Issue 9

Z.H. Qian et alii, Frattura ed Integrità Strutturale, 9 (2009) 105 - 112; DOI: 10.3221/IGF-ESIS.09.11 107 In this paper, the fatigue failures at orthotropic deck bridges are discussed based on the previous investigations. The sensitive connection details to fatigue cracking, rib-to-deck plate, rib-to-diaphragm and rib-to-diaphragm-deck plate, are emphasized through practical cases and numerical analysis. Furthermore, the three improvement techniques, shot peening, fluid bed peening (FBP) and ultrasonic impact treatment (UIT) are detailed presented. This study aims to contribute to the design and reinforcement of orthotropic deck bridges. R IB - TO - DECK PLATE CONNECTION ib-to-deck plate connections are submitted to local transverse bending moments and are therefore susceptible to fatigue cracking. The connections have been studied for a long time, particular in recent years. Fig. 3 shows fatigue cracking at rib-to-deck plate connection [7]. The fatigue tests to rib-to-deck plate connection were carried out by Janss in 1980 in Belgium [8]. Thirty-three small test specimens were manufactured and tested at a frequency of 4Hz. Figure 3 : Fatigue cracks at rib-to-deck plate connection (Xiao, 2008). Through the investigation, it was concluded that the stress range at two million cycles of the transverse stresses at the weld toe in the rib is equal to 80 N/mm 2 when trapezoidal ribs with a thickness of 6mm are welded to deck plates with a thickness of 12mm and the gap between the rib and the deck plate does not exceed 0.5mm. The stress range (80 N/mm 2 ), mentioned above, is certainly a lower limit due to the poor quality of the welds of the test pieces. Based on the investigations of Janss and the others, ECSC research carried out on the optimization of the welding procedure (automatic welding) and the influence of a gap, 0 or 2mm, between the rib and the deck plate [9]. The specimens used in the ECSC experiments were welded with automatic submerged arc welding in an industrial situation. It was found that full penetration welds with a lack of penetration less than 1mm can nearly be achieved without edge preparation. Meanwhile, the fatigue strength significantly increases when using submerged arc welding, which allows larger penetration and larger throat of the weld. With the development of the compute technology, more and more numerical studies are put into practice. Finite element analysis (FEA) provides more results to details compared to the traditional method, such as P-E method. Stress distributions of three different loadcases at the deck plate were shown in Fig. 4 [10] . From the figure, two obvious differences can be concluded. The first one is that the range of high stress is much different, and loadcase1 is larger than the other two. The second one is that both maximal and minimal stresses of loadcase2 and loadcase3 are much higher than loadcase1. The stresses far from the vehicle appear like waves due to the restriction of longitudinal ribs, and are almost zero. Another important point should be noted is that the peak stresses of all these there different loadcases produced near or exactly at the rib-to-deck plate connections. For loadcase1, symmetrical loading, both maximal and minimal stress are exactly at the connections, while for loadcase2 and loadcase3, asymmetrical loading, the maximal stresses are produced at the middle of the rib and maximal negative stresses occur at the rib-to-deck plate connections. This can explain why plenty of fatigue cracks occur at this position in laboratory tests and actual projects. Submodel based on the global FE model is necessary to obtain the accurate results to the fatigue design. Welded joints are not considered in global numerical model, therefore, a submodel is needed to describe the weld geometry influence to orthotropic deck. In comparison, few researches are carried out on the submodel analysis of the rib-to-deck plate connection [7, 11]. High compressive stresses are resulted in the weld toe and root regions, which is due to the large local load used and the abrupt geometry transition modeled in the linear elastic FEA. This analysis may be useful for practical R