Microtremor survey in Talchir, India to ascertain its basin characteristics in terms of predominant frequency by Nakamura's ratio technique

Seismic behaviour of structures built on soft soils is influenced by the soil properties and the foundation type, where the response is significantly different from the fixed base condition owing to the interaction between the ground and the structure. Soil-Structure Interaction (SSI) reduces the natural frequency of the system and increases the effective damping ratio of the system, for typical soils and foundations, in comparison with the fixed-base structure. This can considerably alter the response of the building frames under the seismic excitation by influencing the structural demand of the building as well as amplifying the lateral deflections and inter storey drifts of the superstructure. This amplification of lateral deformations due to SSI may change the performance level of buildings in the performance based design approach, which should be considered with great rigor accounting for the influence of SSI significantly influenced by the foundation type (i.e. shallow and deep foundation), in order to provide safe and cost effective design against the natural disasters such as earthquake.
In this study, in order to provide a benchmark to verify and calibrate the numerical model as well as experimentally investigate the influence of SSI on the seismic response of buildings, a series of shaking table tests on the soil-foundation-structure models are conducted at the University of Technology Sydney (UTS) structures laboratory. Different foundation types such as shallow foundation, floating pile foundation, end-bearing pile foundation as well as fixed base condition, excluding SSI interaction, are physically modelled. A laminar soil container is designed and constructed to simulate the free field soil response by minimising boundary effects. Simulating the superstructure as a multi-storey frame during the shaking table tests makes experimental data unique. Accordingly, in the current shaking table tests, by adopting the same soil properties, same superstructure, same input motions, and same test setup, a clear comparison is provided between the structural responses for different types of foundations. The experimental results indicate that soil-structure interaction amplifies the lateral deflections and inter-storey drifts of the structures supported by different types of foundations. However, the choice of the foundation type influences the structural performance significantly and should be addressed carefully in investigating the influence of SSI on the superstructure response during shaking excitations.
A fully nonlinear three-dimensional numerical model employing FLAC3D is developed to perform time-history analysis and simulate the performance of the superstructure considering the seismic soil-structure interaction. Hysteretic damping of the soil is implemented to represent the variation of the shear modulus reduction factor and the damping ratio of the soil with the cyclic shear strain. Free field boundary conditions are assigned to the numerical model and appropriate interface elements, capable of modelling…

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Effect of dynamic soil-pile-structure interaction on seismic response of mid-rise moment resisting frames

3 high potential are demonstrated. Indeed, it is shown that the partitioned treatment is a feasible and realistic approach for coupled modelling of nonlinear soil-structure interaction problems, providing an integrated interdisciplinary computational approach which combines the advanced features of both structural and geotechnical modelling for a variety of challenging soil-structure interaction problems.

Partitioned analysis of nonlinear soil-structure interaction

Dynamic response of structures sitting on soft soils is influenced by the soil properties, and the response is significantly different to the fixed base condition owing to the interaction between the ground and the structure. In order to study this effect, a fifteen storey moment resisting building frame, representing a conventional type of regular mid-rise building frame, resting on soil type Ee according to Australian Earthquake action code with the shear wave velocity equal to 150 m/s is adopted. The numerical analysis using FLAC2D software is carried out for three different cases, namely: (1) fixed-base structure representing the situation excluding the soil-structure interaction (SSI); (2) structure supported by shallow foundation on soft soil; and (3) structure supported by pile foundation in soft soil. Benchmark earthquakes including the 1995 Kobe, the 1994 Northridge, the 1968 Hachinohe, and the 1940 El Centro earthquakes are adopted. Results indicate that considering soil-structure interaction in both cases with shallow and pile foundations is vital, and the conventional design procedure excluding soil-structure interaction is not adequate to guarantee the structural safety for the moment resisting buildings resting on the soft soil.

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Effects of soil-pile-structure interaction on seismic response of moment resisting buildings on soft soil

The interactive effects of adjacent buildings on their seismic performance are not frequently considered in seismic design. The adjacent buildings, however, are interrelated through the soil during seismic ground motions. The seismic energy is redistributed in the neighboring buildings through multiple structure-soil-structure interactions (SSSI). For example, in an area congested with many nearby tall and/or heavy buildings, accounting for the proximity effects of the adjacent buildings is very important. To solve the problem of SSSI successfully, researchers indicate two main research areas where need the most attention: 1) accounting for soil nonlinearity in an efficient way, and 2) spatial analysis of full 3D soil-structure models. In the present study, three-dimensional finite element models of tall buildings on different flexible foundation soils are used to evaluate the extent of cross interaction of adjacent buildings. Soil nonlinearity under cyclic loading is accounted for by Equivalent Linear Method (ELM) as to conduct large parametric studies in the field of seismic soil-structure interaction, the application of ELM is preferred over other alternatives (such as application of complicated constitutive soil models) due to the efficiency and reliability of its results. 15 and 30 story steel structures with pile foundations on two sandy and clayey sites are designed according to modern codes and then subjected to several actual earthquake records scaled to represent the seismicity of the building sites. Results show the cross interaction of adjacent buildings on flexible soils, depending on their proximity, increases dynamic displacements of buildings and reduces their base shears.

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H. Akagi

Papers

Nonlinear response of surface soil and NTT building due to soil–structure interaction during the 1995 Hyogo-ken Nanbu (Kobe) earthquake