بررسی اثر وضعیت دهانه برخورد بر پاسخ دینامیکی خودرو در اثر برخورد با پل های جاده ای آهنی

The present study investigates the effect of approach span conditions on a bridge’s dynamic response induced by moving vehicles. After developing a 3D bridge–vehicle interaction model for numerical prediction, a dynamic test on a full scale slab-on-girder bridge is conducted with dump trucks to validate the developed numerical methodology. A wooden plank is used to simulate the large faulting between the bridge deck and the approach slab. With consideration of the road surface profile and approach span condition, the predicted dynamic response of the bridge is compared to the experimental results, and they show a satisfactory agreement. The numerical model is also applied to investigate the effect of the approach span condition on the dynamic behavior of the bridge induced by the AASHTO HS20 truck. A parametric study is eventually conducted by changing the road surface condition and the faulting value. The faulting condition of the approach span is found to cause significant dynamic responses for the slab-on-girder bridges and to have a considerable effect on the distribution of impact factors along the transverse and longitudinal directions. Furthermore, impact factors obtained from the numerical analyses are compared with those values specified in the AASHTO codes.

Vehicle-induced dynamic response of bridges is one of the primary problems concerning bridge engineers. Moving vehicles usually produce larger bridge responses than static vehicles do. Bridge vibrations have become one of the causes of deterioration and reduction in long-term serviceability of bridges, although major bridge failures are not usually caused directly by moving vehicles. The effect of moving vehicles on the dynamic response of bridges is of primary importance in the design of these structures. As they play an important role in highway transportation systems, slab-on-girder bridges raised great interest in studying bridge–vehicle interactions. Extensive researches have been conducted to determine the dynamic behavior for this type of bridges (e.g., [10], [15] and [21]). Previous investigations indicated that the dynamic characteristics of bridges and vehicles, and the road surface condition of approach roadway and bridge deck are important factors for bridge dynamic performance induced by moving vehicles [12]. Among these factors, the vehicle initial condition is an important one that affects the dynamic responses of both the bridge and vehicles [17]. The vehicle initial condition is caused not only by the roughness of the roadway, but also by uneven approach span conditions upon entrance to the bridge. The uneven approach conditions are usually caused by the differential settlement of embankment soil and abutments and/or bridge approach deformation. A bridge approach slab is usually constructed to connect the bridge deck with the roadway. It is intended to provide a smooth transition between the bridge deck and the roadway pavement. However, differential settlement often occurs between the bridge abutment and the embankment soil either because the soil underlying the approach slab consolidates or because the embankment soil materials are compressible and the bridge is relatively rigid. When the soil settlement occurs, the approach slabs of bridges lose their contact and support from the soil, and the slabs will bend and deform in a concave manner [5]. Meanwhile, loads on the slab will also redistribute to the ends of the slab, which may result in faulting (or bump) across the roadway at the ends of the approach slab (Δ3Δ3 in Fig. 1). On the other hand, the expansion joint that connects the approach slab and bridge deck will form a faulting (Δ1Δ1 in Fig. 1) due to the differential settlement of the abutments and/or poor maintenance. Full-size image (27 K) Fig. 1. Illustration of approach span deformation. Figure options When a “bump” forms at the bridge end, repetitive movements of traffic vehicles can deteriorate the expansion joint in turn. Thus, a rough transition region has developed with time in some bridge approaches. Consequently, the vehicle receives an initial disturbance before it reaches the bridge. This initial excitation of the vehicle causes an extra impact load on the bridge and affects the dynamic responses of both the bridge and the vehicle. The development and cause of bump-related problems have been commonly recognized and identified; however according to current literature review [3], [8], [9], [13], [22] and [18], the effect of bridge approach span conditions on the bridge dynamic performance has seldom been studied. The objective of this study is to analyze the possible effect of the approach span deformation on the dynamic behavior of slab-on-girder bridges caused by heavy vehicles moving across the bridge. In order to achieve this objective, a bridge–vehicle coupled model, which takes into account the road roughness and approach span conditions, is first developed. Then, the model is adjusted by using experimental results obtained in a full scale field bridge test. Using this validated model, the dynamic behavior of slab-on-girder bridges with different span lengths induced by the AASHTO HS20 truck is investigated. A parametric study is conducted to investigate the influence of approach span conditions on bridge dynamic response. The distribution of impact factors and load distribution factors is also analyzed and compared with values specified in current AASHTO codes.

A fully computerized vehicle–bridge coupled model has been developed. The methodology is validated by field tests on a typical slab-on-girder bridge. The results from the tests indicate that this vehicle–bridge coupled model is reliable for predicting the dynamic response of bridges induced by heavy vehicles with consideration of road surface irregularities. The dynamic responses of four different slab-on-girder bridges under HS20 trucks are obtained in time domain by using the bridge–vehicle coupled model. Parameters such as the bridge span length, road surface conditions, and especially the faulting values are investigated. Based on the results, the conclusions can be drawn as follows: (1) Initial conditions of vehicles entering bridges excited by a large faulting at the approach slab end have a significant influence on bridge dynamic response. The local unevenness of expansion joints at the approach slab ends tremendously increases the dynamic response of shorter bridges. Moreover, the bridge dynamic response under a large faulting condition is much higher than that under the rough road surface with an ISO defined good condition. (2) The same faulting value has a much larger influence on shorter bridges than it does on longer bridges. Longer span bridges have more time for the initial disturbance to be dissipated and for the vehicle to become more stable before it comes to the mid-span of the bridge. (3) The vehicle bumping caused by large faulting condition excites higher modes in the vehicle, which in turn triggers higher modes in the bridge dynamic response. Among these higher modes, the torsion modes contribute more to dynamic response for exterior girders, which results in larger increase in IMs for exterior girders than that for interior girders. (4) The higher modes not only affect the IMs along transverse direction but also affect the IMs along the longitudinal direction. The higher bending modes of bridges excited by the vehicle bumping, may cause larger IMs at the quarter-span than that at the mid-span, which needs to be noticed in the design and evaluation of prestressed concrete girders at sections with harped strands because this section may be close to the quarter-span. (5) Provided that the faulting condition is not larger than 0.020 m, the impact factors of all four bridges are generally smaller than those computed by the AASHTO specifications (both standard and LRFD codes). However, AASHTO specifications may underestimate the impact factors for these bridges with larger faulting conditions at uneven joints at the bridge ends. This situation should be emphasized in practice, especially in rating existing bridges. (6) The bridges with vehicles moving across them under larger faulting conditions have more uniform LDFs than under static loads. The variations in LDFs for all four bridges are similar. The LDFs are consistently lower than those according to AASHTO specifications.