EFFECT OF HIGH-RISE STRUCTURE CHARACTERISTICS ON THE ROCKING OF FOUDATIONS

Rocking of shallow foundation that is a ramification of soil structure interaction has been recently considered by many researchers and earthquake geotechnical performance-based design (PBD) codes. These researches signified the importance of this most unintentional phenomenon in amelioration seismic performance of structures. Elongation of the natural period of the soil-structure system, a decrease in both base shear and ductility demands of superstructures can be named as advantages of this rotational mode; however, unpredictable displacement of the foundation and residual settlement and/or tilting of the foundation concern engineers to allow the base separates from the contacted ground, therefore the ability of realistically modeling the rocking responses of the soil structure interaction is a necessity in this respect. In addition, the responses of soil-structure interaction like settlement, rotation, mobilized moment and dissipated energy and correlation between these results, are completely intricated, e.g. the nonlinearity of rotation-moment relationship is dependent straightly on several factors including the amount of rotation of foundation, the nonlinear behavior of underlying soil, the static factor of safety of foundation (Gaztes et al., 2013), as well, there are some parameters such as the height and the width of structure, in the same vibration, indirectly lead to some significant alterations in the rotation-moment relationship and the other responses of rocking. Therefore, this investigation attempts to study the effects of structure dimensions (height and width) on the rocking of the foundation. Rectangular blocks as superstructure subjected to dynamic loads have been simulated by finite element analysis using ABAQUS. To induce the structure to shake and the foundation to rock, horizontal displacement shown in Figure 1-a is imposed on bedrock. Thisdisplacement time-history was selected in such a way not to led the blocks to slide and the structures vibrated in a controlled way by gradually increasing vibration amplitude. The recorded horizontal acceleration of one node of soil, which is the closest to bedrock is shown in Figure 1-b. Imposed displacement caused peak horizontal acceleration 0.6 g with predominant frequency of 1.75 Hz. For considering structure height, three structures with the same width of 20m, and 30m, 60m and 90m heights were selected. These structures, by supposing the height of each floor is 3m, respectively have 10, 20 and 30 stories. Therefore, the weight of each structure was determined by assuming 1 ton charge loads on per square meter of floors. To provide sufficient data, the supporting soil which the foundation of these structures was placed, assumed in four behavior models namely linear and nonlinear (elastic and elastoplastic). For validation of these numerical modeling, a centrifuge test namely SSG04-9b provided by Gajan et al. (2006) has been simulated. The obtained results were in good agreement with experimental results.

Keywords:

Rocking , high-rise building , nonlinear rotation moment relationship , soil-structure interaction