## 1 Introduction  
Steel is widely used for the construction of a large variety of bridges. Welding operations are commonly used to connect the structural members. Residual welding stresses are introduced through this manufacturing process and the influence of these stresses are assumed to be covered by safety factors during the design. Residual stress distributions can have large stress gradients due to their non-uniform behavior [1]. In a single-pass weld connection, residual stresses are the result of the resistance of the weld metal to the contraction of the weld. The presence of residual stresses promotes fracturing through fatigue, hydrogen cracking, and stress corrosion [2]. Numerous structural connections in bridges also require multi-pass welding. Repeated thermal cycling is conducted on the weld metal which causes multi-mode deformation of this weld metal due to thermal straining. The multi-pass weldment introduces variably distributed residual stress fields across the weld and through the thickness [3]. One of the most important failure modes for steel bridges with welded structural members is fatigue failure. Fatigue is a principal failure mode and it is still less understood than any other failure mode such as strength failure or serviceability failure. For the design of new steel bridges,the fatigue-sensitive bridge components require an accurate fatigue design in order to estimate premature fatigue failure more precisely [4]. Residual stresses have an influence on the fatigue strength of the welded joints in bridge structures. Depending upon the sign and magnitude of the residual stresses, the contribution of these stresses to the loading stresses on the structural element can be harmful or beneficial. For example, the fatigue strength can be increased by the presence of compressive residual stresses [5]. Stress corrosion cracking failure can be either accelerated or retarded by the presence of residual stresses [6]. Therefore, accurate fatigue design of fatigue-sensitive bridge components requires knowledge of the distribution of the residual welding stresses. In order to estimate the effect of residual stresses on the design, a thorough understanding of the residual stresses is necessary. Therefore, measurement of residual welding stresses can be performed. Improvement of the fatigue strength can also be realized with mechanical surface treatments such as shot peening or deep rolling [7]. Different methods have been developed to measure residual stresses. There are three main categories in which residual stress measurements can be classified: non-destructive, semi-destructive, and destructive measuring techniques. For both semi-destructive and destructive techniques, the original stress state is inferred by partially or completely relieving the stress through material removal and the deformations caused by this release are measured. Non-destructive measuring techniques measure a parameter that is related to the residual stress without damaging the specimen [8]. X-ray and neutron diffraction are commonly used non-destructive measurement methods. The distance between atomic planes is measured with the aid of electromagnetic radiation. X-ray diffraction measures the spacing between the atomic planes caused by stressing the material. This measurement is used to calculate the total stress in the material. However, X-ray diffraction is not the most practical method to determine residual stresses in large welds since there is only limited space available on most X-ray diffractometers. This requires the material to be cut in smaller parts for the evaluation of the stresses. The geometry of the measured specimen is also of great importance since the X-ray has to hit the measurement area but also has to be diffracted to the detector without interfering with other material [8]. The neutron diffraction method is similar to X-ray diffraction since the distance between atomic planes is also determined to calculate residual stress.