Building periods for use in earthquake resistant design codes -- earthquake response data compilation and analysis of time and amplitude variations, 2004

Most seismic building codes estimate the design forces in structures based on the seismic coefficient C(T), where T is the "fundamental vibration period of the building." For structures on flexible soil, the relative response is the largest at the first period of the soil-structure system, which should be substituted in the code equation. This period depends not only on the structure itself, but also on the properties of the foundation system, of the surrounding soil, and on the contact conditions between the foundation and the soil. Studies for selected buildings have shown that this period can vary significantly during earthquake shaking as function of the level of shaking, reflecting changes in stiffness of the structure and of the soil (permanent or temporary), and changes in the bonding between the foundation and the soil, and can be very different from the estimates using ambient vibration data. For further refinement of the existing and development of new design code procedures, it is important to understand these changes and estimate their range during strong earthquake shaking, which is done best by analysis of actual earthquake response data for a large number of buildings.; This report summarizes the results of a one year project, which involved compilation of new and gathering and analysis of existing processed strong motion data of building responses in the Los Angeles area, particularly of buildings that recorded the 1994 Northridge earthquake and aftershocks, with the objective to estimate the variation of the first system frequency as a function of the level of shaking and time. Results are shown for 7 buildings for which strong motion data has been archived by U.S. Geological Survey (USGS), and which have been instrumented by USGS or by the building owner. The "instantaneous" system frequency and amplitude of response were estimated by two methods: zero crossing analysis, and from the ridges and skeletons of the Gabor transform. In general, the trend indicated by these data is decrease during the earthquakes that caused the largest levels of response (1994 Northridge main event, 1971 San Fernando earthquake), and "recovery" during the shaking from the aftershocks. For one of the buildings, a significant change that occurred during the San Fernando earthquake (30% reduction) appears to have been permanent. For most buildings, the frequency changed up to 20%, but for two buildings, the change was about 30%. A permanent reduction of the frequency is consistent with permanent loss of stiffness, while a "recovery" to the initial or higher value is consistent with the interpretation that the change was mainly due to changes in the soil (rather than in the structure itself), or changes in the bond between the soil and the foundation. Other causes of the temporary changes include contribution of the nonstructural elements to the total stiffness resisting the seismic forces, and opening of existing cracks in the concrete structures. The degree to which each of these causes contributed to the temporary changes cannot be determined from the current instrumentation, but fortunately, what matters for the building codes is the overall effect.