Design Seismic Forces Calculation: Response Spectrum Analysis Superior to Equivalent Static Force Method

The design seismic forces in the previous code (Equivalent Static Force Method) were specified in terms of story shear forces as a function of building period and soil conditions without the apparent definition of earthquake ground motions. Therefore, the previous design seismic forces were easily applied to the seismic design. However, they become inconsistent. 

The estimated earthquake ground motions were not equal among the different soil conditions, since the previous design seismic forces were specified as the response values of representative buildings. It is also difficult to apply the design seismic forces to new structural systems and construction techniques, such as seismic isolation and structural-control buildings, and to take into account the seismic behavior of surface soil deposits. 

Considering above inconsistency, it was concluded that the seismic design should start with defining the input earthquake ground motions. This methodology coincides with the performance-based structural engineering framework, which aspires toward flexible design. Consequently, new seismic design procedures including the design earthquake response spectrum have been introduced to replace previous procedures.