Implementation of Free-Field Boundary Condition for 2D Site Response in OpenSees

抄録

It is observed from past experiences of earthquakes that local site conditions can significantly affect the strong ground motion characteristics. One-dimensional seismic site response analysis (SH1D) is the most common approach for investigating site response. This approach assumes that soil is homogeneous and infinitely extended in the horizontal direction. Therefore tying side boundaries together is one way to model this behavior, as the wave passage is assumed to be only vertical. However, SH1D cannot capture the 2D nature of wave propagation, soil heterogeneity, and 2D soil profile with inclined bedrock. In contrast, 2D seismic site response modeling can consider all of the mentioned factors to better understand local site effects on strong ground motions. The 2D wave propagation and considering that soil on two sides of the model is not identical in a soil profile clarifies the importance of a boundary condition on each side that can minimize the unwanted reflections from the edges of the model. Regarding site response, for the avoidance of wave reflections in the boundaries, the model size should be sufficiently large to minimize the wave reflection. However, due to computational limitations, increasing the model size is impractical in some cases. Another approach is to employ free-field boundary conditions (absorbing boundaries) to have a non-reflecting behavior in the boundaries while absorbing the wave energy by a factor. These boundary conditions must take into account free-field motion in the absence of the structure at the sides of the model. This research focuses on implementing free-field boundary conditions in OpenSees for 2D site response analysis with considering free-field 1D model results as input for 2D model.