Investigating and Repairing Wood Bowstring Trusses

Abstract

A number of roof collapses in the Chicago area have been associated with the failure of heavy timber bowstring roof trusses that were constructed from the 1920s through the early 1960s in the Midwest. Based on investigation of collapsed and damaged bowstring trusses, several deficiencies and defects are common to this structural system. Performance problems are attributed to deficiencies in the original design of the trusses combined with damage due to exposure and age. The allowable tensile capacity of wood members was generally overestimated in the original design. Furthermore, unbalanced loads caused by drifting snow was generally not considered. Truss bottom chords and bottom-chord splices fail in tension, and web-member–to–chord connections are prone to fail under unbalanced loading conditions. Methods for investigating, analyzing, and designing repairs for wood bowstring trusses are presented. Building owners and structural engineers should be aware of the unique problems commonly associated with this truss type.

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Practice Periodical on Structural Design and Construction

Investigating and Repairing Wood Bowstring Trusses

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