Local Dynamic Response Of Highway Bridges To Moving Vehicles

In recent yeas, heavy vehicles have become larger and have increased in number. At the same time, new materials and improved design methods have resulted in lighter and more flexible bridges. Therefore, highway bridges are increasingly susceptible to vibration. Highway bridges typically have natural frequencies in the same range as those of heavy vehicles. Excitation of one system by the other is significant.Bridge-vehicle interaction does not normally cause major bridge failures. But the wheel loads induce local vibration of the bridge components and contribute to fatigue, surface wear, and cracking which leads to corrosion. Thus, dynamic loads continually degrade bridges, and increase the necessity of regular maintenance.Dynamic amplification factors (DAF) defined in codes have classically been derived from the measurement or simulation of global traffic action effects in the main structural elements of bridges. On the other hand, local dynamic effects in some components such as deck slabs, hangers, expansion joints and so on have not been thoroughly analyzed. Aiming at the problem, some works are done in this dissertation.First, a general and efficient procedure is proposed for the solution of the dynamic interaction problem between bridge and vehicle. The dynamic equation of the coupled system is obtained by Component Mode Synthesis (CMS) method. A home-code program VBDIP (Vehicle Bridge Dynamic Interaction Program) and ANSYS are integrated into the procedure. In this way, a sophisticated tool is formed to investigate vehicle-bridge interaction problem.Second, two thin-walled box-girder bridges due to truck loading are analyzed. The finite element models of the box-girder bridges are set up with shell elements. The analytical vehicle is simulated as a 3D linear vehicle model with 7 independent degrees of freedom. Three different classes of road surface roughness generated from power spectral density function for Perfect, Good, and Poor roads are used in the analysis. The parameter analysis was conducted to analyze the effect of factors such as road surface roughness, vehicle velocity, bridge damping, and vehicle rigidity on the bridge DAFs. The results suggest that vehicle velocity is less important than vehicle rigidity and that road roughness is the most important parameter affecting the dynamic behaviour of deck slabs. Higher damping helps the bridge damp out the local vibration effectively. The DAF of horizontal moment may be above that of longitudinal moment in some circumstances. In order to avoid the use of values that are too conservative, the local DAFs should be defined appropriately.Third, a case study has been carried out on a tied arch bridge. And the vibration and fatigue problems of the hangers are studied. A 3D finite element model is built with shell, beam and link elements together for the bridge. The vehicle model and road surface roughness are identical to those used in the analysis of the box-girders. Results of a parametric study are presented, which suggest that road roughness, vehicle velocity, and vehicle rigidity all significantly affect the dynamic performance of the bridge, especially when the road surface condition is poor. The boundary conditions of the hangers contribute little on the response of the hangers. However, in order to eliminate the bending stress in the hangers, hinge devices are recommended. The local DAFs are normally higher than the global ones. The short hangers are more susceptible to fatigue problem than the longer ones. Furthermore, measures must be taken to improve the design of short hangers.Finally, the problems requiring further studies are discussed.