Flexural properties of structural lumber products after long-term exposure to high temperatures

Abstract

When wood fiber is exposed to significant heat, its strength decreases. It has long been known that prolonged heating at temperatures over 66°C (150°F) can cause a permanent loss in strength. The National Design Specification (NDS) provides factors (Ct) for adjusting allowable properties when structural wood members are exposed to temperatures between 38°C (100°F) and 66°C (150°F) for extended periods of time. The NDS Commentary further states that the use of structural wood members in applications involving prolonged exposure to over 66°C (150°F) should be avoided. Where such exposures do occur, it recommends that adjustments for both immediate and permanent strength reductions should be made. This study presents results on permanent loss in flexural properties for nominal 2x4 solid-sawn and structural composite lumber heated in air at 66°C (150°F) and 82°C (180°F) at two different equilibrium moisture contents (4% and 12% EMC) for time periods that ranged from a few months to up to six years. The study is still underway. The results confirm that significant loses in bending strength can occur at these temperature and humidity conditions. It is recommended that current design recommendations be updated once this test program has been completed.

Citation

Craig, Bruce A.; Green, David W.; Gromala, David S. 2006. Flexural properties of structural lumber products after long-term exposure to high temperatures. WCTE 2006 : 9th World Conference on Timber Engineering, August 6-10, 2006, Portland, OR, USA : program and abstracts. Portland, Or. : World Conference on Timber Engineering, [2006]: [10] pages.