The STAAD.Pro seismic load generator follows the procedure of equivalent lateral load analysis explained in UBC, IBC and several other codes.

It is assumed that the lateral loads will be exerted in X and Z (or X and Y if Z is up) directions (horizontal) and Y (or Z if Z is up) will be the direction of the gravity loads. Thus, for a building model, Y (or Z if Z is up) axis will be perpendicular to the floors and point upward (all Y (or Z if Z is up) joint coordinates positive). The engineer is required to set up the model accordingly. Total lateral seismic force or base shear is automatically calculated by STAAD.Pro using the appropriate equation from the code. IBC 2003, IBC 2000, UBC 1997, 1994, or 1985, IS:1893, Japanese, Colombian, and other specifications may be used.

For load generation per the codes, the user is required to provide seismic zone coefficients, importance factors, soil characteristic parameters, etc. See TR.31.2 Definitions for Static Force Procedures for Seismic Analysis for the detailed input required for each code.

Instead of using the approximate code based formulas to estimate the building period in a certain direction, the program calculates the period using Rayleigh quotient technique. This period is then utilized to calculate seismic coefficient, C.

After the base shear is calculated from the appropriate equation, it is distributed on the various levels above the support level. These levels are up to and including the roof per the code specifications. The load is applied as lateral loads on the nodes at each level in the specified axis. The load case may then be utilized as normal load cases for analysis and design.