3D experimental and numerical analysis of Fault Rupture-Soil-Foundation-Structure interaction

Abstract

In an earthquake, the rupture of an active fault generates two types of ground displacement: transient dynamic waves away from the fault and permanent quasi-static offsets on the fault itself [Ambraseys and Jackson, 1984; Jackson, 2001]. In past decades, earthquake engineering research and practice has generally concentrated on the dynamic response of structures and soil–structure interaction during seismic shaking and less consideration has been paid to the most direct consequence of the faulting process. This is because of the fact that the seismic waves propagate over large distances in the earth and therefore they always affect the ground surface. By contrast, permanent surface ruptures are only important near the fault trace and when rupture extends all the way to the surface. However, in large magnitude earthquakes the fault rupture may be important, imposing significant deformation to overlying structures.

The 1999 earthquakes in Turkey and Taiwan, offering a variety of case histories with structures subjected to large tectonic displacements, have prompted the increased interest of the earthquake engineering community on the subject. While several structures were severely damaged or even collapsed, there were numerous examples of satisfactory performance. Surprisingly, in specific cases the surface fault rupture was effectively diverted due to the presence of a structure.

In order to develop deeper insight into main mechanisms controlling this interlay, this research studies the effects of thrust fault rupture on foundation - structure systems founded on top of a sand soil deposit. This is done through 2-step procedure: The first step is to model the propagation of fault rupture through the soil layers and reaching the ground surface, and the second is simulating the effects of presence of structure founded on top of outcropping fault. Through these two steps, a 3D nonlinear finite element analysis with ABAQUS is performed in order to investigate the response of both a 3-story structure and the foundation to the fault rupture.

The soil constitutive model is calibrated by performing direct shear test. The model in ABAQUS has been verified by experiments which taken place in Laboratory of Soil Mechanics in N.T.U.A. The foundation is modeled as solid element and the columns and beams are modeled as flexural beam elements, while the possibility of sliding and detachment between the foundation and the underlying soil is considered through the use of special interface elements.

The numerical finite element which was verified by some small-scale experiments has been used to study the effects of different parameters like the magnitude of the fault offset and its location on the behaviour of both structure and foundation. The main results for our fault rupture soil-foundationstructure interaction analysis are discussed in terms of the distribution of plastic strains, the vertical displacement profile Δy, the foundation horizontal displacement, the structural drift ratio, structural moment, the foundation and structure rotation, the rigid body rotation in foundation and structure level and the rigid body settlement.

In this dissertation, we achieved good agreement between numerical and experimental results with the presence of the structure especially for larger base vertical displacement. As in the prototype structure which wasn’t comply with capacity design principles and was prone to softstory collapse mechanism, our reduced-scale model in ABAQUS also shows this soft-story failure and having plastic hinges in columns. We also observe that buildings on isolated footings are unable to avoid the direct hit of an underneath outcropping surface rupture. Consequently, the dislocation emerges within the structure causing significant deformation and distress.