Research On The Bearing Width Above Piers Considering Seismic Actions

The girder falling damage of the bridge is common in the devastating earthquake,which is mainly due to the shortage of bearing width above the piers.At present, the provision of "Guidelines for Seismic Design of Highway Bridges" for bearing width in china cites the value of Japan’s new code for seismic design of bridges.Hence,it’s known that the deep study for the bearing width is needed in china.Yet,it should be noted that the calculation formula of the bearing width which gets from the provision is only depended on span,neglecting dynamic characteristic of the whole bridge.In this paper, the relationship between the circular frequency ratio of two adjacent spans and the bearing width was emphasized,obtaining a entirely new formula of the bearing width.Besides that,the correlation between the parameters of the bearing and the bearing width is studied,considering that the bearings of the bridges are used as important components for energy dissipation in seismic design of the bridges.In conclusion,the research results can be concluded:(1)The theoretical research of the bearing width1.A calculation model of bridge is presented for computing the bearing width,and the mass and stiffness matrices for calculating was established. The dynamic response equations can be obtained from the calculation model, and then the equations are disposed by Fourier transformation,i.e,the displacement response equations are transformed from time domian to frequency domain. Hence,the expression of the bearing width in frequency domian can be gotten through these steps.From the random vibration theory,the spatial ground motion model which consists of power spectral density function and coherency function is proposed. The power spectral density function of the bearing width can be gained,combining with the bearing width in frequency domian. According to it,the peak response of bearing width can be solved.2.Using the data processing software "matlab" to integrate the peak response of bearing width,the bearing width can be obtained in different circular frequency ratios of two adjacent spans.Fitting the curve of the value by a least-squares fit,and the approximate equation,i.e,the solution formula of the bearing width is obtained.3.Through discussing and analysing the results of the integrations, some proposals are given:In actual projects,the best site conditions that are chosen should be firm ones, the circular frequency ratio of two adjacent bridges should be above2and the damping ratios of the bridges should be increased.(2)Finite element analysis of the bearing widthIn order to prove the adaptability of the calculation formula, two types of the bridges in the finite element models are established,i.e,the regular and irregular bridges(divided by whether the height of the piers is over30meters).Some conclusions can be gained through doing linear time history analysis on the finite element models:1. It can be found that the bearing width is becoming larger as the design acceleration increases under the motion of the five seismic waves.2.Only if changing the pier height of the model, the needed bearing width doesn’t increase as the height is largened.3.Under the motion of the seismic waves,the bearing width has surpassed the value which "Guidelines for Seismic Design of Highway Bridges" provides as the design acceleration increases. But the bearing width calculated by the solution formula which is proposed in this paper can satisfy the needed value.(3)The research of the bearing performance on affecting bearing widthThe damping coefficient and shear stiffness of the bearing play an controlled role in the seismic performance of bearings.Therefore,carrying on the analysis on these two parameters,some results can be conclude:1.The needed bearing width reduces as the damping coefficient of the bearing is becoming larger.But the descending tendency flattens.2.As the same,the needed bearing width declines as the shear stiffness of the bearing increases.