Seismic Evaluation of Welded Moment-resisting Connections

Abstract

Welded moment-resisting connections are used in the rigid frames of high-rise buildings. Most of the standards specify full penetration butt welds as a suitable type of weld for welding beam flanges to the column flange in seismic areas. New Zealand’s steel design standard is the only standard that allows using fillet welds for the fabrication of welded connections in high seismic zones. However, NZS 3404 standard employs an overstrength factor in designing of fillet welds to assure the reliable performance of welds during earthquake events. Although the previous literature including case history studies of buildings subject to severe earthquakes has shown that fillet welds, designed by the New Zealand standard, perform dependably during earthquakes, the designed fillet weld sizes are larger than those in other standards, particularly for thick beam flanges. Partial penetration butt welds are cheaper than full penetration butt welds and are more cost-effective when compared to large fillet welds. However, partial penetration butt welds are not encouraged in New Zealand seismic design practice. This study intends to remedy the high-cost of fabrication in welded moment connections by examining the feasibility of replacing a complete penetration weld with an effective full penetration T-butt weld. An effective full penetration T-butt weld is a partial penetration butt weld superimposed with two fillet welds. The fillet weld design criteria in different standards are reviewed and the traction stress method is introduced as the most comprehensive approach which considers the angle of shear failure planes and the stress concentration influence on its weld sizing formula. Moreover, the effects related to using different material on the behaviour of welded moment-resisting connections are investigated by replacing carbon-manganese steel with stainless steel. Three large scale T-shaped welded moment connections made by stainless steel laser fused sections and three normal carbon connections fabricated by effective full penetration of T-butt welds are tested under seismic loads. The feasibility of achieving the conditions of effective full penetration T-butt welds is assessed by welding some small-scale mock-up specimens. New conditions and quality control requirements for effective full penetration of T-butt welds are developed by considering the real product situations. The experimental test procedure for determining the shear capacity of fillet welds based on transverse and longitudinal weld shear specimens is explained as well.