Influence of Weld Sequence on the Seismic Failure of Welded Steel Moment Connections in Building Structures

Abstract

Over a decade of research activities after the Northridge earthquake have developed modified designs of welded steel moment connections (WSMCs) with improved ductility performance. Various modifications have been made, since then, to weld access hole and weld details towards improvement of structural performance with emphasis on global parameters such as drift, strength and stability. However the recent research by Castiglioni demonstrates that new designs of WSMCs may still fail in a brittle manner. While test results on the new connections demonstrate significant improvement with regards to ductility, little work has been done to fully understand the localized failure mechanism of WSMCs. Previously a study at NC State by Lu(2003) demonstrated that the localized failure mechanism and fatigue life of welded piping joints is directly influenced by the welding procedure and welding sequence. Hence this current research makes an effort to investigate the influence of weld sequence on the seismic failure of WSMCs. Further an attempt is made to demonstrate a correlation between the localized failure mechanism and global structural performance. The primary goal of this research is to observe and understand the local behavior of the WSMCs through analysis and a set of structural experiments on specially designed tee-joint specimen and full-scale welded joints. A tee-joint specimen consists of a short section of bottom beam flange welded to the column flange on both ends using a complete joint penetration weld. Further a tee-joint specimen yields two set of data for analysis when compared to that of full-scale joint. Two different types of weld sequences are employed in the fabrication of the test specimens to gain an insight of the effect of weld sequence on the fatigue life of the WSMCs. In addition, the cyclic loading tests performed on full-joint welded steel moment connections with weld sequences similar to that of the tee-joints for understanding the relationship between the local and global responses. Finite element simulations of the full-scale WMSCs are conducted using ANSYS with Chaboche and multi-linear material models. These pretest analyses are utilized to develop the loading protocol for the experimental program. Both the full-joint and tee-joint specimens showed brittle failures when subjected to constant amplitude cyclic loading. It is also observed that the fatigue cracks in all the experiments occurred at the weld toe of the complete joint penetration weld. Recorded strain data from the strain gages located near the complete joint penetration welds demonstrated the presence of ratcheting. This observation is further supported by the symmetric strain response (no ratcheting) in the strain gages located away from the welded joint. This strain ratcheting response may also influence the formation of cracks near the welds leading to the brittle failure of the WSMCs. The two tee-joint and full-scale specimens have shown varied fatigue life indicating the affect of weld sequence used in their fabrication. In conclusion, this research investigates local and global failure responses of welded steel moment connections with different weld sequences.