Response of Structures with Nonlinear Soil-Strusture Interaction in Near-Source Earthquakes

Abstract

Current seismic regulations allow earthquake energy dissipation via inelastic behaviour of structures providing that collapse is avoided. Considering that this philosophy can negatively impact due to loss of function of critical structures, mitigation systems are currently being explored by researches. In the last decades a new concept for reducing structural response has emerged. By allowing soil plastic deformations and foundation lift-off, structural performance can be improved. However, more research is required in order to better understand this phenomenon. This thesis concerns an experimental investigation into the structural response of buildings allowed to uplift when subjected to near-source earthquakes considering nonlinear soil –foundation-structure-interaction (NSFSI). Considering that gravity cannot be neglected in scaled models allowed to uplift, a new modelling approach was developed to account for gravity effects. Based on this method, a small scale model of a low rise building was built. Inelastic behaviour of the superstructure was considered by means of replaceable plastic hinge components. To study the benefits of uplift and NSFSI on the structural response, 10 records of actual near-source earthquakes were considered and subsoil was simulated by means of sand in a box. The scaled model constructed using the modelling method developed in this research was validated and it was capable of simulating a more realistic uplift behaviour even at small excitations. The experimental results suggest that uplift is unavoidable and it can significantly reduce the response of the structure. When soil plastic deformation is considered, the response of the structure can be further reduced remaining practically elastic. In contrast, uplift can increase the absolute horizontal displacements of the structure when the supporting base is rigid but marginally increased when soil plastic deformations are permitted. Because of the impulsive loading of near-source earthquakes and soil plastic deformation of subsoil, permanent rotation of the footing was observed after the uplift.