# Designing for Earthquakes

The IBC establishes the minimum lateral seismic design forces for which buildings must be designed primarily by reference to ASCE 7. While ASCE 7 allows use of a number of analysis procedures, the equivalent lateral force (ELF) procedure is most commonly used for seismic design of buildings in the U.S. This is particularly true for low-rise, short-period, wood-frame buildings. The ELF procedure relies on seismic force-resisting system design coefficients such as the response modification coefficient, R (often referred to as the R-factor), deflection amplification factor, Cd, and overstrength factor, Ωo. The R-factor is essential for determining design seismic base shear, V, which is used in the design of elements of the seismic force-resisting system. For short-period, wood-frame structures, seismic base shear, V, is calculated in accordance with Equation 1.

Design seismic base shear is proportional to effective seismic weight, W, the seismic hazard at the site represented by the spectral response acceleration parameter, SDS, response modification coefficient, R, and the importance factor, Ie. Since the R-factor is found in the denominator of the seismic base shear equation, as the R-factor increases for systems being considered, the seismic base shear forces decrease. For wood-frame buildings, values of the R-factor cover a wide range from R=1.5 to R=7.0 depending on the type of wood-frame seismic force-resisting system. (See Table 1.)