Seismic Enhancement of Welded Unreinforced Flange-Bolted Web Steel Moment Connections

Widespread damage to welded unreinforced flange-bolted web (WUF-B) steel moment connections during the 1994 Northridge earthquake led to intensive research study of this connection. Despite the improvements to weld metal and connection details, the post-Northridge WUF-B connection was unable to attain sufficient ductility for use in special moment frames (SMFs). This study presents detailed finite element (FE) analysis of post-Northridge WUF-B connections to better understand the mechanisms which limited connection ductility in laboratory tests. Observations made from the FE analysis led to the development and numerical study of a modified WUF-B connection that combines a new bolted web design with a recently validated technique to promote plastic hinging of the beam away from the connection joint. The proposed connection provides the benefit of reduced field welding and UT inspection without sacrificing connection ductility and seismic performance. Finally, the proposed connection is experimentally validated through full-scale seismic testing. The pilot test specimen exceeded the AISC 341 qualifying 4% interstory drift angle without significant strength loss. No weld or near-weld cracks were observed. Instead, failure of the connection resulted from large local buckling deformation in the plastic hinge away from the welded joint.