Research On Dynamic Pounding Between Girder And Transverse Shear Key Of Bridge Under Earthquake

As a great destructive natural disaster, earthquake has done much serious harm to people's lives and properties. Bridges are important channels of post-disaster rescue operation and reconstruction. The seismic performance of bridges determines whether the rescue personnel and supplies can arrive as quickly as possible or not. Simply-supported beam (plate) bridges and continuous beam (plate) bridges are the most common used, so their seismic performance should be paid more attention. In those bridges, the primary failure leading to collapse is dentified as girders falling which should be avoided. Therefore, many researchers have carried out extensive researches on this subject. But most people pay much attention to the longitudinal girders falling which are in the majority in the girders falling accidents, and develop a lot of prevention measures and equipments. By contrast, studies on the transverse girders falling are very little. Shear keys are the most common transverse prevention measures. Their roles are to avoid girders falling collapse occurring. Shear keys are so important, but disgners have no design codes for reference and only rely on their experience or draw lessons from other projects. Thus, the function that the shear keys play is uncertain during earthquake. Moreover, the pounding between girder and shear key can increase the force on pier, but the specific increase in degree is very difficult to be generally spoken. In order to clarify this issue, some beneficial researches are carried out.This study investigates pounding between girder and transverse shear key of bridges from an analytical perspective. Through the analysis of the simulation and solution of pounding between structures and the discussion of some representative models of contact element approach, a new idea is presented, which uses spring-oscillator model to characterize contact element model. Through solving the equation of motion of spring-oscillator, the cared physical quanlities can be obtained, such as impact force vs. relative displacement, impact force vs. time and relative displacement vs. time, and so on. The improved algorithm is especially effective for solving pounding problem with much energy dissipation. The exact solutions or the approximate solutions that are close unlimitedly to exact solutions can be obtained by the improved algorithm. So, it is very suitable for studying seismic response of bridge considering seismic pounding between girder and shear key. At the same time, the force on shear key can provide data support for the detail design of shear key. Considering the influence of the coefficient of restitution accounting for energy dissipation during pounding and other factors, it is found that there is certain relationship between the maximum impact force and the defromation obtained from Hertz model and those obtained from Hertzdamp model. When other parameters are the same, the relationship only concerns with the coefficient of restitution. One four polynomial is used to fit the relationship, and the result is well.The Hertzdamp model can accurately simulate the pounding process, but as a result of a misalignment existence, the workload will increase greatly, if it is directly used to study seismic pounding between girder and shear key and the transverse seismic response of bridge considering pounding effect. In comparison, Kelvin-Voigt model is more popular because of its simplicity and capacity to consider energy dissipation. But this model has two limitations. One is that two impact objects will contact again after separation; the other is that it is difficult to determine impact stiffness.This study has made certain improvement to this model, and used the Hertzdamp model to calculate the equivalent Kelvin impact stiffness. So the above two limitations can be solved and the calculational accuracy also is guaranteed. On this basis, the calculation methods for seismic response analysis of bridge are derived based on the Kelvin-Voigt model using the central difference method, Newmark method, Wilson-θmethod and SSH method, respectively. The calculation methods can consider the pounding effect between girder and shear key. And the applicability of above four numerical calculation methods in this issue is compared. When the time step is very small, the resultes obtained by the four numerical calculation methods have almost no difference.Using the method and program that established according to the method, the influence of various factors on seismic pounding between girder and shear key and seismic response of bridge is studied, including site types, structure parameters and impact parameters, etc.This study also determines the impact model in which seismic pounding berween girder and shear key is considered and the gap between girder and shear key is filled with rubber bumper, restoring force model and compression stiffness calculation method for rectangular rubber bumper, and derives the numerical calculation procedure. A parameter study is carried out to analyze the aseismic effect of rubber bumper, including rubber damping ratio, hardness, bearing area and thickness. The results show that increasing rubber damping ratio is of advantage to aseismic effect. Rubber hardness affects the aseismic effect through two aspects——it changes the deformation ability and stiffness of rubber bumper. To a certain degree, a high hardness is good, but overhigh hardness will weaken the deformation ability of rubber bumper. Therefore, the rubber with 65 Shore A is recommended in the absence of test data. The influence of bearing area of rubber bumper is similar to the rubber hardness, but the former is more free and flexible in the size selection. The influence of thickness of rubber bumper on the aseismic effect is quite complex. Improper selection may not have the aseismic effect, but also produce disadvantageous effect.Finally, according to the momentum conservation principle and energy conservation principle, the minimum thickness of rubber bumper is given to avoid seismic pounding between girder and shear key. But this value is generally big and difficult to achieve in engineering. In order to enable the findings to directly instruct the design and construction, a conceptual design method and procedure of shear key is proposed to select initial clearance and the thickness of rubber bumper. And it can be realized through a high automatic program based on MATLAB. Designers can select the appropriate combination of initial clearance and the thickness of rubber bumper according to their target. They can also clearly see the difference of pounding effect and seismic response of bridge between this selection and other combination.