and building response collected from this urban event. The wealth of strong-motion data also prompted a 1976 revision to the UBC, which modified the period scaling in the lateral force equation from T–1/3 to T–1/2 and introduced a factor S based on local soil type. The newly formed Applied Technology Council (ATC) organized, with funding from the National Science Foundation (NSF) and the National Bureau of Standards, a national effort to develop a model seismic code. More than 100 professionals who volunteered for the work were organized into 22 committees. In a comprehensive report published in 1978 (175), the ATC proposed a more physically based lateral force coefficient of the form C ~ AvSR–1T–2/3, where Av is the effective peak ground velocity-related acceleration coefficient, S is a site-dependent soil factor, and R is a “response modification factor” dependent on the structure type. At shorter periods, this expression was replaced by a limiting value proportional to the effective peak acceleration coefficient Aa. The report also provides the first contoured maps of the ground-motion parameters Aa and Av, derived from a probabilistic seismic hazard analysis conducted by the USGS.

Strong-motion data from a number of earthquakes, as well as laboratory test data and results of numerical site response models, demonstrated the need to modify the soil factor S to reflect nonlinear site response. The National Center for Earthquake Engineering Research, which NSF established in 1986, led the revision, recommending two sets of amplitude-dependent, site amplification factors derived for six site geology classifications. The factors were first incorporated in the 1994 National Earthquake Hazard Reduction Program (NEHRP) seismic provisions and then into the 1997 UBC.

Strong-motion data from the 1994 Northridge, California (M 6.7), and 1995 Kobe, Japan (M 6.9), earthquakes confirmed observations from several previous earthquakes that motions recorded close to the fault rupture had distinct pulse-like characteristics, which were not represented in the code’s lateral force equation. The effect of these pulse motions was approximated by introducing near-fault factors into the lateral force equation. A single factor N was first introduced in the base isolation section of the 1994 UBC. This representation was replaced by two near-fault factors Na and Nv in the lateral force provisions of the 1997 UBC lateral force requirement. The Na factor was applied to the short-period, constant-acceleration portion of the design response spectrum, whereas the Nv factor was applied to the intermediate- and long-period constant-velocity portion, where the base shear is proportional to T–1. Interestingly, after a 36-year absence, the T–1 proportionality was reintroduced in the 1997 UBC in part because it was judged to be a more accurate representation of the spectral character of earthquake ground motion (176).



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