Seismic response of structures on flexible base under foundation uplift augmented by vertical component of near-field ground motions

Three-dimensional nonlinear dynamic analyses of 3-, 6-, 9-, and 12-story steel buildings with special concentrically braced frame and special moment-resisting frame systems on soft and very soft soils are accomplished using OpenSees software. All buildings are modeled by taking nonlinear soil-structure interaction effects into account, representing a flexible-base case. They are excited under the effect of concurrent two and three translational components of near-field ground motions. Beams and columns are simulated using force-based beam-column elements with nonlinear material behavior at their two ends, and the finite-length plastic hinge approach is used to model the behavior of beam-column joints. By taking P-Delta and imperfection effects into account, braces are modeled using displacement-based fiber elements with distributed plasticity behavior. The Beam-on-Nonlinear-Winkler-Foundation is used to model the flexibility of the base soil. Foundation uplift is free to occur under the overturning moment due to the horizontal components of ground motion and gravity reduction because of the vertical component. The findings indicate that inadequate tensile capacity of the soil fails to prevent detachment of the foundation from the underlying soil especially for the braced frames. Furthermore, the vertical component of ground motion aggravates foundation uplift by as much as 50 % in comparison to the responses observed in buildings affected by horizontal motions. However, the vertical motion does not influence the maximum horizontal drift and base. For the soft soil, the provision provided in ASCE code to consider the vertical component of ground motion in design loading combinations turns out to be unconservative. According to the analysis results, it is recommended that the vertical seismic load effect, Ev, to be regarded as twice the quantity outlined in ASCE guidelines.