Seismic Pounding Responses Of Bridges With High Piers

Limited by the hilly landscape, bridges with high piers are mostly adopted in the West. Because of the variation in pier height leading to conspicuous differential structural dynamic characteristics, the propensity to damage due to pounding at expansion joint greatly increases at the time of earthquakes. Numerous evidences indicated that pounding might cause breakage of girder, peeling of abutment breast wall concrete, destruction of expansion joint, shear breakage of lower support bolt, and even relative displacement of girder so huge as to cause falling. Therefore investigation of pounding effects bears pragmatic significance to enrich seismic design experience in this regard as well as to minimize pounding damage.This article focuses a typical bridge with high piers, whose nonlinear model was established using MIDAS finite element program, with each component adopting beam element simulation. In particular, the main bridge beam is the box girder, within which vertical prestressed steel strand was established. The approach bridge is composed of five T-shape steel section girders. For each pier, the interior of the unit consists of fibre elements for simulating the nonlinearity of piers. Each support adopted the spring system and delay system simulation. At each expansion joint, five gap elements were inserted to simulate real pounding scenario.Upon modulating model parameters and seismic wave input, the following analysis was conducted:Using El Centro seismic wave, the distance between gap elements was set to be 0.17 m. Nonlinear time-history analysis was conducted on the model based on linearity and the model based on fibre element, respectively;Using El Centro seismic wave, nonlinear time-history analysis was conducted under two separate conditions, one without pounding gap element and the other with 0.17 m separation between gap elements, aiming at comparing the responses of each component with and without pounding effect;Using El Centro seismic wave, nonlinear time-history analysis was conducted while varying the peak acceleration (0.25g, 0.30g, 0.35g, 0.40g, 0.45g) to compare the impact of different values on the pier structure;Using El Centro seismic wave, nonlinear time-history analysis was conducted while varying the distance between gap elements (0.01m, 0.02m, 0.03m, 0.04m, 0.05m, 0.06m, 0.09m, 0.10m, 0.11m, 0.14m, 0.17m) to compare the impact of different expansion joint sizes on pier structure; Using Northridge and Parkfield seismic waves after amplitude modulation, nonlinear time-history analysis was conducted while varying the distance between gap elements (0.02m, 0.04m, 0.06m, 0.08m, 0.10m, 0.14m) to compare the impact of different expansion joint sizes on pier structure under different seismic wave types.Via numerical simulation calculation, the following conclusions are made:Based on linear and nonlinear pier models, respectively, the computational results varied greatly under the same seismic wave input. The analysis showed that the consequent deformity and pounding effect were more close to reality when nonlinear model was adopted;Pounding influenced the upper portion of the bridge greatly. Significant pounding force was generated instantaneously at expansion joints. Amplification of acceleration at either end was obvious. Nevertheless, pounding did not have significant impact on individual pier per se;The variation of peak accelerations had significant effect on the upper portion of the bridge. Since the pounding effect of two expansion joints varied, there must be certain relationship and regularity. It is important to note that the increase in peak acceleration will affect piers directly, potentially leading to or complicating the damage to piers;Under fixed peak acceleration, increasing expansion joint gap size led to similar trend in pounding force. In addition, the number of pounding was reduced. Therefore the size can be increased to the extent where pounding is avoided. In practice however, gap distance is limited to ensure smooth and steady traffic. But reduction in pounding can be achieved by adjusting expansion joint size;Under fixed peak acceleration, the responses of bridge structure varied under different seismic waves. This is due to the seismic response character, the natural characteristics, and the spectrum characteristics of the structure per se.