Effect of Low Relative Humidity on Properties of Structural Lumber Products

Wood used in industrial settings, and in some arid parts of the United States, may be subjected to very low relative humidity (RH). Analytical models available for predicting the effect of moisture content (MC) on the properties of solid-sawn lumber imply significant strength loss at very low MC. However, these models are generally valid only for MC above about 10%. The objective of this study was to evaluate the flexural and tensile properties of standard 38- by 89-mm (nominal 2- by 4-in.) dimension lumber equilibrated at 15% RH compared with lumber at 65% RH. Testing was done on solid-sawn lumber, laminated veneer lumber (LVL), and laminated strand lumber (LSL). The results of this study indicate that modulus of elasticity (MOE) in transverse vibration was about the same for lumber equilibrated at 15% and 65% RH for solid-sawn lumber and LVL, whereas LSL, at 15% RH was slightly higher in MOE than that at 65% RH. For solid-sawn lumber, the modulus of rupture (MOR) at 15% RH was found to be up to 8% less than that at 65% RH, whereas there was little difference between the two RH levels for LVL and LSL. The ultimate tensile stress of solid-sawn lumber and LVL was found to be about 10% lower at 15% RH, but there was little difference between the two RH exposures for LSL. The models of the American Society for Testing and Materials standard D1990 predicted the average shrinkage of the solid-sawn lumber, but none of the models adequately predicted mechanical properties after equilibration at 15% RH compared with those at 65% RH. Procedures are suggested for estimating allowable properties at 4% MC from currently assigned properties at 15% MC