چكيده به لاتين
Due to the contribution of rafts in bearing capacity and the role of piles in the load transfering and reduction of settlements, connected piled raft (PR) foundation is known as an efficient system in seismic areas. Under dynamic loads, the super-structure on the piled raft system generates a large inertial force on the raft and high bending moment around pile heads. However, great stresses and cracks at pile heads are the main problem of this system performance. Therefore, the non-connected piled raft (NPR) system has been developed to solve this problem. In this case, the piles work as reinforcement of soft subsoils. However, the horizontal displacements of rafts due to lateral forces are the major concern to this system. In the present research, a series of the physical and numerical modeling are conducted to study the effect of super-structures frequency and excitation specifications on the response of the piled raft system in a dry sand bed. In order to validate the numerical modeling, the simulation results using 3-D finite element analysis are compared with the results obtained from physical modeling using centrifuge tests. Then, the effect of other parameters, such as physical characteristics ofpiled raft system and soil profiles on the dynamic response of the system are studied.
Based on the results, the frequency of the super-structures modeled on the piled raft foundations are smaller than the natural frequency of the fixed-base super-structures. This reduction is more for super-structures with higher frequencies and this is greater under excitation with high acceleration levels. The physical modeling of PR system shows, at the high acceleration level, the dynamic excitation with low frequency leads to the highest responses of the system. The gap between piles and raft in NPR system avoids the complete transfer of the super-structure forces to the piles and then the bending moment along the pile significantly decreases. Therefore, in NPRS, the responses of piles are independent of the type of super-structure. In addition, the presence of proper cushion layer, beneath the raft, prevents the lateral movements of the raft and then the risk of horizontal displacement is reduced. The numerical simulation results indicate that in PR system, the largest responses are produced under the excitation with the same frequency, as the system frequency. Also, the soil type, stiffness of raft and pile length are the parameters affecting the bending moment along the piles, while the soil profile and the structural characteristics are the major factors affecting horizontal displacement in the NPR system.
