Effects of foundation characteristics and building separation gap on seismic performance of mid-rise structures incorporating soil-foundation-structure-interaction

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Seismic waves travel many kilometres and pass through soil layers close to the ground surface before hitting the structures. The seismic induced dynamic behaviour of structures built on soft soil is highly dependent on the soil properties and the foundation type due to their interactions during an earthquake event. The design of building structures needs to consider seismic soil-foundation-structure interaction, where the building responses vary significantly depending upon the fixity of the base condition due to the interaction between the ground and the foundation as well as the building structures. This interaction is called “Seismic Soil-foundation-structure-interaction” (SSFSI). For a typical soil and foundation, SSFSI analysis shows lower natural frequency of the structural system and higher effective damping ratio compared to the traditional analysis with fixed base condition. This can considerably alter the response of the building frames under the seismic excitations by influencing the structural demand of the building as well as amplifying the lateral deflections and inter storey drifts of the superstructure. This phenomenon is highly influenced by the foundation type (i.e. shallow and deep foundation) and may change the performance level of buildings in the performance based design approach. Therefore the interaction should be considered in design of buildings subject to seismic activities so as to provide a safe and cost effective structural system. In this study, a rigorous numerical modelling approach was developed and used to build numerical tests for different foundation types and sizes as well as the pounding effects between buildings. The results consisted of lateral deformation, inter-storey drifts, levelling shear forces of the structures, foundation rocking, impact force and pile responses. These perimeters cover a wide range of earthquake inputs and foundation characteristics. The first step was that the soil-pile-interaction numerical behaviour was investigated in a case study of lateral loaded pile considering the shear plastic deformation of the layered sloping ground including sand and clay layers. Appropriate subroutines were adopted to simulate the soil-pile-interaction which included the incorporation of gapping and sliding (in normal and tangential directions) at the interface. A wide range of parameters for this numerical modelling was validated through comparison with an array of a full-scale lateral loaded pile experiments. Secondly, dynamic characteristics of soil-foundation-structure system were investigated for seismic response of a mid-rise moment resisting building on shallow foundation under four well-known earthquakes. By adopting a direct calculation method, the numerical model can perform a fully nonlinear time history dynamic analysis for three-dimensional numerical model with different foundation sizes where the infinite boundary, sliding and separation in soil-foundation was taken into account. In addition, the influence of foundation sizes on natural frequency and structure response spectrum was also studied. The results confirmed that when the size of shallow foundation is reduced, the natural period would lengthen, the base shear would reduce significantly while the lateral deformation, inter-storey drift and foundation rocking would increase. Thirdly, the comprehensive pile foundation investigation concludes that the type and size of a pile foundation that supports mid-rise buildings in high-risk seismic zones can alter the dynamic characteristics of the soil-pile-foundation system during an earthquake due to soil-structure-interaction. It is not true to believe that longer piles can provide safer condition under earthquake loading. In fact, by increasing the length of floating piles the structure undergoes more maximum lateral deflection, more inter-story drift and more total maximum levelling shear force but less foundation rocking. This can be explained due to the fact that longer pile foundation has higher contact surface with surrounding soil which enable them to absorb extra seismic energy. This finding can be the recommendation for design engineers that the pile should not lengthen too much to reduce the seismic effects. Finally, the separation gap between moment resisting and two shear wall braced buildings on pile foundation under seismic loading was studied. The result from this numerical modelling showed that pounding impact influences the distribution of shear force which disturbs the natural vibration and in extreme case, causes collapse. The outcome of this study provides essential insight to geotechnical and structural engineers when designing neighbouring structures in earthquake prone areas.
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