Displacement ratios for structures with material degradation and foundation uplift

Abstract

In this paper, combined effects of material degradation, p-delta, and foundation uplift are incorporated in a soil-structure-interaction (SSI) framework to assess seismic response of a single-degree-of-freedom system. The considered phenomenological systems represent a column with a lumped mass on top is placed on a rigid foundation. The foundation is mounted on Winkler springs and dashpots to take account of soil-foundation compliance and material/radiation damping. The springs are tensionless to guarantee that uplift is properly modelled. The model is verified for two specific limit cases with the code and literature to make sure that the model is capable of capturing SSI and foundation uplift. Three sets of flexural degradations for capturing nonlinear performance of the column are defined to be categorized as low, moderate, and high degradation levels. The response of each system is then compared to the corresponding one with the elastic–perfectly plastic (EPP) material behavior. More than 150,000 analyses are conducted for several dimensionless parameters and twenty time history ground motions. The results including elastic and inelastic displacement ratios are finally compared with those stipulated by seismic codes for shallow foundations with the flexible-based condition. The results reveal that material degradation can significantly increase the displacement ratios of soil-structure systems and in some cases can lead to structural instability, which cannot be captured by conventional EPP models. More specifically, displacement ratios calculated in accordance with ASCE41-17 by assuming fixed-base model and incomplete data about level of material degradation might be underestimated, once compared to the results of more realistic conditions.