Assessing Impacts of Soil Constitutive Behavior and Water Pressure on Seismic Performance of Buildings on Shallow Foundations
- Publication Type:
- Thesis
- Issue Date:
- 2020
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The growing need for the high rise buildings in the megalopolises necessitates the reliable predictions of the buildings’ performance amidst the earthquakes with the aim of curtailing the severe damage and probable partial or the total collapse of the superstructures. The seismic excitation, experienced by the superstructures, is a function of the seismic source, travel path and local site effects, as well as the Soil-Structure Interaction (SSI) influences. Thus, the undeniable paramountcy of the dynamic soil-structure interaction is evident.
This thesis conducts the three-dimensional elasto-plastic-based coupled SSI numerical simulations in FLAC3D using the direct method with the help of the High-Performance Computer (HPC) at University of Technology Sydney (UTS), taking averagely a few days to a month. The 15-story and 20-story reinforced concrete moment-resisting buildings, as the examples of the typical high rise buildings in the relatively high-risk earthquake-prone zones, are designed considering the relevant Australian codes and in line with the constructability and norms. The plastic moment concept is employed to assign the elastic-perfectly plastic model to the superstructures and their mat foundations. The geometric nonlinearity of the adopted superstructures, capturing the 𝑃 – Δ effect, is accommodated by the use of the large-strain solution mode. The dependency of the soil shear modulus and corresponding damping ratio on the seismically-induced shear strains is also captured. The interaction between the soil mass and building foundation is simulated by the use of the advanced interface element, mimicking the possible sliding, separation, and gapping. The cherry-picked near-field earthquake excitations are scaled by means of the response spectrum matching method.
The medium, underneath the engineering superstructures, influences their dynamic responses. An investigation on the impact of the soil dynamic properties, including the shear wave velocity and small-strain shear modulus, on the seismic performance of the superstructures, supported by a shallow foundation, is conducted. The outcomes show that these soil properties ought to be served with the acute care in any seismic soil-foundation-structure interaction simulation so as to obtain the reliable results. Taking a step further, the variations of the degree of saturation, stemming from the extensive dry climate and floods, could impair the seismic performance of the mat-supported buildings due to exceeding the life safety drift limit, hinging around the post-earthquake damage state. The damp soils are basically softer and so absorb more energy than the dry, stiff soils. After a dry season, during a seismic event, the selected building in this study will experience more load, will move more, will crack more and ultimately will be unsafe whether it remains standing or collapses.
This thesis conducts a host of seismic SSI analyses with the consideration of the hardening hyperbolic concept. It is concluded that incorporating more advanced soil plasticity models, suitable for the seismic analyses of the soil-structure systems, could predict the foundation rocking and structural lateral deflections more accurately, both of which must be strictly overseen in the application of the foundation rocking isolation technique. Examining the geotechnical and structural objectives in this study exhibits that the presence of the water table at the construction site had better not be dismissed in any case as the generation of the excess pore water pressure could markedly weaken the seismic performance of the superstructures by pushing it from the life safety state to the near collapse damage level or even the collapse state. In practice, however, the consideration of the presence of the water table at the construction site is only limited to the drained analysis and undrained shear strength analysis.
The design and practicing engineers, stakeholders, and practitioners are meant to consider the Performance-Based Seismic Design (PBSD) approach as an indicator of the buildings’ performance, subjected to the different levels of the earthquakes. This thesis is devoted to provide them with a clear understanding on the key factors, affecting the relations between SSI, PBSD, and the foundation rocking since an ounce of prevention is worth a pound of cure.
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