How ASCE 7 equivalent lateral force base shear works
ASCE 7 Section 12.8 gives base shear as V = Cs x W, where W is the effective seismic weight, dead load plus the live load portions the standard requires such as storage and permanent equipment. The seismic response coefficient starts at Cs = SDS / (R / Ie). An upper limit of SD1 / (T x R / Ie) keeps long period buildings from being designed for short period acceleration, and a floor of 0.044 x SDS x Ie, never less than 0.01, keeps the force from vanishing.
The approximate fundamental period is Ta = Ct x hn raised to the power x, with height in feet. Table 12.8-2 gives 0.028 and 0.8 for steel moment frames, 0.016 and 0.9 for concrete moment frames, 0.03 and 0.75 for eccentrically braced and buckling restrained braced frames, and 0.02 and 0.75 for all other systems. A period from analysis may be used only up to Cu times Ta, so using Ta here is the conservative choice.
SDS and SD1 are site specific and cannot be hard coded. Get them from the USGS Seismic Design Web Service or ATC Hazards by Location using the project latitude, longitude, risk category and site class. Buildings with plan or vertical irregularities, or above the height limits in Table 12.6-1, may not be permitted to use the equivalent lateral force procedure at all. Treat this as a preliminary check and have a licensed structural engineer confirm the design.
Frequently Asked Questions
The base equation would design a tall flexible building for short period acceleration, which is far more force than it actually attracts. The S_D1 limit follows the falling branch of the design spectrum and prevents that overdesign.
A larger R lowers the design force but demands far stricter ductile detailing, connection qualification and system limitations. You cannot pick R freely; it comes with the seismic detailing requirements attached to that system.