Fatigue in steel highway bridges under random loading

Fatigue damage accumulation in steel highway bridges under random loading i s studied. The fatigue life of welded joints has been determined both exper imentally and from a fracture mechanics analysis. In the experimental part of the investigation, fatigue test series on welded plate test specimens ha ve been carried through. The materials that have been used are either conve ntional structural steel with a yield stress of f(y) similar to 400-410 MPa or high-strength steel with a yield stress of f(y) similar to 810-840 MPa. The fatigue tests have been carried out using load histories, which corres pond to one week's traffic loading, determined by means of strain gauge mea surements on the orthotropic steel deck structure of the Faro Bridges in De nmark. The test series carried through show a significant difference betwee n constant amplitude and variable amplitude fatigue test results. Both the fracture mechanics analysis and the fatigue test results indicate that Mine r's rule, which is normally used in the design against fatigue in steel bri dges, may give results that are unconservative. The validity. of the result s obtained from Miner's rule will depend on the distribution of the load hi story in tension and compression.