Evaluating bridges as part of the traffic system: a case study

Current bridge design and evaluation techniques are based on many assumptions relating to load and bridge behaviour. The current code makes the assumption that the T44 design truck, dynamic load allowances, lateral position of the loads, fatigue and load factors, are representative of the actual situation. Many other factors affect the applicability of these assumptions to the specific site. In terms of bridge behaviour the code is largely based on linear elastic models of structures where assumptions are made regarding material properties, bearing restraints, slab membrane effects and distribution of loads. These assumptions, in addition to bridge condition information, combine to provide conservative design and evaluation procedures with a perceived level of safety. However, increasing vehicle loads and reducing levels of funding are increasing the need to understand these basic assumptions. Modern instrumentation technology is providing opportunities to understand the behaviour of the traffic/road profile/bridge system. This paper presents the results of measuring the response of the bridge over the South Pine River, to the effects of traffic loading over two 24 hour periods. The analysis of this data enables conclusions to be drawn for the specific site regarding the level of loading, the bridge's fitness for purpose, the distribution of the dynamic loads between girders, the predicted levels of overload anticipated for a given return period and fatigue damage accumulation.