Development of improved details for unreinforced welded steel moment connections.

Abstract

Unprecedented failures of welded moment connections found in numerous steel moment-resisting frames caused by the 1994 Northridge earthquake raised questions regarding the validity of the design procedures used at the time for this type of connection. The research presented herein focused on improving the inelastic performance of unreinforced welded steel moment connections. Both analytical and experimental studies were conducted to investigate the effects of three parameters: (1) panel zone, (2) weld access hole length, and (3) continuity plates. The analytical studies included elastic and inelastic finite element analyses of exterior moment connections subjected to monotonic loading. The experimental results used in this study were taken from inelastic cyclic tests of twenty-four full-scale connection specimens carried out by other researchers. The results from analytical studies showed that stress distributions and force flows in the connection region are very complex and significantly different from what had been assumed in past design practice. The beam flanges have to carry a considerable shear force instead of the shear tab. This shear force can cause premature fracture of the beam flanges before they can participate in the inelastic response. Guided by the finite element analyses, the new panel zone design shear strength model is proposed. The ultimate design shear strength of panel zones is a combination of the elastic and strain hardening shear stiffness of the column web panel and the elastic bending stiffness of the column flanges. The proposed equations are shown to extraordinarily capture the whole shear response of pane zones as observed in the analytical and experimental results. The panel zone study was then extended to propose a new design procedure for panel zones. The new design procedure includes the use of the proposed ultimate design shear strength equation and the resistance factor of 0.9. The new procedure was proved to successfully provide a balanced inelastic response between beam and the column panel zone. Based on the analytical results, a modified detail of the weld access hole by increasing the length between the column face to the toe of the access hole was proposed. The optimum design equation for this access hole length is presented. A continuity plate design criterion based on finite element analyses was also developed. The required thickness of the continuity plates as calculated from the proposed equation shows a better correlation with the test results. Finally, the weld details of continuity plates to the column are suggested, primarily based on strain gage data and finite element analyses.