Question
In some cases, the bottom shear force from the response spectrum analysis is excessively larger than the bottom shear force from the static seismic load, why is this?
Answer
There can be several reasons for this, depending on the information input into the model.
Typically, if the response modification factor is reflected in the static seismic load, but the response modification factor is not reflected in the response spectrum analysis, the base shear force caused by the response spectrum may be larger than the base shear force caused by the static seismic load by about the value of the response modification factor.
The response correction factor should also be taken into account when interpreting the response spectrum.
There are two ways to reflect the response modification factor.
1. Use Scale Factor in Response Spectrum Function
2. Use Scale Factor in Response Spectrum Load Cases
For example, if the response modification factor is 4, you would enter 1/4, or 0.25, for Scale Factor.