Effect of boundary conditions on dynamic behaviour of bridges

A moving vehicle, owing to its vibration and mass inertia, has significant effects on the dynamic response of structures. Most bridge codes define a factor called the dynamic load allowance, which is applied to the maximum static moment under static loading due to traffic load. This paper presents how to model an actual truck on bridges and how the maximum dynamic stresses of bridges change during the passage of moving vehicles. Furthermore, an algorithm to solve the governing equation of the bridge simultaneous with the equations of motion of an actual European truck is presented. Subsequently, 32 dynamic analyses of different bridges with different spans, road profiles and boundary conditions are performed and critical influential speeds are obtained. Results, based on these analyses, illustrate that, depending on the bridge boundary conditions, the speed of the moving truck has a significant influence on the entire structural dynamic response. Using the finite-difference method, it is shown that, when vehicle speeds are considerably higher, for example in the case of high-speed railway bridges, current design codes may predict dynamic stresses lower than actual stresses; therefore, the consequences of a full-length analysis must be utilised to design safe high-speed train bridges.