Wood is one of our most sustainable materials, requiring less energy to produce and deliver to the project location than other materials. Timber structures have generally performed well in earthquakes and wind storms if properly constructed. Timber structures often have a cost advantage over other types of structures. Timber structures can be sustainable, but this advantage is reduced when structures fail prematurely, or need substantial repair or replacement early in its life cycle.
In the design of wood structures, it is often assumed that joints fit together perfectly, or that at least minor fit-up problems have been considered in laboratory testing of dowel type fasteners. There is a difference in the quality of fabrication performed in laboratory conditions versus the quality of fabrication at the job site. Laboratory shops usually have some form of climate control, whereas job sites have significant variation in temperature, humidity and weather. These field conditions affect the wood material, the workman and the tools that the workman might choose to use in the field. Laboratory specimens prepared with a table saw will be different in fit up than specimens prepared at a field location using portable saws.
Dowel-type fasteners (nails, screws, etc.) have traditionally been studied through laboratory experiments, which are often costly to perform in quantities adequate to consider individual component variations in strength. Wood scientists and botanists have studied the behavior of wood at the cellular and macromolecular level recently by using finite element models (FEM), achieving good accuracy. Botanists are concerned with the behavior of the tree as a green plant. Green wood has significantly different strength and elasticity when compared to dry wood with a moisture content of 19% or less, commonly used in construction. This paper uses portions of the botany FEM techniques calibrated to dry wood conditions that are typical of light-frame wood construction.
For this study, different types of fabrication errors are considered in evaluating the strength of the wood joint. Sawcuts can be skewed on the strong or weak axis, or shorter or longer than desired for the intended use. Portable saws also permit “dishing” of a sawcut, where there is some curvature across the sawcut in one or two orthogonal directions. In wood connections, bending of the dowel-type fastener is restricted somewhat by the support of the wood fibers, so that the fastener does not have to support all of the load in flexure between the fastener ends which maintain relative fixity. Where fabrication errors are exhibited, the dowel-type fastener must transfer all transverse loading through mechanical action.
Lessons learned from this study can lead to standards that can reduced defects in the future and to enhancing the sustainable nature of wood construction. This paper will broaden the knowledge of practicing engineers, preparing them for the challenges of the 21st century. It will assist them to be knowledgeable in factors affecting sustainability of wood connections and assist them to be better stewards of materials and the built environment.