How
...
is the response-spectrum scale specified?
Answer: 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 CdCd / I (ASCE 7-05, Section 12.9.2).
Therefore, the response-spectrum scale factor shall be is I g *I / R, where g is acceleration due to gravity (use 386.4 in/sec2 for models in kips-inch units kip-in 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 users should review the base shear due to all modes, reported in the Response Spectrum Base Reaction Table. If reported the dynamic base shear reported is more than 85% of the static base shear then , no further action is required. However, when if dynamic base shear is less than 85% of the static base shear, then readjust the scale factor to match should be adjusted such that the response-spectrum base shear equal to matches 85% of the static base shear (i.e., . In this case, the new scale factor = would be (I g *I / R) * (0.85 * static base shear / response-spectrum base shear). Use this readjusted Analysis should then be rerun with this scale factor specified in the response spectrum case and rerun the 2nd analysis-spectrum case.
References
- American Society of Civil Engineers. ASCE 7-05, Minimum Design Loads for Buildings and Other Structures. Reston, VA: ASCE Publications, 2006. Print.