Effects of slip on eccentrically loaded multiple-ply girder timber trusses

Recent roof failures in South Africa, of multiple ply nail plated trusses, have necessitated a rethink about the way in which timber trusses are analysed. The authors believe that the relative slip between plies caused by a torsional moment as a result of eccentric loading is a possible reason for some of the failures. The design assumption in South Africa and America is that all the plies share the applied load equally; Eurocode 5 (BS EN 1995-1-1:2004) does not cover the design of multiple ply trusses; and BS 5268-3:2006 recommends additional load factors to be applied to eccentrically loaded truss components with the outer ply being loaded more than the inner plies, as a result of the torsion induced by an out-of-plane eccentricity. In this paper, a matrix stiffness method is used to show that, by ignoring the eccentric loading and relative slip between the trusses, the member and plate force may be underestimated by a factor in excess of two for a three ply girder truss. A single three-ply girder truss test is used to illustrate the merits of the method. The required load factors calculated also show that the magnification factors in BS 5268-3:2006 are lower than they should be.