## Question

How is the Story Shear calculated in Response Spectrum Analysis?

## Answer

For response spectrum, the Story force obtained for each mode(from Eigenvalue Analysis) is combined using suitable modal combination method.

Shear Force represents the sum of shear forces of all the upper stories as well as the corresponding story, and Story Force represents the shear force acting on the corresponding story. For response spectrum case is obtained from combination of each mode using modal combination method.

The story inertia forces and story shear forces from the response spectrum analysis could be obtained in format of a table.

From the main menu select Result > Table > Result Tables > Story > Story Shear ( Response Spectrum analysis)

After analysis, users should review the base shear due to all modes, reported in the Response Spectrum Base Reaction Table. If the dynamic base shear reported is more than certain percentage of the static base shear depending on the code provision, say for instance 85% as per UBC, no further action is required. However, if dynamic base shear is less than 85% of the static base shear, then the scale factor should be adjusted such that the response-spectrum base shear matches 85% of the static base shear.

As per UBC, The response spectrum scale factor is I / R. Hence for case where the above condition is not met, the new scale factor would be (I g / R) * (0.85 * static base shear / response-spectrum base shear). Analysis should then be re-run with this scale factor specified in the response-spectrum case. The Response spectrum scale factor in GEN could specified from the main menu select Model > Load > Load Type > Dynamic Load > RS load case

The value for each force-related design parameter of interest, including story drifts, support forces, and individual member forces for each mode of response, shall be computed using the properties of each mode and the response spectra defined in specified code.