What is steel bridge fatigue

Due to the advantages of light weight, high strength and fast construction, the orthotropic steel bridge deck composed of welding between longitudinal ribs, cross beams and cover plates has been widely used in modern highway steel bridges with large and medium spans at home and abroad. However, due to the local action of the vehicle directly, the stress influence line is short, and several stress cycles will be caused when the vehicle passes by. In addition to the stress concentration of the welded joints and the inevitable welding defects, the steel bridge deck is prone to fatigue cracking.

Steel bridge fatigue is the cumulative damage process caused by the repeated actions of vehicles with various daily loads. Therefore, the load used in the fatigue calculation should be as consistent as possible with the actual situation. This requires on-site traffic investigation to obtain the load of various typical vehicles and their appearance. The relative frequency of, that is, the frequency spectrum of typical vehicle load, and on the basis of the survey combined with the forecast analysis to get the estimation of the annual or daily average traffic volume. Although the load spectrum obtained by the investigation may not include all models, it mainly reflects the typical models with a higher occurrence rate. However, there are generally as many as a dozen types according to the number of axles, wheelbases, and loads. The calculation workload is very large during the fatigue calculation. It is large, so it can be further simplified. The methods are as follows:

(1) A standard fatigue car is used to represent the load spectrum, that is, the fatigue damage caused by various vehicles in the original load spectrum to the steel bridge is analyzed, and a standard fatigue car is extracted based on the most severely damaged vehicle. The load of various vehicles in the original spectrum is expressed by its ratio to the load of the fatigued vehicle. In this way, the stress conditions generated by the various vehicles in the original spectrum can be calculated proportionally through the one-time loading of the fatigue car during the verification. The British BS5400 adopts this simplified method.

(2) Use the load spectra of several model vehicles to represent the original load spectra, that is, the various vehicles in the original spectra are combined and classified according to the same number of axles, and several types of vehicles in each category are separately classified according to the equivalent fatigue damage principle. Effective as a model vehicle.