Seismic Soil-Structure Interaction in Buildings. I: Analytical Methods; Seismic Soil-Structure Interaction in Buildings. II: Empirical Findings

The two papers describe trends of the apparent system periods, , and damping factors, , in recorded response of 56 ˜˜ T z0 buildings (A1 to A45, and B1 to B14 in Table 1 of part II) to 15 California earthquakes. The building, foundation, and soil are viewed as an equivalent linear system, excited by horizontal ground motion. Only inertial interaction is considered as ‘‘the more important effect for foundations without large, rigid base slabs or deep embedment’’ (by allowing one translation and rocking of the foundation). The kinematic interaction (modification of the free-field motion by the foundation in absence of inertial forces) is neglected as ‘‘second order.’’ Rocking and torsional excitation components of the incident motion, differential motions of extended foundations, warping of flexible foundations, and nonlinear soil response due to soilstructure interaction (SSI) are not mentioned. The papers aim to verify the ‘‘simplified analytical procedures similar to those in the BSSC and ATC codes.’’ This discussion (1) requests some clarifications; (2) suggests that time-dependent changes in (caused by nonlinear response of soil during SSI) should ˜ T have been included in the analysis; and (3) comments on the conditions when kinematic SSI should not be neglected. In view of the bold simplifications adopted in parts I and II, one would expect at least fair to good agreement of the predicted trends with the data, as a proof (albeit qualitative and rough) that the assumptions are justified. However, as the authors admit, there is ‘‘significant scatter in the data.’’ Table 1 in part II lists the confidence (A-acceptable; L-low) characterizing the quality of the available geotechnical data and the accuracy of identification. An L was assigned 15% of the 44 cases with base acceleration 0.25g. Does the proposed simplified analysis break down for progressively larger strains in the soil, e.g., base accelerations >0.15g?