12.6 ANALYSIS PROCEDURE SELECTION
The structural analysis required by Chapter 12 shall consist of one of the types permitted in Table 12.6-1, based on the structure’s Seismic Design Category, structural system, dynamic properties, and regularity. The analysis procedure selected shall be completed in accordance with the requirements of the corresponding section referenced in Table 12.6-1.
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The four basic procedures provided in Table 12.6-1 are the equivalent lateral force (ELF, Section 12.8), the modal response spectrum(MRSanalysis, Section 12.9.1), the linear response history (LRH, Section 12.9.2), and the nonlinear response history (NRH, Section 16.1) analysis procedures. Nonlinear static pushover analysis is not provided as an “approved” analysis procedure in the standard.
The use of the ELF procedure is limited to buildings with the listed structural irregularities because the procedure is based on an assumption of a gradually varying distribution of mass and stiffness along the height and negligible torsional response. The basis for the 3.5Ts limitation is that the higher modes become more dominant in taller buildings (Lopez and Cruz 1996, Chopra 2007a,b), and as a result, the ELF procedure may underestimate the seismic base shear and may not correctly predict the vertical distribution of seismic forces in taller buildings.
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