A Detailed Appraisal Of The Strength Of TraditionalTimber Truss Eaves Joints

Abstract

The structural engineer working on the refurbishment of buildings with traditional timber joints is often faced with the task of assessing the strength of the truss in its present condition, with possible damage or deterioration from years of service. Current practice is to avoid replacement or strengthening where possible (Hume, 1991). Modern codes of practice do not refer to the strength of traditional joints, and simple methods of equilibrium are unduly conservative. A study was undertaken to investigate the strength of two commonly occurring types of eaves joint, using both laboratory tests on scaled joints, and computational analyses of detailed stress distributions in the timber. Materials tests on the shear capacity of timber showed that the along-grain shear strength is not single-valued but is best defined by a Mohr-Coulomb model, and that measured values defining a shear envelope were considerably in excess of values recommended by British Standard 5268 (1990). The computational analysis indicated that the stresses along a plane of shear failure in an eaves joint was highly non-uniform. Close correlation was achieved between the failure loads of several scale model truss joints and detailed finite element analyses of stresses along and normal to the failure plane combined with the Mohr-Coulomb envelopes. Introduction For several hundred years, the construction of joints in timber roof trusses followed a traditional pattern based upon notches, tenons, laps, dovetails, Transactions on the Built Environment vol 15, © 1995 WIT Press, www.witpress.com, ISSN 1743-3509 286 Architectural Studies, Materials & Analysis scarfs and dowels (Harris, 1978, and Hewitt, 1980). The carpenter sized the timbers and selected the joint type and proportions by experience. No structural analysis was conducted, because the mathematical skills were not generally available, and the loads and timber strengths were completely unknown. Despite this situation, failures were rare because the timber sizes were chosen to be generous, and the craft guilds ensured apprenticeships for trainee joiners, so that safe practices were handed down. In addition, the requirement of adequate stiffness to limit deflections led to the use of heavy sections. In the present day, the structural engineer may need to assess, in building refurbishment, the factor of safety of timber trusses which have deteriorated with time, and which may be required to carry additional loads based upon modern Standards. BS 5268 Section 7.3 (1990), offers some guidance on modern stress graded timbers and gang nail joints, but this is a very different form of construction to the traditional roof truss (Yeomans, 1992). Two types of eaves joint became widely used in roofs of halls and larger houses, and are often to be assessed during refurbishment work. They are the simple notch (sometimes also dowelled, tenoned, or strapped), and the inclined notch (sometimes dowelled or strapped), see figure 1. While the loads impinging upon an eaves joint of a Victorian timber truss may now be readily calculated by the structural engineer, the strength of a notched joint is less definitive. A simple approach would be to assume a shear failure along the grain due to the horizontal component of the rafter force, resisted by an average shear strength of the timber. This, however ignores the detailed mechanics of the joint, and the complex pattern of stresses around the joint. Figure 1 Two types of eaves joint, and the plane of shear failure.