|Earthquake response of a structure on relatively soft soil is influenced by the soil material properties and type of foundation i.e. shallow or deep foundation. The design recommendations of Eurocode 8 provide a conservative method to determine seismic loads for spectral analysis approach assuming a fixed base structure. This thesis attempts to evaluate the influence of the piles on the response of a structure in terms of acceleration and base shear force. The shear force induced at the shallow base is determined using finite element program PLAXIS 2D and compared with the calculation based on Eurocode 8. The aim is to investigate the correspondence of numerical analysis in PLAXIS 2D with simplified analysis method of Eurocode 8 (NS-EN 1998-1:2004+A1:2013+NA:2014).
A representative soil model is constructed with proper boundary conditions for dynamic analysis in PLAXIS 2D based on previous researches and recommendations. The choice of boundary conditions is tested with a simpler material model and a harmonic motion and verified with theoretical solution of amplitude factor. The next step is to perform a free field site response analysis of the main soil model for an input motion of 0.1g to evaluate the soil behavior during earthquake. The results are compared with one dimensional ground response analysis in DEEPSOIL and reasonable agreement is observed.
The soil model is then provided with the structures to evaluate the effect of the piles. Four cases consisting of two different structures with two different foundation systems (shallow foundation and piled raft) are simulated on the given soil condition. Free vibration analyses are carried out for all the cases to determine natural frequencies or period of vibration of the structures. The obtained results are compared with logarithmic decrement and Eurocode 8. The resulting natural period of vibrations are 0.33 seconds for the single-story structure and 0.6 seconds for the four-story structure. This indicates that the numerical analysis results are consistent with theory and Eurocode 8.
Results of full dynamic analyses show that the foundation type has considerable influence on seismic response of a structure. For a single-story structure, the acceleration increases over 40% and the for the four-story structure it is over 45%. Calculated shear force at the bottom of the rigid base is increased by almost 100% due to the piles for both structures. This highlights the fact that specific analysis for design base shear is required for pile supported structures to ensure safety against earthquakes. However, the obtained values of base shear for all the cases are lower than the values calculated based on Eurocode 8 as expected. Although for shallow foundation, Eurocode 8 provides overestimation which may not be cost effective. Therefore, it can be inferred that the numerical analysis using PLAXIS 2D demonstrates reasonable correspondence with Eurocode 8. Further studies can be conducted to obtain spectral behavior for different kinds of foundations.