Abstract Summary
The design of structures to seismic excitation in which soil-structure interaction (SSI) is not considered with enough detail can lead to overdesigned solutions and, in the case of large structures, lead to significant increases in the overall budget of the project. For such large structures, spending more effort in modelling SSI to derive less conservative solutions (yet, solutions that satisfy the seismic demand) can be advantageous. In this work, we report on the method recently adopted at Royal Haskoning DHV (RHDHV) to assess the seismic response of a large building, which is partly deeply embedded in soil and partly founded on piles. Due to the critical nature of the project, SSI is not just of financial interest, but also a requirement from the relevant authorities. The method described in this work consists of a substructuring technique, where the structure is decoupled from soil. This enables each part to be analyzed with the most suitable domain: frequency for soil and time for the structure. The two parts are thereafter re-coupled in a time-implicit FEM solver. The division between substructures is made at the edges of the floors and walls in contact with soil (for the embedded parts of the structure) and at the foundation beams for the more superficial parts of the buildings. Piles are included in the part containing soil. For the soil part (and piles), a frequency-domain FEM strategy is adopted. This limits the analysis to linear elasticity. Nevertheless, the soil properties can be made linearly equivalent based on the non-linear ‘free-field’ response (i.e., without the presence of the building), as specified in some standards, e.g., ASCE-4-16. In order to consider appropriate reflection-free boundaries and radiation of waves, perfectly matched layers (PML’s) are used at the edges of the FEM model. A Python program has been developed for this purpose, and parallel computing was used to take advantage of the multiple cores available at calculation servers. For the upper structure, an implicit time stepping scheme is used, in which structural non-linearities (e.g., sliding between components) can be modelled. The structural model is created and solved using ABAQUS software. The coupling between the upper structure and soil is done after transforming the soil response to the time-domain. This results in time convolutions at the interaction surfaces, which in turn can lead to an unstable stepping algorithm. Therefore, one must be critical and careful when deciding the time-step to use in the simulation. In this work, we further present our approach to analyze the stability of the system (without having to solve it in its entirety, which is a time consuming task), and present a strategy/simplification which showed to improve the convergence of the coupled algorithm.
