2001 ASD Supplements - unprotected PDF - American Wood Council

GL-56OTHER CONSIDERATIONS6.1 GeneralThe section contains information concerning physicalproperties of glued laminated timber members. Otherdesign considerations, such as fire protection, are alsogiven.6.2 Specific GravityTable 6.1 provides specific gravity values for someof the most common wood species used for glued laminatedtimber. These values are used in determining variousphysical and connection properties. Further, weight factorsare provided at four moisture contents. When thecross-sectional area (in. 2 ) is multiplied by the appropriateweight factor, it provides the weight of the glued laminatedtimber member per linear foot of length. For othermoisture contents, the tabulated weight factors can be interpolatedor extrapolated.Glued laminated timber members often are manufacturedusing different species at different portions of thecross section. In this case the weight of the glued laminatedtimber may be computed by the sum of the productsof the cross-sectional area and the weight factor for eachspecies.Table 6.1Average Specific Gravity and Weight FactoraWeight Factor bSpecies Combination Specific Gravity a 12% 15% 19% 25%California Redwood (Close Grain) 0.44 0.195 0.198 0.202 0.208Douglas Fir-Larch 0.50 0.235 0.238 0.242 0.248Douglas Fir (South) 0.46 0.221 0.225 0.229 0.235Eastern Spruce 0.41 0.191 0.194 0.198 0.203Hem-Fir 0.43 0.195 0.198 0.202 0.208Red Maple 0.58 0.261 0.264 0.268 0.274Red Oak 0.67 0.307 0.310 0.314 0.319Southern Pine 0.55 0.252 0.255 0.259 0.265Spruce-Pine-Fir (North) 0.42 0.195 0.198 0.202 0.208Yellow Poplar 0.43 0.213 0.216 0.220 0.226Specific gravity is based on weight and volume when oven-dry.b Weight factor shall be multiplied by net cross-sectional area in in. 2 to obtain weight in pounds per lineal foot.6.3 Dimensional Changes Due to MoistureDue to the hygroscopic nature of wood, it changesdimensions as its moisture content is altered below thefiber saturation point. For most species the longitudinalshrinkage of normal wood drying from fiber saturationpoint to oven-dry condition is approximately 0.1 to 0.2percent. However, certain atypical types of wood mayexhibit excessive longitudinal shrinkage and these typesshould be avoided in use where longitudinal stability isimportant.The change in radial (R), tangential (T) and volumetric(V) dimensions are computed as:where:X= X o( ∆MC)e ME[6.1]X oe ME= initial dimension or volumeX = new dimension or volume= coefficient of moisture expansion (in./in./%MCfor linear expansion, in. 3 /in. 3 /%MC forvolumetric expansion), as given in Table 6.2∆MC = moisture content change (%), as defined asfollows:APA – The Engineered Wood Association