2001 ASD Supplements - unprotected PDF - American Wood Council

GL-58OTHER CONSIDERATIONSTable 6.3 provides the numerical values for e TE forthe most commonly used commercial species or speciesgroups.Wood that contains moisture reacts to varying temperaturedifferently than does dry wood. When moist woodis heated, it tends to expand because of normal thermalexpansion and to shrink because of loss in moisture content.Unless the wood is very dry initially (perhaps 3 or 4percent MC or less), the shrinkage due to moisture losson heating will be greater than the thermal expansion, sothe net dimensional change on heating will be negative.Wood at intermediate moisture levels (about 8 to 20 percent)will expand when first heated, then gradually shrinkto a volume smaller than the initial volume, as the woodgradually loses water while in the heated condition.Even in the longitudinal (grain) direction, where dimensionalchange due to moisture change is very small,such changes will still predominate over correspondingdimensional changes due to thermal expansion unless thewood is very dry initially. For wood at usual moisturelevels, net dimensional changes will generally be negativeafter prolonged heating.Computation of actual changes in dimensions can beaccomplished by determining the equilibrium moisturecontent of wood at the temperature value and relative humidityof interest. Then the relative dimensional changesdue to temperature change alone and moisture contentchange alone are computed. By combining these twochanges the final dimension of lumber and timber can beestablished.Table 6.3Coefficient of Thermal Expansion, e TE, for Solid WoodsaSpecies Radial (10 -6 in./in./°F) Tangential (10 -6 in./in./°F)California Redwood 13 18Douglas Fir-Larch a 15 19Douglas Fir, South 14 19Eastern Spruce 13 18Hem-Fir a 13 18Red Oak 18 22Southern Pine 15 20Spruce-Pine-Fir 13 18Yellow Poplar 14 18Also applies when species name includes the designation “North.”e TE6.5 Fire ConsiderationsFires do not normally start in structural framing, butrather in the building’s contents. These fires generallyreach temperatures of between 1,290°F and 1,650°F. Gluedlaminated timber members perform very well under theseconditions. Unprotected steel members typically suffersevere buckling and twisting during fires, often collapsingcatastrophically.Wood ignites at about 480°F, but charring may beginas low as 300°F. Wood typically chars at 1/40 in. perminute. Thus, after half an hour’s fire exposure, only theouter 3/4 in. of the glued laminated timber will be damaged.Char insulates a wood member and hence raises thetemperature it can withstand. Most of the cross sectionwill remain intact, and the member will continue supportingloads during a typical building fire.It is important to note that neither building materialsalone, nor building features alone, nor detection and fireextinguishing equipment alone can provide adequate safetyfrom fire in buildings. To ensure a safe structure in theevent of fire, authorities base fire and building code requirementson research and testing, as well as fire histories.The model building codes classify Heavy Timber as a specifictype of construction and give minimum sizes for roofand floor beams.The requirements set out for Heavy Timber constructionin model building codes do not constitute one-hourfire resistance. However, procedures are available to calculatethe glued laminated timber size required for projectsin which one-hour fire resistance is required (see Section16.2 of NDS and AF&PA Technical Report 10 available atwww.awc.org). The minimum depths for selected gluedlaminated timber sizes that can be adopted for one-hourfire ratings are given in Table 6.4 for glued laminated timberbeams.APA – The Engineered Wood Association