Analysis of timber girder trusses for eccentric loading

Abstract

Recent timber trussed roof failures in South Africa, especially in Gauteng, of large-span trusses have necessitated a rethink about the analysis of timber trusses in general, but especially primary girder trusses, which support major loads from secondary trusses. The failures include roofs that collapsed (these having been reported in the press) to roofs where the bottom chord of multiple-ply girder trusses had rotated to such an extent that nail plates on the outer ply failed. These roofs were repaired before collapse could occur. Some of these failures are still under investigation and to divulge their location would be inappropriate. The author has identified eccentric loading as one of the possible reasons for some of the failures. In this paper he investigates all the possible factors that may influence the strength and stiffness of the trusses and he uses these to analyse three different span of girder truss. The author shows that by ignoring the eccentric loading the plate force may be underestimated by a factor in excess of 5. He shows a simple way of determining member and plate forces and suggests that these should be used when girder trusses are designed. This may then reduce the number of failures and should lead to timber roof structures that are more robust and more capable of accommodating erection errors.