Study On Seismic Displacement Response Characteristics And Seat Width Of Multi-span Simply Supported Skew Bridges

Lots of simply supported skewed bridges with plate rubber bearings suffered severe superstructures in the Wenchuan earthquake,especially the displacement and even the falling down of the main girders,which further led to a series of consequences such as the collision of structures,the damage of shear keys,the slippage of bearings and so on.From the perspective of unseating prevention,the risk of the falling down of girder can be reduced effectively by setting a reasonable seat width.Chinese criterion mainly refers to the seismic criterion of Japan and the United States when calculating the seat width.However,in the aspect of the layout and forms of bridges,Chinese small and mediumspan bridges are different from those in Japan and the United States.This is mainly reflected in the fact that the plate rubber bearings are not anchored and reinforced concrete shear keys which are used in the transverse direction.etc.Therefore,the seismic displacement response characteristics of the simply supported skew bridges and the reasonable setting of the seat width are also different from those in Japan and the United States.Based on the above reasons,this dissertation first studies the dynamic characteristics of multi-span simply supported skew bridges.Then it analyzes the structural displacement response characteristics research from the characteristics of the structure itself and the ground motion and reveals the mechanism of typical earthquake displacement damage of multi-span simply supported skew bridges.Finally,in order to specifically establish the calculation formula of the minimum seat width of like the bridges above,it uses the quadratic regression orthogonal combination design method to develop a parameter study on the seat width of multi-span simply supported skew bridges.The formula proposed in this dissertation is also be optimized by the Monte Carlo simulation method.The main research contents and conclusions of this dissertation are as follows:(1)This dissertation takes a 5×20m multi-span simply supported skew T-beam bridge as a reference bridge example.Then it studies and establishes its nonlinear finite element analysis models and corresponding linear elastic analysis models with the help of SAP2000,considering some nonlinear factors such as the slippage of plate rubber bearings,the plastic hinge of piers,the collision of adjacent girders at expansion joints and the performance degradation of the reinforced concrete shear keys.To analyze the influence of some parameters such as skew angle,pier height and span on dynamic characteristics of multi-span simply supported skew bridge,linear elastic analysis models are used in this dissertation.The results show that,skew angle,pier height and span have great influence on the natural vibration frequency and vibration mode of the bridges;The vibration forms of skew bridges and orthogonal bridges are obviously different and the former often produce significant coupling phenomenon between translation and torsion;Thanks to the flexibility of plate rubber bearings,the pier height and span mainly affect the numerical value of the natural vibration frequency of the bridges,while they have a slight impact on the vibration mode of the bridges.(2)Considering the bi-direction ground motions,this dissertation uses nonlinear analysis models to discuss the influence of ground motion input direction,bearing friction coefficient,the strength of shear key and ground motion intensity on the seismic displacement response of multi-span simply supported skew bridges.The typical seismic displacement response mechanism of multi span simply supported skew bridge is revealed from three aspects of uneven collision at expansion joints,the slippage of bearings and the performance degradation of the shear keys The results show that,the displacement response of longitudinal direction is more sensitive to the change of seismic input direction than that of transverse direction;The friction coefficient of bearings has little influence on the displacement of main girders and shows different influence rules in the longitudinal and transverse directions;Improving the strength of shear keys can effectively limit the transverse displacement of main girders;With the increase of the seismic intensity,the development law of the displacement response of the main girders can basically reflect the whole process of the structure from the elastic state to the ultimate collapse state;Eccentric collision,bearing sliding and the performance degradation of the shear keys are the main reasons for the displacement of the main girders.(3)Different provisions of different countries’ seismic criterions on the seat width are compared and analyzed in this dissertation.It also takes the span,pier height and skew angle as the test factors and then carries out the test scheme design based on the quadratic orthogonal combination test to establish a regression equation between the above parameters and the minimum seat width.The effect of each test factor is discussed in this dissertation.Finally,it uses the Monte Carlo simulation method to optimize the proposed formula.The results show that,span,pier height and skew angle have no obvious interaction effect on the minimum seat width;The influence of span,pier height and skew angle on the minimum seat width and the characteristics of the minimum width law which are varied with those parameters.are different.The optimized formula of the minimum seat width presented in this dissertation has good applicability for orthogonal bridges and multi-span simply supported skew bridges.