Strengths and Weaknesses of Wood-Framed Structures

With its significant strength and low weight, wood has been a versatile building material for centuries; however, wood-framing — like many other structural materials — is negatively influenced by a multitude of factors, and as a result, wood-framing in older structures often exhibits distress. Distress in wood structures can often be traced to underestimation of environmental loads such as snow, wind, or rain; decay due to exposure to moisture, fungal attack, or insect infestation; the presence of eccentric loads on connections that were not designed for eccentricity; structural overload caused by inappropriate addition of dead loads; and overestimation of allowable stresses. Also due to the wide variability in the physical properties of wood and due to the large number of repetitive members in a typical wood-framed structure, the laws of probability dictate that a few members may well experience distress over the life of a given structure. Generally, however, although one or several members in many types of wood-framed structures may become damaged, collapse of part or all of the structure remains unlikely due to the repetitive nature of the framing, its relatively light weight, and/or the inherent redundancy of these structures. Consequently, these structures often do not pose a significant life safety hazard even if they do not meet current code requirements; however, a lack of understanding on the part of building owners, building officials, or even engineers can result in overreaction regarding the significance of distress that may occur. This paper sets forth a much needed philosophy for evaluating older wood-framed structures in light of the facts that outright collapses are rare and that the significant redundancy in most of these structures generally provides considerable warning before structural damage becomes life-threatening.