Durability of Structural Lumber Products at High Temperatures. Part I. 66°C at 75%RH and 82°C at 30%RH
Keywords:
Lumber, laminated veneer lumber, laminated strand lumber, modulus of rupture, modulus of elasticity, long-term temperature exposureAbstract
Background. The effect of temperature on properties can be separated into reversible and permanent effects. The National Design Specification (NDS) provides factors (Ct) for reducing properties for reversible effects but provides little guidance on permanent effects.
Objectives. The primary objective of this paper is to evaluate the effect of prolonged heating (permanent effect) on the flexural properties of solid-sawn and composite lumber products exposed at 66°C and 75% relative humidity (RH) and at 82°C and 30% RH. A second objective is to determine how to estimate total effects.
Procedures. Solid-sawn lumber, laminated veneer lumber (LVL), and laminated strand lumber (LSL) were heated continuously for up to 6 years. After each exposure period, the lumber was conditioned to room temperature at the specified RH and then tested on edge in third-point bending. Some lumber was also tested hot at 66°C after 48 h of exposure and after 3 years of exposure.
Results. After 3 years of continuous exposure at 66°C and 75% RH, solid-sawn Spruce-Pine-Fir (SPF) and Douglas-fir retained about 72% of their original modulus of rupture (MOR) and southern pine about 47%. For the first 2 to 3 years of exposure, changes in MOR of LVL were similar to that of solid-sawn SPF and Douglas-fir. After almost 6 years of exposure, SPF retained about 67% MOR and LVL 26% to 49%. The MOR of LSL was more sensitive to duration of exposure than was the MOR of either solid-sawn lumber or LVL, with a residual MOR of 47% after 28 months. After 21 months at 82°C and 30% RH, solid-sawn lumber retained 50% to 55% MOR, LVL 41%, and LSL 45%. For all products, modulus of elasticity was less sensitive to thermal degradation than was MOR.
Conclusions. The effect of temperature on MOR of solid-sawn lumber is independent of grade. Composite lumber is more sensitive than solid-sawn to change in strength due to thermal degradation. The difference in MOR between species and product types may be less at low humidity levels than at high. The total effect of temperature on MOR can be estimated by adding the reversible plus the permanent effects. Available literature suggests that the wood used in attics of residential construction is not likely to experience significant accumulation of exposure at temperatures ≥66°C over the life of the structure.
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