Performace Based Seismic Design Of Small And Medium Span Girder Bridges

Laminated rubber bearing is widely used in highway bridges in China. For installation and construction convenient, laminated rubber bearing was put on the cap beam or pad stone at the top of pier directly, which bridge superstructure and substructure are not bolted with bearings. The Wenchuan earthquake damage showed that, the bridges which using laminated rubber bearing experienced superstructure shift, bearings damaged under earthquake, the damage rate of substructure was lower. To study the damage mechanism and reasonable seismic design methods, aiming at the structure system of small and medium span highway bridges in China, the performance design of the bearing as the fuse element and the method of multilevel fortification and hierarchical energy dissipation were carried out systematically, the main contents and conclusions are introduced as follows:1. Reveals the distribution regularities of bridge seismic damage and bridge structure system, found the reason of lower damage rate of bridge pier under Wenchuan earthquake. The load path of bridge system with laminated rubber bearings is interrupt under earthquake, which reduces the seismic force transmitting to the substructures, bearing plays a role of isolation and the energy dissipation. On this basis, put forward the seismic design methods of small and medium spans bridge considering bearing friction slipping.2. Conducted the experimental study of the seismic performance of laminated rubber bearings under different anchoring conditions firstly. The test results show that, when the laminated rubber bearing was bonded to top and bottom plate, the bearing fracture failure mode was rubber layer tears, and the ultimate shear strain reaches 300%～400%, the bearing shear stiffness hardening phenomenon emerged when load displacement is large. When the laminated rubber bearing was bonded to one plate, bearing force can be divided into two phases:before and after sliding, the sliding characteristics related with vertical load and the type of bearing, and the energy dissipation of laminated rubber bearing considering friction slipping is closed to lead rubber bearing. On this basis, design formulas of equivalent stiffness and equivalent damping ratio of laminated rubber bearing considering friction sliding according to the experiment results are put forward.3. The stiffness ratio control equations of laminated rubber bearing considering friction slipping-pier serial system are established which consider the axial compression ratio, bearing thickness, bearing displacement and the nonlinearity of the bridge piers or bearings, obtains the limiting stiffness ratio value of bearing damaged first prior to pier, also the bridge displacement controllable research of piers according the parameters of excessive displacement limiting structures on laminated rubber bearing is studied, and the reasonable gap value range is determined.4. Based on the energy analysis method, the hysteretic energy dissipation of piers and bearings between the bridges with laminated rubber bearing considering friction slipping and lead rubber bearing were compared and analyzed, the good seismic performance of laminated rubber bearing considering friction slipping is verified, and the factors that influence the seismic performance of the bridge are analyzed. Also, the seismic performance of different components of different bridge modes under earthquake through IDA (incremental dynamic analysis) is studied, and the reasonable bridge damage mode of bridges with laminated rubber bearing was put forward.5. Adopted equivalent linearization concept, the displacement design method of bearing-pier double degree of freedom model which considering friction slipping and multiple lines of bridge performance based seismic design method are established, also the performance objectives and performance requirements of different bridge components under the different level earthquakes were put forward. At last, an example was established to verify the seismic design method of multilevel lines under different level earthquakes.