New Site Coefficients and Site Classification System Used in Recent Building Seismic Code Provisions

Recent code provisions for buildings and other structures (1994 and 1997 NEHRP Provisions, 1997 UBC ) have adopted new site amplification factors and a new procedure for site classification. Two amplitude‐dependent site amplification factors are specified: F a for short periods and F v for longer periods. Previous codes included only a long period factor S and did not provide for a short period amplification factor. The new site classification system is based on definitions of five site classes in terms of a representative average shear wave velocity to a depth of 30 m (V¯ s ). This definition permits sites to be classified unambiguously. When the shear wave velocity is not available, other soil properties such as standard penetration resistance or undrained shear strength can be used. The new site classes denoted by letters A ‐ E, replace site classes in previous codes denoted by S1 ‐ S4. Site classes A and B correspond to hard rock and rock, Site Class C corresponds to soft rock and very stiff / very dense soil, and Site Classes D and E correspond to stiff soil and soft soil. A sixth site class, F, is defined for soils requiring site‐specific evaluations. Both F a and F v are functions of the site class, and also of the level of seismic hazard on rock, defined by parameters such as A a and A v ( 1994 NEHRP Provisions ), S s and S l ( 1997 NEHRP Provisions ) or Z ( 1997 UBC ). The values of F a and F v decrease as the seismic hazard on rock increases due to soil nonlinearity. The greatest impact of the new factors F a and F v as compared with the old S factors occurs in areas of low‐to‐medium seismic hazard. This paper summarizes the new site provisions, explains the basis for them, and discusses ongoing studies of site amplification in recent earthquakes that may influence future code developments.