Consideration of Wet Service Conditions on the Performance of Bowstring Trusses

This paper presents a study to determine the cause of failure of a bowstring truss roof. This work is the result of a structural engineering investigation that was conducted to determine the cause of failure of a commercial building’s roof structure in Chicago, IL during an unusually snowy and wet winter season in 2011. The subject building was built in the 1930s with prefabricated long-span bowstring roof trusses. The study comprised an engineering analysis to evaluate the roof structure’s capacity to withstand gravity loads in combination with loads imposed by the weather conditions during the days leading up to the failure. Multiple load cases were considered and analyzed to determine the cause of the failure, including the effect of wet service condition. The effect of wet service condition (as defined in National Design Specification for Wood Construction, NDS) on failure of mechanically fastened components in wood bowstring trusses is discussed in detail. Engineering analysis showed that under wet service condition, tensile forces in the truss web members exceeded the ultimate withdrawal capacity of nails holding the top chord laminate assemblage together. The pattern of observed damage which indicated a distinct localized unzipping of top chord laminates at the location of failure was consistent with results of the engineering analysis. This study re-emphasizes the adverse impact of wet service condition on mechanical fasteners in wood construction and re-iterates the importance of proper connection detailing as well as maintenance of water barrier components to prevent moisture from reaching mechanical fasteners in wood structures.