Effects of Foundation Uplift on the Dynamic Response of Steel Frames

This paper examines the effects of foundation uplift on the dynamic response of steel frames supported by shallow foundations. In order to achieve this goal, the dynamic responses of a number of two-dimensional concentrically braced steel frames with fixed base, flexible base preventing uplift, and flexible base allowing uplift, are compared. The steel frames are modeled by considering material nonlinearities and P-Delta effects, while the soil-foundation system is modeled through the Winkler foundation approach. The dynamic analyses are conducted using 15 far-fault ground-motions with various intensities and different response parameters are monitored. According to the achieved responses, the presence of foundation uplift generally increases the total displacement demands and reduces the inter-story drift ratio and ductility demands. The important parameters affecting the responses of uplift-permitted frames are also introduced and examined.