2001 ASD Supplements - unprotected PDF - American Wood Council

L-8 DESIGN ADJUSTMENT FACTORS4.1 GeneralTwo approaches may be used to generate memberdesign capacities. With the calculation method, tabulateddesign values are multiplied by adjustment factors andsection properties to obtain member capacities. With thetabulation method, values for a given set of conditionsare presented in the tables.The member selection tables in Chapter 5 include precalculatedASD capacity values for reference conditionsof use. For other conditions, the calculation method is usedrequiring the use of tabulated design values (from NDSSupplement), modified appropriately by the adjustmentfactors discussed in this Chapter.The adjustment factors provided in this Chapter arefor applications outside the reference end-use conditionsor for member configuration effects (i.e., flat use). Adjustmentfactors for the effects of load duration, moisture,temperature, size, flat use, and incising are provided inthis Supplement.4.2 Use of Adjustment FactorsTensionThe design capacities for tension members can becomputed by the following equation:T′ = F ′ Awhere:tT′ is the allowable tension design capacity, F t ′ is theallowable tension design value and A is the cross sectionalarea.The allowable tension design value is computed bymultiplying the tabulated design value by a series of adjustmentfactors.The tabulated design values for F t are provided in theNDS Supplement.and:F ′t = FtCDCMCt CiCFCompressionThe design capacity for compression members canbe computed by the following equation:P′ = F ′c Awhere:P′ is the allowable compression design capacity, F c ′is the allowable compression design value and A is thecross sectional area.The allowable compression design value is computedby multiplying the tabulated design value by a series ofadjustment factors.The tabulated design values for F c are provided in theNDS Supplement.and:F ′c = Fc CDCMCt CiCPC D is 1.0for normal (ten years) load duration, or asshown in Table 4.1 otherwiseC D is 1.0for normal (ten years) load duration, or asshown in Table 4.1 otherwiseC M is 1.0for 19% MC and less, or as shown in Table4.2 otherwiseC M is 1.0for 19% MC and less, or as shown in Table4.2 otherwiseC t is 1.0unless temperatures exceed 100°F for extendedperiods of time, or as shown in Table4.3 otherwiseC t is 1.0unless temperatures exceed 100°F for extendedperiods of time, or as shown in Table4.3 otherwiseC i is 1.0for members that are not incised, or as shownin Table 4.6 otherwiseC i is 1.0for members that are not incised, or as shownin Table 4.6 otherwiseC F is as shown in Table 4.4C F is as shown in Table 4.4AMERICAN WOOD COUNCIL