Performance-Based Design Method And Effect Of Environmental Condition On Earthquake Response For Seismically Isolated Bridges

Many bridges have suffered severe damage during strong earthquakes. Seismic isolation is an effective technology for improving seismic performance of bridges in highly seismic region. However, the research and application of seismic isolation for bridges in China is still limited, and there is a large developmental space in the designing concept, idea and method for isolated bridges. To develop the performance-based seismic design theory for isolated bridges, the displacement-based design method considering controlling of pier damage, and the structural fuse-based design method or functional segregation-based design method, are proposed. The main work and results are summarized as follows:(1) The seismic behavior of isolated bridges during the past earthquakes were summarized. And special attentions were paid on the behavior of isolated bridges and bearings during 2008 Wenchuan earthquake and 2011 great east Japan earthquake. The lessons and some issues from past earthquakes were summarized, including designing scheme of isolated bridges, design method and detailing requirements of seismically isolated bridges, challenge in design of isolated bridges in near fault zones, and durability of seismic isolated device. And a comparative study about the design codes for seismic isolation in China, Japan, and USA, is made.(2) Based on the guide specifications for seismic design of bridge and isolation devices in China, a displacement-based design method for medium span length highway bridges with seismic isolation is developed. First, the methods for choosing appropriate size of isolated bearing from the technical standard and estimating the design target displacement are proposed. Second, the displacement distribution of pier and bearing for one pier-bearing-deck system is carried out according to mechanics characteristic of system in series, and the pier-bearing-deck system is replaced by a single degree of freedom (SDOF) system by the equivalent linearization method. Finally, the method involves iteration until the target displacement and displacement demand are achieved convergence. The proposed method allows the piers to yield, but the damage index is strictly controlled to ensure the post-earthquake performance of bridges. Taking 36 bridges with different layout and different span length for examples, the validation studies of the feasibility and effectiveness through nonlinear time-history analyses are conducted.(3) To improve the seismic performance of bridge in transverse direction, the bridge bents by using buckling-restrained brace (BRB) was proposed based-on the structural fuse concept. The relationship between length of the BRB core and shear span ratio, the ratio of intercolumniation and diameter of the pier is derived. The inelastic response spectrum including structural fuse is developed. And the constant ductility inelastic spectra of strength redution factors and ductility demand spectra for structures with fuse are studied. Many parameters and aseismic ratio of BRB are investigated. The displacement-based design method of bridge bents with BRB is developed. A bigger ratio of yielding force of BRB and bridge bent and a smaller BRB yielding displacement, would lead to a better the seismic performance of bridges with BRB as structural fuse.(4) For the issue of isolated bridges under near-fault ground motions with velocity pulse-like effect, a new isolated system including steel energy absorbers and cable restrainers for high-speed railway bridges, or lead rubber bearings (LRBs) and cable restrainers for highway bridges, was proposed. The steel energy absorbers or LRBs can reduce the seismic response of bridges due to their hysteretic dissipation capacity; the cable restrainers were substantially effective in controlling relative displacements between girder and piers. Under the framework of performance-based design, the design methods for steel energy absorbers, LRBs, and cable restrainers were introduced under low-level earthquake, design earthquake and high-level earthquake, respectively. And the isolated design for high-speed railway bridges and highway bridges near fault zones has been achieved.(5) To count for the effects of aging, ambient temperature, and scragging (which are collectively called "aging and environmental conditions") on mechanical properties of bearings and seismic response of isolated bridges, the property modification factors, system property adjustment factors and bounding analysis are introduced based-on the previous research. Combined with characteristics of the rubber bearing in China, the modification procedure to consider the effect of aging and environmental conditions on mechanical properties of the bearing is suggested. The seismic response of isolated bridges under different ground motion intensity is analyzed using incremental dynamic analysis (IDA). The displacement response of varied height piers and its discrete state in different bridges are investigated. The analysis results indicated that the seismic response of isolated bridges is susceptible to influences of aging and environmental conditions, especially for lower piers when the isolated bridges are under the ground motion of PGA from 0.2g to 0.4g; If the effect of aging and environmental conditions are not considered, the displacement demand of lower piers was underestimated by about 10%,50%,100%and over, when the LRBs, HDRBs, SHDRBs are adopted in the isolated bridges.(6) According to the damage characteristics of skew bridges during earthquakes and the variation of stiffness and strength of rubber bearings under low temperatures, the seismic behavior of a skew bridge isolated with lead rubber bearings (LRBs) was studied. By investigating the relationships between seismic response of piers and skew angles, and various ambient temperature, it is shown that the seismic response of isolated skew bridge is increased due to mechanical properties of LRBs changed by low temperature. If the effect of low temperatures is not considered, the shear strength and bending moment of piers was underestimated by about 10%,20%and 40% when the isolated bridge at low temperatures of 0℃、-10℃ and -30℃ than the case at nomal temperature.