ground). To approximate this dynamic behavior, a committee of the American Society of Civil Engineers and the Structural Engineers Association of Northern California proposed in 1952 that the lateral force requirement be revised to vary inversely with the building’s fundamental period of vibration (C ~ T–1). With only a handful of strong-motion recordings available at the time, the decrease in the response spectral accelerations with period remained uncertain. Particular attention was focused on the band from 0.5 to 5.0 seconds, which includes the fundamental periods of vibration for most midrise to high-rise buildings as well as many other large structures.

The lateral force coefficient was recast in the 1961 UBC with a weaker (inverse cubed root) dependence on the response period: C ~ ZKT–1/3. This version introduced a seismic zone factor Z that represented the variability of the seismic hazard throughout the United States and a structural factor K that depended on building type and accounted for its dynamic response. The parameters were chosen to reproduce as well as possible the response spectral accelerations measured in previous earthquakes, which were still sparse. The uncertainties in the empirical coefficients remained high, but the form of the lateral force requirement did establish a firm connection between strong-motion measurements and the requirements of earthquake engineering.

The dearth of strong-motion data ended when the San Fernando earthquake (M 6.6) struck the Los Angeles region on February 9, 1971. It subjected a community of more than 400,000 people to ground accelerations greater than 20 percent of gravity and triggered in excess of 200 strong-motion recorders, more than doubling the size of the database. San Fernando provided the first well-resolved picture of the temporal and spatial variability of ground shaking during an earthquake (170). Short-period (0.1-second) accelerations varied widely, even among nearby sites with similar geologic conditions, while long-period (10-second) displacements were coherent over tens of kilometers (171). More important, this earthquake demonstrated that the ground motions could substantially exceed the maximum values observed in previous events. A strong-motion instrument on an abutment of the Pacoima Dam, 3 kilometers above the fault plane, recorded a sharp, high-amplitude (100-centimeter-per-second) velocity pulse in the first three seconds of the earthquake, as the rupture front passed under the dam (Figure 2.18). Four seconds later, after the rupture had broken the surface 5 kilometers away in the San Fernando Valley, the Pacoima instrument recorded an acceleration pulse exceeding 1.2 gravity in the horizontal plane. This value more than doubled the highest previously observed peak ground acceleration (PGA), measured during the 1966 Parkfield earthquake (M 5.5) on the San Andreas fault (172). The short acceleration pulse observed at Pacoima Dam

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