Earthquake response analyses of bi-linear pile supported superstructures
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The use of piles is a widespread support system for superstructures located on unstable surfaces, which have strict behavioural requirements. The requirements from Eurocode 8 for pile supports is that they have to be designed to remain elastic. The investigation in this thesis is related to the structural response of a system supported by piles designed with insufficient properties, i.e. designed with properties which include inelastic behaviour. This is conducted by comparing the results from simulated models in SAP2000 with elastic and inelastic properties, which get exposed to an earthquake load. The analysed structure is a shear wall from a performed case study of a typical Norwegian apartment building. The seismic event is implemented to the systems according to NS-EN 1998-1-1, appendix B. This method uses the static pushover curve for the equivalent SDOF system of the analysed structure to calculate the response. A parameter study is conducted to investigate how piles designed with perfect elastoplastic properties influence the entire structure. This is done by investigating the results from the SAP2000 models, where the chosen parameters are inserted and changed between each case. The findings in this thesis are based on the results of the parameter study. The findings are done by comparing the results with the response of the superstructure supported by elastic piles. The applied parameters are the total capacity of the support system, λ, and the reduced capacity of the two piles in the middle of the wall, ι. The reduced capacity ι is a percentage reduction of the estimated reaction force for one elastic pile. The parameter study consists of four main sections, where the total capacity of the system, λ, changes between each section. The calculated requirement for the elastic support system is applied as a baseline to give the systems sensible capacities. Further, the main sections are divided into four subsections, based on the degree of ι. Through this parameter study, a general finding is that all the investigated system remains stable. Nevertheless, some of the systems get permanent displaced because of inelastic behaviour, but the total capacity of the supports is not exceeded. If perfect elastoplastic piles are applied with one of the total capacities in the parameter study, this seems to give an improved response regarding the roof displacement. It has also been found that piles described with perfect elastoplastic properties remain elastic if the ι value is set to the lowest percentage reduction investigated. Designing for perfect elastoplastic properties also give a lower total base shear if the system is designed with a total capacity λ 10 % lower than the estimated requirement for the elastic supported system. The conducted investigation leads to the conclusion that designing for inelastic behaviour of piles exposed to an earthquake seems to improve the structural behaviour, given certain conditions.