Residual Stresses in Welded Moment Frames and Implications for Structural Performance

Abstract

Welding-induced residual stresses are usually regarded as secondary stresses and therefore not accounted for in the structural analysis. This treatment, however, was brought into question with the welding research community by the discovery of a widespread structural damage in the 1994 Northridge earthquake. Brittle fracture was identified in many of the prequalified weld joints in steel moment frames. Welding-induced residual stresses are believed to be one of the factors contributing to the brittle fracture. In this paper, a detailed investigation on residual stresses in welded moment frame connections is presented. Both the actual connection configurations of the moment resistant frames and local weld details are considered by means of advanced residual stress analysis techniques. Then, the effects of triaxial residual stress state on plastic deformation capacity are discussed based on finite-element results on a wide-plate specimen loaded in tension. Finally, a series of fracture mechanics analyses were carried out to assess the effects of residual stresses on the fracture behavior. As a result, a great deal of insight has been obtained on the fracture behavior of the moment frame welds. Among other things, the results indicate that welding-induced residual stresses significantly increase the fracture driving force due to the presence of high tensile residual stresses. The high triaxial residual stress states identified in these joints can greatly reduce the plastic deformation capability, consequently promoting brittle fracture. Therefore, it seems reasonable to postulate that weld residual stresses could have played an important role in the occurrence of brittle fracture in the pre-Northridge weld joints.