Response Spectrum Analysis as per IS1983 in Staad Pro. V8i

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Response Spectrum Specification as per the Indian IS:1893 (Part 1)-2002 Code for Dynamic Analysis
The design lateral shear force at each floor in each mode is computed by STAAD in accordance with the IS: 1893 (Part 1) -2002 equation and

Qik = Ak*Oik*Pk*Wi

STAAD utilizes the following procedure to generate the lateral seismic loads.

1.      User provides the value for  Z/2*I/R as factors for input spectrum
2.      Program calculates time periods for first six modes or as specified by the user.
3.      Program calculates Sa/g for each mode utilizing time period and damping for each mode.
4.      The program calculates design horizontal acceleration spectrum Ak for different modes.
5.      The program then calculates mode participation factor for different modes.
6.      The peak lateral seismic force at each floor in each mode is calculated.
7.      All response quantities for each mode are calculated.
8.      The peak response quantities are then combined as per method (CQC or SRSS or ABS or TEN or CSM) as defined by the user to get the final results.

Soft Storey Checking
For details on the Soft Storey Checking option, see the chapter detailed in Section Definition of Lateral Seismic Load per Indian IS:1893 (Part 1) – 2002 Code.