Paraic Rattigan

Assessment dynamic ratio for traffic loading on highway bridges

The determination of characteristic bridge load effect is a complex problem. Usually, statistical extrapolation of simulated static load effects is used to derive a lifetime characteristic static load effect. However, when a vehicle crosses a bridge, dynamic interactions occur which often cause greater total load effect. This total load effect is related to the static load effect through a dynamic amplification factor (DAF). Specifications often recommend a conservative level for DAF, based on bridge length, number of lanes, and type of load effect only. Therefore significant improvements in the accuracy of this calculation are possible if a DAF, specific to the considered bridge, is applied. In this paper, the authors develop a novel method that considers site-specific bridge and traffic load conditions and allows for the reduced probability of both high static loading and high dynamic interaction occurring simultaneously. This approach utilises multivariate extreme value theory, in conjunction with stat...

Influence of pre-existing vibrations on the dynamic response of medium span bridges

Critical static bridge loading scenarios are often expressed in terms of the number of vehicles that are present on the bridge at the time of occurrence of maximum lifetime load effect. For example, 1-truck, 2-truck, 3-truck, or 4-truck events usually govern the critical static loading cases in short and medium span bridges. However, the dynamic increment of load effect associated with these maximum static events may be assessed inaccurately if it is calculated in isolation of the rest of the traffic flow. In other words, a heavy vehicle preceding a critical loading case causes the bridge initial conditions of displacement and acceleration to be nonzero when the critical combination of traffic arrives on the bridge. Failure to consider these pre-existing vibrations will result in inaccurate estimation of dynamic amplification. This paper explores these dynamic effects and, using statistical analyses, outlines the relative importance of pre-existing vibrations in the assessment of total traffic load effects.

Bridge Traffic Loading: The Implications of Some Recent Findings

The maintenance of highway infrastructure requires major expenditure in many countries. By minimizing the repair or replacement of highway bridges in particular, this cost can be reduced significantly. Of the two bridge assessment components, loading is more difficult to estimate than strength, due to its more variable nature. Consequently, bridge traffic loading has been an area of intensive research in recent years. Recent research has focused on assumptions inherent in previous work and the results are presented and discussed. In this paper, the latest statistical analyses adapted for use in the bridge traffic loading problem are reported. Comparisons to the previous state of the art are made and it is shown that a revised approach reflects the underlying phenomenon of bridge traffic loading more accurately. A method which is shown to reduce the variability of the statistical extrapolation process is also presented. Of more significance, a statistical approach which joins the dynamic and static effects of traffic loading is presented. An assumption inherent in much previous research in this area is that free-flowing traffic with coincident dynamic effects is more critical than congested traffic (which has practically no dynamic effect) for short- to medium-length bridges. Given that about 90% of bridge stock is of this length, this assumption has critical implications for the expenditure on bridge rehabilitation. A sample application of the proposed statistical method is presented and the results are shown to be of great significance. It is shown that the level of dynamic interaction is not sufficient for freeflowing traffic to govern and that it is congested traffic that may govern the vast majority of bridges. The implications of the cumulative effect of these various findings are discussed with reference to the future direction of research into bridge traffic loading and current practice in bridge assessment for traffic loading.