Analyses Of Earthquake Responses Of Frame Structure Clusters Near Causative Fault

It has been shown from several earthquakes such as the1994Northridge earthquake, the1995Hyogo-ken Nanbu earthquake and2008Wenchuan earthquake that earthquake may result in severe seismic hazard of near-fault cities. Earthquake responses of near-fault structure clusters are one of important reasons resulting in severe seismic hazard of near-fault cities. So there is practical significance to study earthquake responses of structure clusters in near-fault city. Actural earthquake responses of near-fault structure clusters are generated by seismic wave propagation in sturctures and earth medium. The wave numerical methods can reproduce the macro-laws of dynamic responses of building clusters during earthquake, which can explain clearly the phenomena of earthquake disasters. Therefore, it is a meaningful work to deeply carry out research on wave numerical method, which is used for studying earthquake responses of near-fault structure clusters by considering simultaneously structure clusters and earth medium as well as causative fault into the same computational model. This dissertation mainly includes the following contents:(1) The point and finite-fault seismic source models as well as viscoelastic earth medium model were briefly reviewed. This chapter focused on research status of analytical methods and numerical methods, which are used for simulating seismic wave propagation, and study on earthquake responses of near-fault building clusters. Finally, purpose and main content of research in this dissertation were given.(2) Finite-fault seismic source model used for implementing rupture process of fault and the Generalized Zener Body (GZB) model used for implementing attenuation of earth medium were introduced in detail. Moment tensor for plane strain problem was given and empirical formulas involving finite-fault seismic sources were collected. The relationship between viscoelastic parameters of two-mechanism differential-type GZB model and relaxation times of two-mechanism integral-type GZB model was derived and given for the first time. Two-mechanism differential-type GZB model, L-mechanism GZB with memory variables and L-mechanism GZB with history variables for plane strain problem have been given from the3-D relevant GZB models. The collected and given contents are used for the following researches.(3) Three investigated lump methods for simulating viscoelastic seismic wave propagation in earth medium have been proposed by introducing two-mechanism differential type of the Generalized Zener Body (GZB) model, L-mechanism GZB with memory variables and L-mechanism GZB with history variables, respectively. For each proposed method, the following three examples are used to verify correctness and validity of it. Firstly, this chapter simulated wave propagation in infinite viscoelastic earth medium and compared the numerical results with analytic solutions. Secondly, this chapter simulated near-fault ground motions due to rupture of fault and compared the numerical results with the results calculated by discrete wave method. Finally, by using half-space earth medium with irregular surface topography, this chapter calculated displacement responses of the receiver due to a single force and compared the numerical results with the results obtained by FEM. Comparison results verify the correctness and validity of each proposed method in calculating viscoelastic wave propagation, simulating near-fault ground motion due to rupture of causative fault and dealing with wave propagation problem with irregular surface topography. Comparison results show also good accuracy of each proposed method.(4) This chapter applied the method, which is based on two-mechanism differential-type GZB model and finite-fault seismic source model, proposed in Chapter3for simulating viscoelastic seismic wave propagation in earth medium into ground motions simulation in Beichuan town due to rupture of Yinxiu-Beichan fault. In this numerical simulation, the rupture process of the Yinxiu-Beichan fault of Wenchuan earthquake, viscoelastic attenuation of earth medium, inhomogeneous earth media and actual surface topography have been considered. The research results shows that ground motions on hanging wall are abvious stronger than those on footwall. The’wave’phenomenon appears for the PGA values with respect to distance from the surface rupture, no matter on the hanging wall or on the footwall. The viscoelasticity of earth medium below Beichuan and the topography around Bechuan town have a great influence on ground motions in Beichuan town. Frequency characteristics of ground motions and distribution characteristics of the PGA values with respect to spatial variation are in line with actual seismic hazard in Beichuan town. The method proposed in this dissertation provides an effective means to simulate near-fault ground motion.(5) This chapter proposed an integrated system consisting of frame structure clusters, viscoelastic half-space earth medium and causative fault. Based on this system, this chapter developed an integrated simulation method for implementing seismic wave propagation in frame structure clusters and viscoelastic earth medium. Different from two-mechanism differential-type GZB model used in Chapter4, in this integrated simulation method, the L-mechanism GZB model with memory variables was used for implementing viscoelastic wave propagation in earth medium. In order to implement flexural wave propagation in frame structure, this chapter established governing equations of investigated lumps in frame structure and gave algorithm implementation of recursive evaluating in time domain for flexural wave propagation in frame structure. In order to implement bidirectional wave propagation between soil and structure, this chapter established governing equations of a new type of the investigated lumps for structure-soil connection and gave algorithm implementation of corresponding recursive evaluating in time domain. A test example was performed to verify validity of the flexural wave propagation algorithm proposed in this dissertation. This chapter has simulated earthquake responses of frame structure clusters during a Mw6.0hypothetical earthquake by using the proposed integrated simulation method. The numerical results showed that the resonance between structure and site is the main reason resulting in large earthquake responses of the structure when the PGA values of ground motions on the sites of two structures with different natural frequencies are close. The non-zero displacements of the near-fault frame structures relative to their original positions appear after the earthquake. Deformation patterns of frame structure show the proposed wave method in structure can simulate efficiently flexural wave propagation in frame structure. When seeing the structure clusters towards fault strike direction, the orbit of the structures in the cluster located between the epicenter and rupture forward is in anticlock motion induced by rupture of reverse fault. This integrated simulation method provides an effective tool for calculating internal forces and displacements of the members of structure in structure clusters induced by rupture of causative fault.(6) Based on the proposed integrated system, this chapter proposed an integrated simulation method for wave propagation in viscoelastic earth medium and frame structures in mountain city. Different from two-mechanism differential-type GZB model used in Chapter4and Z-mechanism GZB with memory variables in Chapter5, the viscoelastic model in this integrated simulation method is X-mechanism GZB with history variables to implement attenuation of earth medium on seismic waves. The algorithms for flexural wave propagation in frame structure and bidirectional wave propagation between structure and soil are same as Chapter5. By using this integrated simulation method, this chapter studied earthquake responses of structure clusters situated in different sites of mountain and hill in Yuzhong district of Chongqing during a Mw6.2hypothetical earthquake. It is shown that the effects of the dip angle of causative fault on earthquake responses of12-storey structures are great. When seeing the structure clusters towards fault strike direction, the orbit of the structures in the cluster located between the epicenter and rupture forward is in anticlock motion induced by rupture of reverse fault. In the four structure clusters situated at different sites in mountain city, the3-storey structure with maxiumun peak beam-end bending is on the hilltop and the12-storey structure with maxiumun peak beam-end bending is on the mountainside. The method proposed in this chapter can be used for selection of construction site and determination of risk positions of frame structures in strucuture clusters due to rupture of causative fault.(7) Summary of the entire desertation and prospects of further research and work were given.