Review of design methods for round notched timber sections subjected to flexure and shear

Timber is an extremely versatile and relatively inexpensive natural material. It is widely used in the construction of timber structures in Australia. An interesting feature is that girders used in these structures are often round in profile. Round timber girders are usually notched at both ends. Notching (or sniping) of the timber girders at the end support areas is required for seating purposes and to create levelness in the top of the structure. Notching reduces the strength of the girder in the vicinity of the connection due to a concentration of high shear and cross-grain tensile stress at the re-entrant corner. Extensive literature has been published on the design and behaviour of notched rectangular sections, but there is a dearth regarding round sections. Very little research has been undertaken with regard to the behaviour of, and design procedures for, notched round timber sections. The techniques available for design rely on research relating to rectangular sections with area properties from truncated round sections substituted into the equation. Another common assumption in design is that the profile of shear stress parallel to the grain is parabolic. This is not the case, as finite element analysis simulations show high concentrations of both tension perpendicular to the grain and shear stress parallel to the grain occur at the notch corner. Currently asset managers and engineers must rely on patchy and often anecdotal assumptions when considering sniped timber girders. Due to the current lack of knowledge and the state of liability laws, some timber girders are removed from service long before any issues arise. This deficiency in understanding leads to unnecessary and expensive intervention that should not be occurring as timber, if maintained, should last hundreds of years.