2001 ASD Supplements - unprotected PDF - American Wood Council

PP-24OTHER CONSIDERATIONS6.1 GeneralWith proper detailing and protection, timber poles andpiles can perform well in a variety of environments. Onekey to proper detailing is planning for the natural shrinkageand swelling of wood products as they are subjectedto various drying and wetting cycles (Table 6.1). Whilemoisture changes have the largest impact on product dimensions,some designs must also check the effects oftemperature as well.In addition to designing to accommodate dimensionalchanges and detailing for durability, another significantissue in the planning of wood structures is that of fire performance.Several facets of this issue are discussed in thisChapter.6.2 Dimensional Changes6.2.1 Approximate Moisture and Thermal Dimensional ChangestDescriptionRadial or Tangential Direction1Dimensionalchange due to moisture content change 1% change in dimension per 4% change in MC2Dimensionchange due to temperature change ≈ 0 x 1062 - in/in per degree F1Corresponding longitudinal direction shrinkage/expansion is about 0.1 to 0.2 percent.2Corresponding longitudinal direction coefficient is about one-tenth as large as radial and tangential.6.2.2 Equations for ComputingMoisture and ThermalShrinkage/Expansionwhere:M o= initial moisture content % (M o≤FSP)M = new moisture content % (M≤FSP)Due to Moisture ChangesFor more precise computation of dimensional changesdue to changes in moisture, the change in radial (R), tangential(T), and volumetric (V) dimensions due to changesin moisture content can be computed as:X = X ( ∆MC)ewhere:oX oME= initial dimension or volumeX = new dimension or volumeFSP = fiber saturation pointValues for e ME are tabulated in Table 6.1.Due to Temperature ChangesFor more precise computation of dimensional changesdue to changes in temperature, the shrinkage/expansionof solid wood including lumber and timber can be computedas:X = X o( ∆Te ) TE∆MC = moisture content change (%)where:e ME= coefficient of moisture expansion linearX o= reference dimension at T oand:(in./in./%MC), or volumetric (in. 3 /in. 3 /%MC)X = computed dimension at TT o= reference temperature (°F)∆MC = M - M oT = temperature at which the new dimension iscomputed (°F)e TE= coefficient of thermal expansion (in./in./°F)AMERICAN WOOD COUNCIL