Study of Period Lengthening Effects in Soil-Structure Interaction Systems

Natural period of structures can be elongated by the inclusion of a flexible foundation system. Period lengthening arising from the inclusion of soil-structure interaction (SSI) changes seismic design of structural and geotechnical systems, since period elongation may lead to larger demands in the structure. This paper parametrically studies the main variables that influence period lengthening characteristics of tall buildings. Variables such as the fixed-base natural period of buildings, structural aspect ratios, and effective soil shear wave velocity of supporting soils were included in the analysis using a direct approach. Three-dimensional (3D) numerical simulations including SSI effects were included to show the effects of period lengthening of tall buildings when compared to more common fixed-base 3D analyses. The analyses were performed using OpenSees by applying ground motions in two directions. Supporting soils are modeled by means of a direct approach in a fully coupled continuum soil-structure system formulation to represent in a more realistic manner the SSI effects. It is concluded that period lengthening of the first natural period of the structure arising from including SSI effects in the numerical framework produced more flexible structures. SSI generally increases the engineering demand parameters when compared to fixed-base models.