Lumber Property Relationships for Engineering Design Standards

Data from the In-Grade Testing Program for visually graded dimension lumber are used to identify lumber property relationships for engineering design standards. The properties studied are modulus of rupture (MOR), modulus of elasticity (MOE), ultimate tensile stress parallel to grain (UTS), and ultimate compression stress parallel to grain (UCS). The relationships identified between UCS and either MOR or UTS vary little with species, lumber grade, and lumber width. The data show a closer relationship between UCS and MOR than between UTS and MOR. The historical basis for assigning allowable properties to machine stress-rated (MSR) lumber is reviewed. The UCS-MOR and UTS-MOR relationships presented in the paper are shown to be consistent with historical data obtained with visually graded lumber but different from relationships currently assumed for MSR lumber. The effects of species, moisture content, test span, lumber width, and presence or absence of pith on strength property relationships are discussed. Strength property relationships are shown to be a result of inherent growth characteristics of the lumber. Finally, a procedure is provided for estimating coefficient of variation as a function of grade that may be useful in reliability-based engineering design standards.