Characteristic dynamic traffic load effects in bridges

Abstract

When formulating an approach to assess bridge traffic loading with allowance for Vehicle-Bridge Interaction (VBI), a trade-off is necessary between the limited accuracy and computational demands of numerical models and the limited time periods for which experimental data is available. Numerical modelling can simulate sufficient numbers of loading scenarios to determine characteristic total load effects, including an allowance for VBI. However, simulating VBI for years of traffic is computationally expensive, often excessively so. Furthermore, there are a great many uncertainties associated with numerical models such as the road surface profile and the model parameter values (e.g., spring stiffnesses) for the heavy vehicle fleet. On site measurement of total load effect, including the influence of VBI, overcomes many of these uncertainties as measurements are the result of actual loading scenarios as they occur on the bridge. However, it is often impractical to monitor bridges for extended periods of time which raises questions about the accuracy of calculated characteristic load effects.Soft Load Testing, as opposed to Proof Load or Diagnostic Load Testing, is the direct measurement of load effect in bridges subject to random traffic. This paper considers the influence of measurement period on the accuracy of soft load testing predictions of characteristic load effects, including VBI, for bridges with two lanes of opposing traffic. It concludes that, even for relatively short time periods, the estimates are reasonably accurate and tend to be conservative. Provided the data is representative, Soft Load Testing is shown to be a useful tool for calculating characteristic total load effect.

Vehicle-Bridge Dynamic Interaction Using Finite Element Modelling

“…Thus, bridge codes recommend higher dynamic allowances for shorter spans. Nevertheless, a number of authors have shown that the sitespecific dynamic amplification derived from VBI simulation models and field data can represent a significant reduction with respect to these recommendations and save an existing bridge from unnecessary rehabilitation or demolition (Gonzalez et al, 2008a;OBrien et al, 2009). …”

Direct field measurement of the dynamic amplification in a bridge

“…In particular, it can be seen that the 3 rd , 4 th and 5 th largest static strains exceed the 3 rd , 4 th and 5 th largest total strains. It should also be noted that there is a general trend of decreasing dynamics with increasing static load and this is expected [15,16].…”

“…OBrien et al [15,16] suggest that DAF does not recognize the reduced probability of both maxima occurring simultaneously, i.e., static load effect and dynamic amplification. They point out that the maximum values of DAF tend to result from lighter vehicles and are not relevant when seeking characteristic maximum values and propose the concept of Assessment Dynamic Ratio (ADR).…”

A State of the Art Review of Modal-Based Damage Detection in Bridges: Development, Challenges, and Solutions

“…Furthermore, Brady et al [94] observed that the amplification of frequency response due to a single passing vehicle is considerably more than for two vehicles simultaneously passing in opposite directions, thereby indicating the influence of vehicle direction. In a subsequent study, O'Brien et al [95] allocated the cause of this phenomenon to single vehicle crossing behaviour mirroring a bridge's first mode shape pattern, thus amplifying the dynamic response. Other, more complex bridge traffic crossing events, where vehicle locations and velocities are such that they induce dynamic amplification, are much less probable.…”