A REVIEW OF TRAFFIC LOADS AND STRESSES IN STEEL BRIDGES

This report is a review concerned with traffic loads and the dynamic stresses they cause in highway bridges. Special attention is given to techniques of measurement and to the nature of the load and stress spectra. The review was undertaken as part of a programme of research relevant to wholly steel bridges. The different types of dynamic weighbridge used to measure wheel loads are summarised. These measure total (dynamic plus static) load and must be sited in a smooth straight road to minimise dynamic effects. Units which can be fixed to the road surface as pads are very convenient but not very accurate except as indicators of general trends. Units which are installed in pits can give accuracies of plus or minus 10 per cent. Measurement of dynamic stresses requires considerable care to eliminate features such as noise and temperature drifting. In cases where fatigue is in question it is necessary to measure the full range of stresses. Stress histories can be complex and constituent cycles are counted using a method such as range-pairs or a variation. For lightweight steel decks, stresses close to the deck plate are reduced by the structural action of asphalt surfacings; the degree of reduction is affected by temperature and vehicle speed. Dynamic behaviour of vehicles is considered. Variations of wheel load occur even on a very smooth road surface so that the traffic induced stresses are affected by factors such as vehicle speed, and type of suspension. (Author/TRRL)