Impact of Soil Structure Interaction on the Seismic Design of Reinforced Concrete Buildings with Underground Stories G. Saad, F. Saddik & S. Najjar American University of Beirut, Lebanon ABSTRACT: Current building codes lack explicit recommendations on how to simulate the seismic performance of high-rise buildings with multiple underground stories. Designers are typically basing their analyses on subjective engineering judgment and experience. Some model and analyse the buildings cropped at the ground floor level, others include a partial number of basement floors, while a few include all the underground floors. This paper studies the seismic behaviour of reinforced concrete buildings with multiple underground stories. It seeks to provide recommendations on the number or percentage of underground stories to be accounted for in the analysis of reinforced concrete shear wall buildings. A base-case where the buildings are modeled with a fixed condition at ground level is adopted, and then the number of basements is incrementally increased to investigate changes in performance. The Beirut local site conditions are used for the analysis. The base shear, inter-story shears and moments are evaluated in order to quantify the effects of soil structure interaction on the design process. Keywords: Soil Structure Interaction, Seismic Design, Reinforced Concrete Buildings 1. INTRODUCTION A controversial issue in the seismic analysis and design of buildings with multiple underground stories lies in incorporating the effects of these underground stories on the seismic response of these structures. Building codes lack recommendations concerning this controversy; thus, the designers are basing their analysis on approximations, engineering judgment and experience. Some model and analyze the building cropped at the ground floor level, others include a certain number of basement floors, while few include all the underground floors. This has been an active area of research throughout the past decade (Dutta and Roy, 2002, Dutta et al., 2004, Shakib, 2004, Naim et al., 2008, El Ganainy and El Naggar, 2009, Raychowdury 2010, Tabatabaeifar and Massumi, 2010). El Ganainy and El Naggar investigated the seismic performance of moment-resisting frame steel buildings with multiple underground stories. Their study was tailored for the governing site conditions in Vancouver, Canada, and the Beam-on-a-Nonlinear Winkler Foundation approach was used to simulate the important aspects of the nonlinear behaviour of the foundation and side soil. Raychowdhury also used a similar approach to study the response of low-rise steel moment resisting frame buildings. Tabatabaiefar and Matssumi (2010) used a 3D finite element model to simulate the effects of soil structure interaction on reinforced concrete moment resisting frames. While current research mainly aims at understanding the effects of soil structure interactions, this study has the ultimate goal of finding appropriate recommendations concerning the inclusion of underground stories in the modelling and analysis of reinforced concrete shear wall buildings and optimizing their design. The impacts of the building substructure on its seismic performance are gauged by explicitly incorporating the underground stories, basement walls, foundation and side soil in the structural analysis model. In accordance with the geotechnical map of Beirut, the soil types considered in modelling the subsurface conditions are assumed to consist of medium dense and very