TMCP steels produced by thermo-mechanical control process are now spot lighted due to the excellent combinations of strength, toughness and weldability. Recently, in Korea, high strength SM570-TMCP steel whose tensile strength is 600MPa has been developed and applied to steel structures due to its excellent nature. But, for the application of the TMCP steel to steel structures, it is necessary to elucidate not only the material characteristics but the mechanical characteristics of welded joints. In this study, high temperature tensile properties of the SM570-TMCP steel were investigated through the elevated temperature tensile test and the characteristics of residual stresses in welded joints of the TMCP steel were studied through the three-dimensional (3-D) thermal elastic-plastic finite element (FE) analysis on the basis of mechanical properties at high temperatures obtained from the experiment. The results are then compared with the conventional quenched and tempered high strength SM570 steel.

In recent years, the replacement repair welding, through which damaged bridges are repaired by replacing of the damaged sections with new steel plates through cutting and welding under in-service conditions, has been highlighted for its brilliant features i.e. it can be achieved without incurring traffic dislocation. However, the validity of the repair method is not fully verified because of its lack of safety and reliability. Moreover, repair method by welding inevitably involves cutting and welding process, and the weldability of the replacement repair welded joints has not yet been clarified. In this paper, the weldability of the replacement-repair-welded joints was investigated in order to improve reliability in the repair welded joints of the aged steel bridge. A Steel plate was extracted from the steel bridge after the period of prolonged service, and was welded with a new plate, using replacement repair welding technique, in order to construct a specimen. Flux Cored Arc Welding (FCAW) process was used to fabricate the double "V" butt joints. Welded joints were characterized on the basis of chemical composition analysis, hardness test and micro-structural analysis.

In recent years, the replacement repair welding, which repairs damaged steel girder bridges by replacement of the damaged sections with new steel plates through cutting and welding under in-service conditions, is spotlighted for its brilliant features, i.e. it can be achieved without incurring traffic dislocation. However, the mechanical behavior of the welded joints under cyclic loads due to the traffic which passes along the girder bridges is not clarified. In this paper, the fatigue strength of the replacement repair welded joints was investigated in order to improve reliability in the repair welded joints. Steel plate was extracted from the old steel girder bridge; then was welded with new steel plate to construct specimen with the replacement repair welded joints. Flux Cored Arc Welding (FCAW) process was used to fabricate the double "V" butt joints. Fatigue tests were conducted using a servo hydraulic controlled 50tonf capacity UTM with a frequency of 5Hz under constant amplitude loading. The fatigue strength of the replacement repair welded joints was analyzed in detail.