Keywords: critical plane, damage, fatigue, prediction, structural life, welds.

Steel trusses are commonly used for railway bridges. When the chord members of a steel truss are jointed together at the gusset plate connections by welding, the welding process may lead to a complicated stress state at the joint. Practical projects have shown that fatigue cracks may occur at these welded joints due to the long-term dynamic loading of moving trains [1]. This may endanger the safety of the bridge if the fatigue cracks cannot be detected and repaired timely.

There are three different methods for the fatigue strength analysis for the welded structures of steel truss bridges [2]: the nominal stress assessment method, the local stress-strain method, and the fracture mechanics method. The nominal stress assessment method is widely used for the high-cycle fatigue, the local stress-strain method is good for the low-cycle fatigue, and the fracture mechanics method is effective for the welded structures with initial defects and cracks. Because the stresses in the steel trusses are normally lower than the yield stress of the steel, the nominal stress assessment method may be suitable. However, in the welded structures, welding defects inevitably exist in welds, and local areas of some major joints are often subjected to the stress concentration and plastic deformation. From this viewpoint, the local stress-strain method may be better.

All these methods ignore the effect of welding residual stresses on the fatigue strength. Although the moving loads are usually lower than load-carrying capacity of the steel truss, the maximum cyclic stress at some major joints may exceed the yield stress of the steel because of the combined actions of the welding residual stresses and the stresses produced by the dynamic loads of moving trains and the self-weight of the steel truss [3]. In addition, stress concentration and plastic deformation may also occur in the welds of these joints. All of these actions may cause the initiation of fatigue cracks at the joints and lead to fatigue failure of the steel truss. Although there are no satisfactory models for predicting the structural life of the steel truss prior to multiaxial fatigue, the critical plane method has been proven to be commonly acceptable for estimating the structural life prior to the multiaxial fatigue damage. This method considers the plane with the maximum weighted-average shear strains as the critical damage plane and uses the shear and normal strains in the plane to establish the multiaxial fatigue damage parameter for predicting the structural life prior to the multiaxial fatigue damage.

This paper proposes a computational method for detecting the fatigue crack initiation at the welds of the major joints of the steel truss by accounting for the welding residual stresses. A finite element method is used to simulate residual stresses due to welding. A critical damage plane method is then developed by combining the effects of the dynamic loads of moving trains (including the self-weight of the truss) with the welding residual stresses to obtain the strain histories for determining the fatigue crack initiation at the welds of the major joints of the steel truss. The method is then applied to the fatigue damage assessment for the Poyang lake steel truss bridge.

1

H. Agerskov, J.A. Nielsen, "Fatigue in Steel Highway Bridges under Random Loading", Journal of Structural Engineering, ASCE, 125(2), 152-162, 1999. doi:10.1061/(ASCE)0733-9445(1999)125:2(152)

2

BSI, "Fatigue Design and Assessment of Steel Structures", BS 7608, London, British Standards Institution, 1993.

3

C. Polizzotto, "On the Conditions to Prevent Plastic Shakedown of Structures", ASME, Journal of Applied Mechanics, 60, 15-19, 1993. doi:10.1115/1.2900739

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