Residual welding stress in medium and highly restrained weldments

Restrained welded structures hold significant residual strains that preload the structural system, robbing the structural capacity expected by design. Shrinking weld metal and thermal joint displacement combine to create residual stress in the welded connections. Residual welding stress combines with other processing stresses remaining in the individual members or preassembled components. Whether the combinations of all residual stresses result in a compressive or tensile applied force depends upon the geometry of the structural members and the direction of applied internal loads. Regardless of magnitude, residual stresses become part of the forces applied during service. Safety factors incorporated into structural design, typically 1.65, are thought to address the impact of residual stress. Residual welding stresses in heavy weldments are tensile in all three planes. The reaction of the massive joint against displacement increases the magnitude of resultant tension. Tension along all three axes, triaxiality, promotes a state of plane-strain stress in the weldment. Steels at plane-strain stress, where metal cannot stretch or yield, can fail at loads below those anticipated by design. Triaxial tension reduces the ductility of structural systems. The heat of the welding arc drives residual welding stress. The hottest spot is located immediately beneath the arc and temperatures decrease to ambient temperatures at some distance from the weld. Cooling rates are very fast under the arc but slow in the regions surrounding the arc as the thermal gradient declines. The welding arc furnishes the energy that causes the weld metal to shrink and the surrounding base metal to expand at the same time. Welding Codes identify three joint restraint conditions: low, medium, and high. Production costs increase dramatically as joint restraint increases. No welding Code, however, offers defining attributes for restraint conditions. This thesis evaluates residual strain in a moment-frame while welding the beam-to-column connections. Data obtained during three experiment series describe the welding effects on pinned and fixed joint configurations. Experimental data will characterize joint restraint in medium and highly restrained connections. A structural analysis of an actual large-span truss shows the effect of numerous small displacements at each truss connection node.