Analysis of the influencing factors of dynamic loads acting on the operating systems of beam balanced bascule bridges

There are functional movable bridges in The Netherlands dating since the 1950s. If these bridges were to be reassed based on the current design code for movable bridges, then more than 90% will not meet the exigencies stated there. However, if these bridges were to be reassed based on field measurements, then this percentage will be lowered considerably. Then why should existing bridges be reassed with the rules for new bridges? This research intends to lower the conservativeness of the current design codes based on an analytical approach, by including both the theoretical considerations behind the standards and the real data given by field easurements of an existing movable bridge. The aim is to create a dynamic model which reduces the gap between theory and reality. Since the available measurements were performed on a beam balanced type of movable bascule bridge, only this type is analysed. The model is created for a straight rack type of mechanism, and the parameters are afterwards adjusted so that the behaviour of a curved rack type of operating mechanism is simulated properly. The bridge is modelled by means of equations of motion which are intended to simulate its behaviour in two situations: a complete opening cycle of the bridge and an emergency brake. The entire structure is considered as a three degrees of freedom model, interconnected by spring-dashpot elements that consider the stiffness and damping of the mechanical components of the bridge. The results are obtained at the level of the electro-motor, the movable deck and the balance part which includes the counterweight. The model predictions are obtained in terms of angular displacements, angular velocities and angular accelerations of the deck and/ or the balance part and the electro-motor torque. When the results obtained for the deck and balance part are compared with the same variables obtained from either the design standard or the measurements, there is an almost perfect match. The model predictions are always below the theoretical values, showing that the code is indeed conservative regarding these parameters. Moreover, the model predictions are also close to the measured variables, proving that the model is closer to the reality. The outcome in terms of the electromotor torque cannot be properly verified. The data from measurements is insuffiecient and characterized by some uncertainty, thus pertinent conclusions in terms of the electro-motor torque cannot be drawn. The only viable conclusion observable for the electro-motor torque is that the predictions are always below the design electro-motor torque, proving again the conservative nature of the standard with regard to this variable. The research proved to be accurate in modelling the behaviour of the bridge structure, including the rack, the movable deck, the hanger, the balance beam and the counterweight, but the model can be improved further on the side of the electro-motor. Recommendations are given at the end in terms of additional modelling procedures, other measurements that should be performed and more components that could be accounted for.