EARTHQUAKE RECORD INTERPRETATION FOR SOIL-STRUCTURE INTERACTIONS EFFECTS OF TWO INSTRUMENTED BUILDINGS

Kinematic soil-structure interaction (SSI) results from the incoherent nature of ground motions, which causes the motions of foundation slabs to generally be reduced from those in the free-field at high frequencies. We compare two models of kinematic SSI utilized in engineering practice: one based on finite element analysis and the second based on a semi-empirical approach. Predictions from both approaches are compared for two well-instrumented structures to observed transfer functions. The results show (1) the two models produce very similar transfer function estimates and (2) the model predictions are generally consistent with observations. Introduction It has long been recognized that foundation-level and free-field seismic ground motions have different characteristics as a result of the spatial variations in ground motions, even on sites with relatively uniform site conditions (e.g.,Yamahara, 1970; Scanlan, 1976; Newmark et al., 1977). The spatial variation of ground motions in the horizontal plane causes foundation motions to be reduced in amplitude relative to free-field motions, an effect termed “base slab averaging”. Likewise, the spatial variation of ground motions vertically leads to an “embedment effect”. Both base slab averaging and embedment are kinematic soil-structure interaction (SSI) effects that influence foundation-level ground motions. Two types of models have been independently developed over the years to predict kinematic SSI effects. The first is implemented in the computer program SASSI (a System for Analysis of Soil-Structure Interaction; Lysmer et al., 1981, 1999; Ostadan, 2005). SASSI computes the relative motion of the foundation to the free-field (input motion) in terms of a transfer function expressing the frequency-dependent ratio of foundation motion amplitude to free-field motion amplitude. Recent versions of SASSI (Ostadan, 2005; Ostadan and Dang, 2007) allow the free-field motion to have a specified level of coherency (producing spatial variation in phase angles). The second approach for prediction of transfer functions from base slab averaging is based on a theoretical solution by Veletsos and co-workers (1989, 1997) coupled with empirical calibration of an incoherence parameter by Kim and Stewart (2003). The Kim and Stewart approach includes both base slab averaging and embedment effects. SMIP12 Seminar Proceedings 20 In this paper we compare predictions from the SASSI and semi-empirical models for the conditions present at two instrumented buildings: the Factor building in Los Angeles, California and the Atwood building in Anchorage, Alaska. Both buildings are well suited to these analyses for several reasons: i. The buildings have shallow foundations (mat, footing and grade beam) appropriate for use with the kinematic SSI models; ii. The buildings consist of moment-resisting steel frames that are not conducive to significant inertial SSI effects, thus simplifying data interpretation for kinematic effects; iii. The buildings are well instrumented, including foundation sensors and nearby reference arrays with both surface and downhole high resolution sensors; and iv. Data have been recorded during multiple events, allowing investigation of event-to-event variability. A limitation of the data is that strong shaking has not been recorded. Hence, the structures' responses are essentially in their elastic range; the noise levels are also high, occasionally leading to low signal-to-noise ratios. Our motivation for these comparisons is to (1) investigate the degree to which the results from the two kinematic SSI models differ from each other and from data and (2) to add new data points to the empirical data set available for the calibration of the semi-empirical model. Site and Building Descriptions Factor Building The Doris and Louis Factor Health Science building is a 17-story special momentresisting steel frame structure located on the University of California, Los Angeles campus (UCLA). As shown in Figure 1, the building has 15 stories above ground and two basement levels. The Factor building was constructed in the 1970s and was designed under the 1973 building code. It has a footprint of approximately 22.4 m by 38.6 m (73.5 ft by 126.5 ft) from the top basement floor to the 9th floor inclusively. The lower basement (level B) has a smaller footprint of 22.4 m by 29.7 m (73.5 by 97.5 ft). A diagonally-braced overhang at floors 10-15 increases the East-West dimension by 7.4 m (24.2 ft) on the west side and by 3.2 m (10.5 ft) on the east side. The ground surface has a gradient uphill towards the north side, making the first floor at the ground surface on the south side and the second floor at the ground surface on the north side. The building rests on shallow foundations. Figure 1 shows the underlying soil conditions, which consist of Pleistocene alluvium ranging from sands to clays having Vs  400600 m/s beneath the foundation. The Factor building was instrumented following the 1994 Northridge earthquake with a 72-channel sensor array configured as shown in Figure 1. The array includes vertical sensors on the foundation that enable evaluation of rocking in the east-west direction. A 100 m deep vertical array configured with triaxial accelerometers at 100m depth and at the ground surface is located 25 m from the edge of the building in a botanical garden. Additional details on the building and borehole arrays are provided in Goulet et al. (2011). SMIP12 Seminar Proceedings 21 Figure 1. Schematic of the Factor building, showing the sensor array and the subsurface soil profile. Arrows indicate sensor orientation with empty circles pointing into the page and filled circles pointing out of the page. The depths shown on the velocity profile are presented in Table 1. Soil profile information after LeRoy Crandall and Associates, 1976.