Chapter 18Design and Detailing for Earthquake Loads

18.1 General

Steel frames shall be so designed and detailed as to give them adequate strength, stability and ductility to resist severe earthquakes in all zones classified in 5 of Part 6 'Structural Design, Section 1 Loads, Forces and Effects' of the Code without collapse. Ductility requirements in 18.4 to 18.11 and 18.13 need not be satisfied in case of non-ductile frames as defined in 18.12. Frames, which form a part of the gravity load resisting system but are not intended to resist the lateral earthquake loads, need not satisfy the requirements of this section, provided they can accommodate the resulting deformation without premature failure.

18.2.1 Earthquake loads shall be calculated as per 5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code, except that the reduction factors recommended in 18.3 may be used.

18.2.2 In the limit state design of frames resisting earthquake loads, the load combinations shall conform to Table 4.

18.2.3 In addition the following load combination shall be considered as required in 18.5.1.1, 18.7.3.1 and 18.11.2.2:

a) 1.2 Dead Load (DL)+ 0.5 Imposed Load (IL) ± 2.5 Earthquake Load (EL); and

b) 0.9 Dead Load (DL) ± 2.5 Earthquake Load (EL).

NOTE — The above combinations need to be considered for only the design axial forces of members, ignoring simultaneous bending moments.

For structures designed and detailed as per the provision of this section, the response reduction factors specified in Table 23 may be used in conjunction with the provision in 5 of Part 6 ‘Structural Design, Section 1 Loads, Forces and Effects’ of the Code for calculating the design earthquake forces.

The braced and moment resisting frames can be designed as non-ductile frames, if their overall height to overall width ratio does not exceed 1.0, such as in industrial buildings. Such frames need not meet the ductility requirements of other types of frames as specified in 18.4 to 18.11 and 18.13.