Performance-Based Seismic Design Of Bridge Structure Excited By Near-fault Earthquake Using Energy Concept

Serious earthquake disaster close to the active fault brings forward bran-new challenge. Records with velocity pulse of long duration near fault region result in a new test for modern structure seismic design theory. There is an urgent need to understand the major characteristics of near-fault earthquake and improve existing seismic design methods. Along with technological advances in digital equipment, a number of near-fault earthquake records have been obtained. Necessary scientific foundation of this study is accessed. Based on the understanding that the mechanism of structure resisting earthquake impact is essentially a process of seismic energy consumption, energy-based performance evaluation method for bridge structure under near-fault earthquake was proposed, and seismic design method with exact performance goal to consider both cumulative energy and maximum deformation demand was developed in this paper. The main contents are as follows:1. Relied on selection of typical earthquake records with velocity pulse of long duration, the control parameter I_p was extracted as index of velocity pulse destructive capacity from dynamical computation results of an isolated continuous bridge, and a pulse simulation model using trigonometric function was introduced and regressed based on the collection of horizontal earthquake records all around the world. By introduction the concept of cumulative energy and instantaneous energy, energy spectrum analysis shows that pulse period and intensity are key factors for structure peak response during various parameters of near-fault earthquake. Cumulative input energy is affected significantly by high-frequency component, and instantaneous energy depends mainly on pulse velocity. Example analysis of isolated piers indicates that instantaneous input energy is an effective indicator to characterize the earthquake damage capacity. From the distribution characteristic of cumulative energy dissipation ratio and ratio of instantaneous dissipation energy, it can be drawn that energy dissipation efficiency of dual-linear isolation bearings can not exceed 0.6, and is about 0.4 under normal conditions, which means the energy dissipation capacity of LRB isolated structure excited by near-fault pulse needs to be strengthened, and reliable energy absorbing approach is requisite.2. In summing up simplified models for evaluation of input energy, a computation method based on response spectrum velocity was advised according to the demerit of each existing model, and the accuracy of this method was verified by near-field records. Based on record division, mean-value spectrum and 84% envelope curve of input energy was calculated, as a result the energy input design spectrum (EIDS) was recommended applicable to 0-15km fault zone, and was verified by actual records. Compared to Japan Seismic Codes(1985,2001) and Akiyama,Climent models, it can be concluded that the proposed design energy spectrum can reflect betterly the potential energy demand of near-fault earthquake. From the proposed simplified calculation model of input energy, input energy spectrum compatible to major China and USA seismic codes (GB50011-2001,UBC97) was derived. Contrast results shows that GB50011-2001 Code can reflect the amplification effect of near-fault only for HK IV soil types. On the contrary, UBC97 Code can match the recommended input energy spectra in most conditions except for soil S_A , which means the necessity of N_A,N_V and the rationality of the proposed EIDS in this paper from another aspect.3. The decisive factors for distribution of input energy among damping and deformation dissipation energy was discussed, on the basis the energy ratio taking into account structural damping and displacement ductility was introduced considering a decline in long period region. Based on hysteretic model considering stiffness degradation, the relationship between seismic input energy and inelastic seismic deformation of structure was studied. A function expression considering soil classification was proposed by piecewise linear method and was verified by example of MDOF bridge structure. In making certain the total input energy and hysteretic energy demand of earthquake, as well as energy dissipation and distribution ability of MDOF structure, an energy-based damage assessment method for bridge was developed and a validation computation was conducted by 3 piers of Canada.4. An improved dual-parameter Park-Ang damage model was developed to assess the exact damage of stiffness degradation SDOF system designed by constant ductile strength reduction factor, and it can be drawn that such method can not effectively consider the effect of cumulative damage from stiffness degradation, maybe being somewhat on the unsafe side. Based on the definition of strength reduction factor of constant performance goal, large numbers of cases of stiffness degradation SDOF system were accounted by self-writing computation program and a regression model under 5% damping was extracted. On the basis, constant performance goal non-elastic displacement spectrum compatible with UBC97 Seismic Design Code was educed, a performance-based seismic design method for bridge pier was proposed and its feasibility was checked by example. Finally, multi-phase modes effect of flexible bridge pier with high slenderness and MDOF full-bridge structure was discussed, and decomposition theory of multi-degree of freedom response spectrum was introduced to develop a performance-based seismic design method for full-bridge structure.