Crack Study And Local Fatigue Analysis Of Orthotropic Steel Decks On Bridges

Fatigue problems of highway bridges become more and more prominent because traffic loads have been significantly increased over years. Researches have been conducted on this topic, however, the local fatigue problem has not been well explored. Fatigue cracks were found in a highwany bridge in south, which has been put into service only for about ten years. Such cracks significantly affect the safety and functionality of the bridge, raising a lot of concern from the society.In this paper, first, comprehensive studies were carried out to investigate the causes of the cracks. Cracks of the bridge were carefully investigated. Statistics on the traffic load were conducted, particularly on the heavy trucks. Finite element analyses were conducted by using detailed modeling. Through the finite element analyses, stress distribution of the deck plates was obtained, and effects of stress concentration and residual stress caused by welding were also discussed. Major conclusions obtained are as follows: (1) the cracks at the deck plate are the most severe cracks; (2) The traffic loads are significantly larger than those considered in the design; (3) High transverse stress occurs in the region of weld joints between trough profiles and deck plates, and stress concentration effects in the trough to deck plate connections are significant, especially at the location having crossbeams; High residual tensile stress exists in both trough to crossbeam connections and trough to deck plate connections.Second, taking the bridge as an example, a novel procedure to evaluate local fatigue of highway bridges was developed. The procedure starts from the vehicle survey, acquires the probabilistic distribution of vehicle load data, and simulates the random vehicle traveling by Monte Carlo simulation. A local modification factor was suggested to describe the effects of wheel position on local stress when generating local stress samples, so that the uncertainty associated with vehicle loads was considered. Based on the relationship between equivalent stress range and loading cycles, the fatigue reliability of the bridge was evaluated, and the results agreed well with physical inspection results of the bridge.Third, another procedure was developed to evaluate the local fatigue of the butt joints of troughs of the bridge. Based on the probabilistic distribution of vehicle load and the results of finite element analysis, the stress time history of the butt joints was simulated by Monte Carlo simulation. Stress cycles were counted by using a rain-flow method. Based on the relationship between equivalent stress range and loading cycles, the fatigue reliability of the bridge was assessed, and the results also agreed well with the physical inspection results of the bridge.