Forensic Investigation of a Warehouse Roof Collapse Due to Snow Load

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Eighth Congress on Forensic Engineering

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The warehouse is located in East Multnomah County, a suburb of Portland, Oregon. Building roof dimensions are 384 ft long by 352 ft wide. Roof contains open web steel joists, 30 inches deep, 48 ft long, 8 ft on centers. The structure was designed based on 1997 UBC and completed in 2001. In December 2008, substantial rain and snow fell in the Portland metropolitan area. The accumulation of snow on the roof, according to some eye witnesses, was close to 3 to 5 ft at the parapets and about 2 to 3 ft on the interior sections of the roof. Late December, 2008, 24 roof trusses collapsed. Forensic investigation findings: 18 trusses failed in their connections by weld failure, and 6 by concrete breakout failure. Causes of failure were: snow load that surpassed the design load, wind direction that accumulated snow against the tall parapets (snow drift), construction defects, and each collapsing truss adding more load on the neighboring truss, i.e., a progressive collapse. The snow load requirements by ASCE 7-05 were higher than 1997 UBC, and if the trusses had been upgraded for the higher snow loads, failure most probably would not have occurred. As the requirements for snow load increases in subsequent building codes, it is recommended that the structural engineer of record advise the building owner that the structure be evaluated for the anticipated increased loads.


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