Seismic Soil Foundation Structure Interaction in Urban Environments
The interactions between a structure, its foundation, and the surrounding soil during an earthquake are referred to as soil foundation structure interaction (SFSI) The interactions between multiple structures, and their foundations, through the surrounding soil are collectively known as structure soil structure interaction (SSSI) Modern design codes in use in the United States, and abroad, provide guidance for considering SFSI during the seismic design of structural systems. However, these same codes do not provide any guidance for considering SSSI. This situation is a direct result of the current paucity of research into the effects of SSSI on structural performance. This dissertation describes the results of four centrifuge experiments designed to study the influence of SSSI on the seismic performance of building foundation systems. Following these experiments, numerical models were developed and their efficiency at reproducing measured response evaluated. The experimental program involved two pairs of tests. During Test 1 and Test 2, the SFSI and SSSI influenced responses of two three dimensional inelastic frame structures were recorded. During Test 3 and Test 4, the interactions between an inelastic frame structure and an elastic rocking wall arranged in a variety of orientations were recorded. In each of the pair of test series, one configuration was devoted to the evaluation of the response of the model frame structures far from any neighboring structure. Ultimately, the experimental results demonstrate that when structures are placed next to each other, the seismic demands in inelastic frame structures can increase. As such, seismic structural performance may be negatively impacted by SSSI. The tests reveal that footings of buildings placed nearest to other buildings can be physically restrained when loaded towards the other building a physical mechanism that had not previously been observed. This asymmetrical physical restraint resulted in a stiffened hysteretic response of footings nearest to adjacent buildings and nominal increases in seismic demands to superstructure elements. It is also demonstrated that wave based analytical solutions to the SSSI problem alone are not adequate for modeling the interactions between building foundation systems with highly nonlinear foundation responses. During the numerical phase of this research, available tools for modeling SFSI effects (i.e. absent the effects of neighboring structures) in OpenSees were first refined. The shallowFoundationGen command was redeveloped to give the user greater flexibility. Subsequently, it was demonstrated that the updated modeling technique provides an adequate means to model the experimentally observed coupling of the vertical footing force and overturning moment load combinations for shallow foundations attached to inelastic frame structures. Finally, an available methodology for modeling wave based SSSI effects was implemented in OpenSees and its capability to predict the experimentally measured seismic demands of an SSSI influenced inelastic structure was evaluated. Ultimately, it is concluded that the use of foundation to foundation connection springs, which are based on wave based solutions, is insufficient for capturing the seismic response of adjacent structures with highly nonlinear individual SFSI responses