STRENGTH • An example to explain the difference between the member capacity and the applied load is that of a bottle. • This bottle may have a fixed liquid retaining capacity of suppose 1 litre. • However, it may be empty at times meaning that the amount of liquid retained in it is zero litres but the capacity of the bottle still remains the same.

  31. DESIGN STRENGTH • Any amount of liquid may be poured in this bottle that is not exceeding 1 litre. • Similarly, load capacity of a member exists with a fixed value. • The applied load may have a different value with only one condition that the applied load must be lesser than or equal to the member capacity for stability.

  32. CAPACITY ANALYSIS OF STRUCTURES • Knowing the material properties and dimensions of the member, finding the maximum loads that can be applied on the member using the design equation is called Capacity Analysis or Analysis of Structures.

  33. DESIGN OF STRUCTURES • Knowing the expected loads and span lengths of the members in the basic design equation, finding the required material properties and cross-sectional dimensions is called Design of Structures. • In steel structures, the design mainly consists of a selection out of already available sections in the market.

  34. DESIGN OF STRUCTURES • Structural Design may be defined as “a mixture of art and science, combining the experience and intuitive feeling for the behavior of the structure with a sound knowledge of the principles of statics, dynamics, mechanics of materials, and structural analysis, to produce a safe economical structure which will serve its intended purpose.”

  35. Objectives of Structural Designer • Design is a process by which an optimum solution is obtained satisfying certain criteria. • Minimum cost • Minimum weight • Minimum construction time • Minimum labour • Maximum efficiency of operation

  36. Objectives of Structural Designer • The structural designer must learn to arrange and proportion the parts of his structures so that they can be practically erected and will have sufficient strength and reasonable economy. • These important items, called safety, cost and practicability are discussed next:

  37. Objectives of Structural Designer • The structure must safely support the loads to which it is subjected. The deflections and vibrations should not be so excessive as to frighten the occupants. • The designer must keep the construction, operation and maintenance costs at the lowest levels without sacrificing the strength.

  38. Objectives of Structural Designer • Designers need to understand fabrication methods and should try to fit their work to the available fabrication facilities, available materials and the general construction practices. Some designers lack in this very important aspect and their designs cause problems during fabrication and erection.

  39. Objectives of Structural Designer • Designer should learn everything possible about the detailing, the fabrication and the field erection of steel besides the loads, mechanics, and the expected material strengths. • The designer must have information concerning the transportation of the materials to site, labor conditions, equipment for erection

  40. Objectives of Structural Designer problems at site, field tolerances and the required clearances at the site. • This knowledge helps to produce reasonable, practical and economical designs.

  41. Procedure of the Structural Design • The structural framework design is the selection of the arrangement and sizes of structural elements so that service loads may be safely carried. • Structural designer has to complete the following steps to get a successful design:

  42. Procedure of the Structural Design • The general layout of the structures. • Studies of the possible structural forms that can be used. • Consideration of loading conditions. • Analysis of stresses and deflections, etc. • Design of parts. • Design of assembly and connections. • Preparation of design drawings.

  43. The above design procedure for a whole structure requires iterations and the main steps are listed below: • The functions to be performed by the structure and the criteria for optimum solution of the resulting design must be established. This is referred to as the planning stage. • The general layout of the structure is decided.

  44. Different arrangements of various elements to serve the functions in step 1 are considered. The possible structural forms that can be used are studied and an arrangement appearing to be best is selected for the first trial, called preliminary structural configuration. Only in very rare cases, it has to be revised later on.

  45. Loading conditions are considered and the loads to be carried by the structure are estimated. • Based on the decisions of the earlier steps, trial selection of member sizesis carried out depending on thumb rules or assumed calculations to satisfy an objective criterion, such as least weight and cost.

  46. Structural analysisinvolving modeling the loads and the structural framework to obtain internal forces, stresses and deflections is carried out. • All strength and serviceability requirements along with the predetermined criteria for optimum are checked. If any check is not satisfied, the member sizes are revised. This stage is called evaluation of the trial member sizes.

  47. Repetition of any part of the above sequence found necessary or desirable as a result of evaluation is performed in this stage called redesign. • The rivets, bolts and welds along with other joining plates and elements are designed. The process is termed as the design of assembly and connections.

  48. It is determined whether or not an optimum design has been achieved, and the final decision is made. • Drawings are prepared to show all design details. An estimate for the required quantities is also made. This stage of design is called preparation of design documents.

  49. Procedure of the Structural Design • The important sub-steps in the design of parts (step 7 above) are shown in the form of a flow chart in Fig 1.1 • Objectives of the design must always be kept in mind while using this flow chart. • The selection of trial section in step 2 depends on the main objectives, availability of material, construction requirements and compatibility with other members.

  50. Collect and list all the known data Select trial section based on assumed stresses/ effectiveness of cross-sectional alternatively, selection tables may be used Apply all stability checks Perform strength checks Perform serviceability checks Accept section if all checks are satisfied, other-wise revise Write Final Selection

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