Seismic Base Shear (ASCE 7 §12.8 Equivalent Lateral Force)
The earthquake demand on a regular building reduced to one equivalent static base shear: Cs = SDS / (R / Ie), capped at SD1 / (T x (R / Ie)) for T <= TL and floored at the code minimum, times the seismic weight. In much of the western US the seismic demand, not the wind, governs the lateral system - the shear walls, braced frames, hold-downs, and anchor bolts. A taller, more flexible building draws a smaller base shear because the period cap recognizes it rides the short-period spectral peak less. SDS / SD1 are from the USGS maps; R is from Table 12.2-1. A lateral-demand estimate, not a stamped seismic design.
Formula and source
cs_basic = sds / (R / Ie); cs_cap = sd1 / (period_s x (R / Ie)); cs_min = max(0.044 x sds x Ie, 0.01); cs = max(cs_min, min(cs_basic, cs_cap)); base_shear = cs x weight.
ASCE 7 §12.8 equivalent lateral force (Cs = SDS / (R / Ie), the cap Cs <= SD1 / (T x (R / Ie)) for T <= TL, the minimum Cs >= max(0.044 x SDS x Ie, 0.01), V = Cs x W), by name.
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