## 1 Introduction
These methods employ the finite element method and estimate not only the ultimate strength but also the pre- and post-collapse regime behavior. The simulation accuracy of welding-induced residual stress by these methods is relatively satisfactory and mainly relies on the accuracy of modelling of heat source. However, the nonlinear analysis of thermo-mechanic coupling would result in poor computational efficiency; thus, they cannot be applied in engineering designs. According to the major effect of residual stress on structural behavior under external loads, an idealized distribution of residual stresses was presumed [13; 16] based on a large number of experiments and numerical simulations, where the magnitude of the tensile residual stress is assumed to equal the flow limit of the material in HAZ and its width is determined by the thickness of plate and stiffener and the maximum welding heat input in multi-pass welding, and the corresponding residual stresses in compression can be obtained by demanding zero resultant forces. This simplified approach can be easily applied and directly used in design, and they estimate only the ultimate strength of steel structure subjected to compressive load [15]. Other simplified models have also been proposed, and these methods may be less accurate than the Finite Element Method (FEM), while being particularly useful in situations like early design stages, for instance, in the recent studies of [PERSON] and [PERSON] [17] and [PERSON] et al. [18]. The buckling behavior and ultimate strength of stiffened plates are affected by many factors: component scantlings, mechanic properties of materials, initial imperfections, load types and boundary conditions, etc. Many studies have been carried out by scientists and researchers to investigate these factors [19]. [PERSON] and [PERSON] [20] and [PERSON] et al. [21] investigated on the influence of geometry and boundary conditions on the collapse behavior of stiffened panels under combined loads. [PERSON] et al. studied the influence of initial distortion on the structural stress in 3-mm-thick stiffened panels [22]. [PERSON] et al. investigated the influence of crack on the residual ultimate strength of stiffened plates [23]. Among these factors, material is one of the most influential parameters on the ultimate strength. With the increase in ship size and requirement of lightweight structures in recent year, the use of high-strength steels instead of medium strength steel has been increasing for their higher'strength to weight ratio'. The high-strength steels commonly used for merchant ships are Steels S315, S355 and S390, etc. There exist in the literature several studies of the influence of material on the ultimate strength, while most of them focused on the effect of material constitutive models and variation of mechanical properties of particular materials on the buckling behavior [6; 24] and ultimate strength of structures. For instance, [PERSON] et al. determined the ultimate strength of stiffened plates made of thick and high-performance steel by nonlinear FEA, and investigated the probabilistic distribution of ultimate buckling strength [7]. The ultimate strength of stiffened plates of high strength steels is to be systematically studied, and the influence of steel grade on the ultimate strength is to be investigated as well. The objective of the present study is to investigate the effects of welding residual stress and materials on the ultimate strength of stiffened plates fabricated in Steels S235, S315, S355 and S390, for they are the most commonly used steels in shipbuilding, no matter merchant or military vessels. The ninety stiffened plates fabricated with three types of stiffeners calculated in the ISSC'2000 VI.2 benchmark research are employed in the present study. The fabrication-related imperfections, such as initial deflections and residual stresses, are simulated in the numerical models. The Finite Element Method using ABAQUS is verified against the benchmark calculations.