How to specify response spectrum scale?
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 either ASCE 7-05 Section 11.4.5 or 21.2 divided by the quantity R/I. The value for displacement and drift quantities shall be multiplied by the quantity Cd/I (ASCE 7-05 Section 12.9.2).
Therefore, response spectrum scale factor shall be g*I/R where g is acceleration due to gravity (use 386.4 in/sec2 for models in kips-inch units and 9.81 m/sec2 for models in kN-m units). After analysis is performed, review the Response Spectrum Base Reaction Table and refer to the Base shear due to all modes. If reported dynamic base shear is more than 85% of the static Base shear then no further action is required. However, when dynamic base shear is less than 85% of static base shear then readjust the scale factor to match the response spectrum base shear equal to 85% of static base shear
. Use this readjusted scale factor in Response Spectrum and rerun the 2nd analysis.
