A set of reinforced concrete structures with gravitational loads and mechanical properties (strength and stiffness) representative of systems designed for earthquake resistance in accordance with current criteria and methods is selected to study the influence of dynamic soil-structure interaction on seismic response, ductility demands and reliability levels. The buildings are considered located at soft soil sites in the Valley of Mexico and subjected to ground motion time histories simulated in accordance with characteristic parameters of the maximum probable earthquake likely to occur during the system's expected life. For the near-resonance condition the effects of soil-structure interaction on the ductility demands depend mainly on radiation damping. According to the geometry of the structures studied this damping is strongly correlated with the aspect ratio, obtained by dividing the building height by its width. In this way, for structures with aspect ratio greater than 1.4 the storey and global ductility demands increase with respect to those obtained with the same structures but on rigid base, while for structures with aspect ratio less than 1.4 the ductility demands decrease with respect to those for the structures on rigid base. For the cases when the fundamental period of the structure has values very different from the dominant ground period, soil-structure interaction leads in all cases to a reduction of the ductility demands, independently of the aspect ratio. The reliability index is obtained as a function of the base shear ratio and of the seismic intensity acting on the nonlinear systems subjected to the simulated motions. The resulting reliability functions are very similar for systems on rigid or on flexible foundation, provided that in the latter case the base rotation and the lateral displacement are removed from the total response of the system. Several groups of ductile reinforced concrete beam-column frame buildings are studied. They include six-, ten-, fourteen-and twenty-storey systems. There are two ten-storey systems, with two and three bays, respectively; three fourteen-storey systems, with two, three and five bays. All six-and twenty-storey systems have three bays in width. In all cases the bay width is equal to 7.0 m, centre to centre, and the storey height is equal to 3.5 m for the bottom storey and 3.0 m for all the others, except for the twenty-storey system, for which these heights are 0.2 m greater. Sketches of the building configurations are shown elsewhere [2]. The responses obtained for the three-bay systems are applied to the study of the variation of the influence of the DSSI with the period of the structure for the different soil conditions considered. The results obtained for the ten-and fourteen-storey systems, with different numbers of bays, are used to study the influence of the aspect ratio.Each system is designated by the number of stories (#S) and the number of bays (#B). (For instance, the ten-storey three-bay system is designated as 10S-3B). All systems are regular...

SUMMARYA criterion is developed for the simulation of realistic artiÿcial ground motion histories at soft-soil sites, corresponding to a detailed ground motion record at a reference ÿrm-ground site. A complex transfer function is deÿned as the Fourier transform of the ground acceleration time history at the soft-soil site divided by the Fourier transform of the acceleration record at the ÿrm-ground site. Working with both the real and the imaginary components of the transfer function, and not only with its modulus, serves to keep the statistical information about the wave phases (and, therefore, about the time variation of amplitudes and frequencies) in the algorithm used to generate the artiÿcial records. Samples of these transfer functions, associated with a given pair of soft-soil and ÿrm-ground sites, are empirically determined from the corresponding pairs of simultaneous records. Each function included in a sample is represented as the superposition of the transfer functions of the responses of a number of oscillators. This formulation is intended to account for the contributions of trains of waves following di erent patterns in the vicinity of both sites. The properties of the oscillators play the role of parameters of the transfer functions. They vary from one seismic event to another. Part of the variation is systematic, and can be explained in terms of the in uence of ground motion intensity on the e ective values of sti ness and damping of the artiÿcial oscillators. Another part has random nature; it re ects the random characteristics of the wave propagation patterns associated with the di erent events. The semiempirical model proposed recognizes both types of variation. The in uence of intensity is estimated by means of a conventional one-dimensional shear wave propagation model. This model is used to derive an intensity-dependent modiÿcation of the values of the empirically determined model parameters in those cases when the ÿrm-ground earthquake intensity used to determine these parameters di ers from that corresponding to the seismic event for which the simulated records are to be obtained.

SUMMARYA previously developed simpliÿed model of ground motion ampliÿcation is applied to the simulation of acceleration time histories at several soft-soil sites in the Valley of Mexico, on the basis of the corresponding records on ÿrm ground. The main objective is to assess the ability of the model to reproduce characteristics such as e ective duration, frequency content and instantaneous intensity. The model is based on the identiÿcation of a number of parameters that characterize the complex ÿrm-ground to soft-soil transfer function, and on the adjustment of these parameters in order to account for non-linear soil behavior. Once the adjusted model parameters are introduced, the statistical properties of the simulated and the recorded ground motions agree reasonably well. For the sites and for the seismic events considered in this study, it is concluded that non-linear soil behavior may have a signiÿcant e ect on the ampliÿcation of ground motion. The non-linear soil behavior signiÿcantly a ects the e ective ground motion duration for the components with the higher intensities, but it does not have any noticeable in uence on the lengthening of the dominant ground period.