Which framing material is dimensionally unstable yet can be used structurally?

• A. Wood
• B. Light-gauge metal
• C. Both
• D. Neither

Answer: (A)

Wood is dimensionally unstable because it swells and shrinks with changes in moisture content. It absorbs moisture from the air and expands; when it dries, it contracts. This movement is not uniform in all directions, since wood is anisotropic, so dimensions can change in width and thickness more than along the length.

Despite that, wood is routinely used for structural framing because these movements can be anticipated and accommodated in design and construction. Lumber is dried to a target moisture content and acclimated on site, and joints are detailed to tolerate shrinkage and swelling (for example, allowing for some movement at connections, using appropriate fasteners, and planning for seasonal changes). By accounting for moisture-driven movement, wood serves as a reliable framing material even though it is dimensionally unstable. Light-gauge metal framing, in contrast, is more dimensionally stable with humidity, so it doesn’t fit the given condition as well.