Bridge joint as pavement surface unevenness

Road bridge joint faulting is due to progressive structural decay under the stress produced by heavy vehicles. This faulting creates an unevenness on road surface that is undesirable for two reasons: − the generation of vertical accelerations on the vehicle. These accelerations decrease or cancel the skid resistance between road and tyre, with dangerous consequences on traffic safety; − the generation of impact actions on the joint, due to the punctual unevenness. These actions increase the speed of faulting process. Besides, there are some particular structural conditions that often emphasise the problem, for example: local cracking due to the settlement of subgrade near the shoulder; the discontinuity due to the change of elastic moduli of the different material on the road (asphalt concrete, cement concrete, iron, etc.); the possible differential settlements near the beam heads, even if they are well connected with the ceiling. The vehicle-bridge interaction can be studied by means of numerical models using the pavement profile data. The moving vehicle can be represented with an array of rigid bodies characterised by masses and suspensions. These are represented with simple or combined rheologyc schemes. The application of the models strongly depends on the exact knowledge of road surface including the bridge joint. The coming vehicle already has vertical oscillation motions that can be amplified when it pass through the joint. As the speed increase, as an elevation change in a short length produces a bump action. The valued displacements and stresses are much greater than the equivalent static values. Therefore even if there are not specific standards about this topic, the design of a joint without considering the traffic loads dynamic effects underestimates the real service condition and the fatigue resistance. These dynamic effect must be considered also in the bridge structural design. The definition of the dynamic actions between vehicle and bridge is possible also by means of the analysis of the vertical accelerations measured on a heavy vehicle axle running on the joint. This paper deals with the analysis of a case history of a bridge expansion joint. The three- dimensional representation of the pavement surface and the acceleration measurements on a heavy test vehicle were performed to analyze the joint behaviour under traffic.