Experimental investigation of the dynamic behavior of slender structures supported on combined pile-raft foundation: towards of performance-based design

In this research, the experimental investigation of the inertial interaction of soil-pile raft structure has been conducted for slender structures supported by the combined pile-raft foundation with emphasis on the new concept of design method (performance-based design). Most of the former studies based on this concept have focused on the surface foundation, where the surface foundation's rocking motion acts as a source of energy dissipation to protect the superstructure. Meanwhile, less attention has been paid to the surface foundation combined with piles (Combined pile-raft foundation) as an economic support system for high-rise and heavy structures. Mainly, the focus of optimizing these foundations through parametric analysis has been on variables such as pile arrangement and pile length for vertical static loading. When the heavy structures are subjected to the lateral load caused by the earthquake, the foundation experiences significant inertial moments. Thus, the nonlinear behavior of the foundation is not far from expected. The present research intends to examine the rocking behavior of combined pile-raft foundations as the foundation of slender structures. Evaluating the response of the superstructure and its possible benefit from the nonlinear behavior of the foundation is the principal goal of this research. In this regard, using experimental models, some characteristics of combined pile-raft foundations, such as the arrangement of piles and the relative length of the piles, have been investigated on the response of the superstructure. Three physical models were constructed in the laboratory. Each model contained a single degree of freedom superstructure supported by a floating pile raft foundation in sandy soil. Two characteristics were considered for evaluating pile raft characteristics: pile configuration and pile length ratio. The superstructure was identical in all three physical models. An experimental procedure based on forced vibration tests was presented to assess the dynamic response of the models at different levels of foundation nonlinearity. According to the experimental measurements, the nonlinear behavior of the foundation has a significant role in the response of the superstructure. Dynamic demand reduction as well as drift reduction are the two most important factors that benefit the superstructure from foundation nonlinearity. Accordingly, the dynamic behavior of the models is divided into two individual phases. Also, comparing the results of the models showed that the arrangement of piles and the relative length of the piles in the combined pile-raft foundation have a significant impact on superstructure response.