Contextualizing the Load Duration-Dependent Strength of Wood Members

Strength is defined as the capacity of a material or element to resist load and can be graphically portrayed on a load-displacement curve by the point at which the maximum load is sustained, regardless of the displacement at which it occurs. It has long been recognized that the strengths of structural materials typically exhibit some sensitivity to load duration. Most strength of materials textbooks, for example, touch on the subject of strength versus load duration and indicate that structural materials subjected to very short term loading exhibit strengths that are enhanced relative to slower loading regimens. Less well-recognized is the fact that the strengths of some structural materials under very long duration loading are substantially reduced relative to what are typically taken to be their ultimate capacities, although for years, the wood engineering community has incorporated a factor into the codes that explicitly acknowledges wood's reduced capacity under long duration loads. This paper addresses the often misunderstood phenomenon of load duration effects in wood members.