What Is Steel Bridge Fatigue?

The orthotropic steel bridge deck, which is composed of longitudinal ribs, cross beams and cover plates welded together, has been widely used in large and medium-span modern highway steel bridges at home and abroad due to its advantages of light weight, high strength and fast construction. However, because it directly bears the local action of the vehicle, the stress influence line is short. The passing of the vehicle will cause several stress cycles. Coupled with the stress concentration of the welded nodes and inevitable welding defects, the steel bridge deck is prone to fatigue cracking.
Steel bridge fatigue is a cumulative damage process caused by the repeated action of vehicles with various loads in daily life. Therefore, the loads used in fatigue verification should be as consistent as possible with reality. This requires on-site traffic surveys to obtain the loads of various typical vehicles and their occurrence. The relative frequency, that is, the frequency spectrum of typical vehicle load, is combined with prediction analysis to obtain an estimate of the annual or daily average traffic volume based on the survey. Although the load spectrum obtained by the survey cannot include all car models, it only mainly reflects the typical car models with a high occurrence rate. However, there are generally more than a dozen types according to the number of axles, wheelbase, and load. The calculation workload during fatigue verification is very heavy. is large, so it can be further simplified by the following methods:
(1) Use a standard fatigue vehicle to represent the load spectrum, that is, analyze the fatigue damage caused to the steel bridge by various vehicles in the original load spectrum, and extract a standard fatigue vehicle based on the vehicle with the most severe damage. The loads of various vehicles in the original spectrum are expressed by their ratio to the load of fatigue vehicles. In this way, the stress conditions generated by one load of various vehicles in the original spectrum can be calculated proportionally through one load of the fatigue vehicle during the calculation. The British BS5400 adopts this simplified method.
(2) Use the load spectra of several model vehicles to represent the original load spectrum, that is, combine and classify various vehicles in the original spectrum according to the same number of axles, and classify several vehicles in each category according to the equivalent fatigue damage principle. The result is a model vehicle.