Metal-strip bracing versus diagonal timber bracing in timber trussed tiled roofs

Abstract

Diagonal bracing is a common method used in South Africa for the bracing of timber trussed roofs with spans of less than 9 m. In the typical diagonally braced roof system, the brace is placed on the underside of the compression chord, which is as far as it can be placed away from the battens that brace the trusses not directly connected to the diagonal brace. Some of the stiffness of the bracing is lost because the low torsional rigidity of the top chord as well as the batten to top chord nailing is all part of the structural system. In this paper the authors investigate bracing of the entire roof system and contend that it is better to connect the bracing member closer to the battens as this improves the stiffness and bracing ability of the system. The authors study both the diagonal brace as well as the alternative bracing method that uses criss-cross metal strapping, sometimes called speed brace. The speed brace is nailed to the top of the compression chord and is therefore much closer to the battens. Two different truss spans were studied to ascertain the buckling length of the top chord for both the diagonal bracing as well as speed bracing. By moving the bracing system closer to the battens the authors show that the speed brace gives a stiffer system which thereby reduces the buckling length of the top compression chord.