Investigation of the suitability of finger jointed structural timber for use in nail plated roof trusses

Abstract

The most common method of roof framing employed by Australian builders in modern construction is the use of pre-fabricated nail plated timber roof trusses. These trusses are predominantly manufactured from structural framing timber limited in length to a maximum of 6 metres. The style and size of houses increasingly preferred by Australian homeowners means that trusses are regularly required to span further than 6 metres. Truss manufacturers therefore use larger or additional nail plates to splice members during fabrication, and the assembly process becomes far more complex. Finger jointing of sawmill off-cuts and other short lengths of timber is a means of manufacturers economically producing timber in longer lengths. This dissertation investigates the suitability of using finger jointed structural timber for the fabrication of nail plated roof trusses. Physical testing and statistical analysis has been used to compare the performance of finger jointed structural timber with standard structural framing timber normally used in truss fabrication. This study involved characterizing the mechanical properties of the timber, as well as assessing the performance of joints including mechanical fasteners. These methods, along with the static modelling of loading situations, were also used to quantify the probability of inducing failures unique to finger jointed timber, during the truss fabrication and erection process. These investigations concluded that finger jointed timber could be produced with equivalent mechanical properties to standard framing timber. Joints manufactured from finger jointed and solid structural timber also exhibited no significant difference in performance. Furthermore, failures unique to finger jointed timber could occur during fabrication and erection, however, the probability of these, under normal use conditions, is generally quite low.