Design of Steel Structure
General Design Requirements
DESIGN OBJECTIVE
The objective of design is the achievement of an acceptable probability that structures will perform satisfactorily for the intended purpose during the design life. With an appropriate degree of safety, they should sustain all the loads and deformations during construction and use and have adequate resistance to accidental loads and fire.
METHODS OF DESIGN
Structure and its elements shall normally be designed by limit state method as per IS 800-2007. Where the limit state method cannot be conveniently adopted; the working stress method shall be used.
LOADS AND FORCES
For the purpose of designing any element, member, or a structure, the following loads and their effects shall be taken into account, where applicable, with partial safety factors and combinations :
(a) Dead loads
(b) Imposed loads; (Live load, crane load, snow load, etc.)
c) Wind loads
(d)Earthquake loads
(e) Accidental loads such as those due to blast
(f) Erection loads
(g) Secondary effects due to contraction or expansion resulting from temperature changes, differential settlements of the structure as a whole or of its components, eccentric connections.
LOAD COMBINATIONS
The following load combinations with appropriate load factors may be considered in designing
(a) Dead load + Imposed load
(b) Dead load + Imposed load + Wind or Earthquake load
(c) Dead load + Wind or Earthquake load
(d) Dead load + Erection load
Note: Wind load and earthquake loads shall not be assumed to act simultaneously.
GEOMETRICAL PROPERTIES
IS 800-2007 gives the concept of the gross and effective cross-sections of a member.
- The properties of the gross cross-section shall be calculated from the specified size of the member or read from the appropriate table.
- The effective cross-section of a member is that portion of the gross cross-section that is effective in resisting the stresses.
IS 800-2007 has classified the members' cross-section as follows.
CLASSIFICATION OF CROSS SECTIONS
Basis of Classification
- The plate elements of a cross-section may buckle locally due to compressive stresses.
- When the plastic analysis is used, the members shall be capable of forming plastic hinges with sufficient rotation capacity (ductility) without local buckling to enable the redistribution of bending moment required before the formation of the failure mechanism.
- When the elastic analysis is used, the member shall be capable of developing the yield stress under compression without local buckling.
On the above basis, four classes of sections are defined as follows:
(a) Plastic: Cross-sections, which can develop plastic hinges and have the rotation capacity required for the failure of the structure by the formation of a plastic mechanism.
(b) Compact: Cross-sections, which can develop the plastic moment of resistance, but have inadequate plastic hinge rotation capacity for the formation of a plastic mechanism.
(c) Semi-compact: Cross-sections, in which the extreme fibre in compression can reach yield stress, but cannot develop the plastic moment of resistance, due to local buckling,
(d) Slender: Cross-sections in which the elements buckle locally even before reaching yield stress.