Practical Suggestions for Evaluating Existing SMRF Structures

Since the January 17, 1994 Northridge earthquake, practicing engineers, academics, metallurgists, and inspectors alike have been engaged in uncovering the cause of cracking in welded beam-to-column joints in over 250 steel moment resisting frame (SMRF) structures throughout the Los Angeles area. As a result, thousands of dollars a day are being spent testing, analyzing, inspecting, and repairing steel connections using a variety of sharply contrasting methods and criteria. Despite the Interim Guidelines published by the SAC Joint Venture in 1995, the current approaches within the industry for the evaluation and repair of steel moment frame structures are largely governed by engineering judgement, costs, and tenant relations rather than systematic approach and protocol. As an aid to the structural engineer and building owner, this paper outlines several factors that should be taken into account when determining the scope of investigation for an existing SMRF. The evolution of FEMA 267 guidelines from 1995 to 1999 and the implications for SMRF investigations are discussed. Practical insights regarding the various components of an existing SMRF evaluation, such as connection preparation, visual inspections and ultrasonic inspections are discussed to equip the engineer and building owner for better decision making. Drawing on the lessons learned from investigating damaged SMRFs throughout Los Angeles, the authors discuss the differences between a post-Northridge initial evaluation with that of a more costly, yet complete investigation.