Load Duration Effect on Failure of an Overloaded Wood Truss Structure

This paper presents the analysis used to demonstrate how load duration affects the time required to cause failure of overloaded wood structures. A bowstring roof truss of a commercial building partially collapsed two months after remodeling. During remodeling, several industrial HVAC units and a new suspended ceiling were attached to the roof structure. However, failure of the truss did not occur immediately after the introduction of the new loads; it took approximately two months for the truss to collapse. A computer model of the roof trusses was created and a linear-staged analysis was performed. The load duration curve of wood developed by U.S. Forest Product Laboratory was used to determine the time-to-collapse after application of additional loads. The analysis showed it would take approximately two months for the truss to fail. The calculated time-to-failure closely matched the actual time of collapse of the bowstring trusses. Load duration curve shows that when the stress level in a member exceeds 60% of the ultimate strength of wood, failure is imminent.