Northridge Postscript: Lesson on Steel Connections

by Mehdi S. Zarghamee, (F.ASCE), Principal; Simpson, Gumpertz & Heger Consulting Engineers, Arlington, MA,
Rasko P. Ojdrovic, (M.ASCE), Staff Engineer; Simpson, Gumpertz & Heger Consulting Engineers, Arlington, MA,

Serial Information: Civil Engineering—ASCE, 1995, Vol. 65, Issue 4, Pg. 68-71

Document Type: Feature article


Immediately after the Northridge earthquake in January 1994, the general concensus was that steel-framed buildings performed extremely well. Later, inspectors were surprised to find weld cracks in more than 100 welded steel moment-resisting frame buildings. More damage may be found after several hundred similar buildings are examined. These buildings are all standing, but the potential for catastrophic failure during future earthquakes has prompted the profession to reevaluate the connections and has led Los Angeles to draft an ordinance for the inspection of welded steel moment resisting frame buildings. Simpson Gumpertz and Heger, Arlington, Mass. undertook one such inspection and has designed repairs for a seven-story office building with a welded steel moment frame. Our conclusions from this investigation hold important lessons for the engineering community as it addresses the condition of other steel-framed buildings. Among the key findings: 1.) concealed connection failures that are not detected and repaired could compromise a building's resistance to future earthquakes; 2.) Failure of moment connections may not necessarily be discovered by an inspection of only a handful of connections. A large sample of connections should be inspected to help ensure that connection failures have been detected. 3.) The design of American Institite of Steel Construction-recommended seated beam connections used typically in unbraced steel frames is not suitable for buildings subjected to earthquake loading with cyclic loads and significant joint rotations. 4.) Conventional welded moment connections lack the necessary ductility for the expected magnitude of plastic deformation of moment-resisting steel frames in a design earthquake and, as a result, are not suitable for buildings subjected to earthquake loads. 5.) Ultrasonic testing of groove welds that connect beam flanges to columns does not reliably detect cracks in the welds, especially where the backup bar is in place, and in the zone near the beam and column webs. 6.) The jumbo steel sections typically used in fabrication of of the columns of unbraced steel moment frames have a notch toughness that is not acceptable for buildings subjected to the shock loads that may be imparted by a strong earthquake.7.) The flux-core arc welding process used to field fabricate moment connections of a steel frame building may produce weld metals with relatively low notch toughness.

Subject Headings: Steel frames | Steel columns | Moment (mechanics) | Earthquakes | Bars (structure) | Steel beams | Welded connections | Load tests | Los Angeles | California | United States

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