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lrfd 025

# lrfd 025 - 32 DESIGN ADJUSTMENT FACTORS 4.1 General Two...

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Unformatted text preview: 32 DESIGN ADJUSTMENT FACTORS 4.1 General Two alternative approaches may be used to generate member design resistance. With the calculation method, reference strengths are multiplied by adjustment factors and section properties to obtain member resistances. With the tabulation method, values which are fully factored and adjusted for a given set of conditions are presented in the tables. The member selection tables in Chapter 5 include pre- calculated LRFD factored resistance values for reference conditions of use. For special design cases, an alternative approach permits the use of reference strength values (from Chapter 3), modiﬁed appropriately by the adjust- ment factors included in this chapter. The adjustment factors provided in this chapter are for applications outside the reference end-use conditions or for member conﬁguration effects (i.e., ﬂat use). When one or more of the specific end—use or member conﬁgura- tion conditions fall outside the range of the reference con- ditions deﬁned in AF &PA/ASCE 16—95 Section 2.5, these adjustment factors shall be used to modify the reference strengths. Adjustment factors for the effects of moisture, temperature, size, shear stress and flat use are provided in this supplement. 4.2 Use of Adjustment Factors Tension The unfactored resistances for tension members can be computed by the following equation: T' = F; A Where T' is the adjusted tension resistance (unfactored), Ft’ is the adjusted tension strength and A is the cross sec— tional area. The adjusted tension strength is computed by multi- plying the reference strength by a series of adjustment factors. The reference values for Ft are tabulated in Chapter 3 of this supplement for all species and grades listed in the 1991 NDS. Fri = Ft (CMCGCzCrtCF) and CM is 1.0 for 19% MC and less, or as shown in Table 4.1 otherwise CI is 1.0 unless temperatures exceed 100°F for ex— tended periods of time, or as shown in Table 4.2 Otherwise Cpt is 1.0 for members that are untreated or preservatively treated in accordance with ap- proved standards for members that are not fire-retardant treated. For fire—retardant treated members, the value of Crt shall be established by the supplier of the treated products or treatment chemicals CIt is 10 CF is as shown in Table 4.3 Compression The unfactored resistances for compression members can be computed by the following equation: #:ch A where P ’ is the adjusted compression resistance (unfactored), F c’ is the adjusted compression Strength and A is the cross sectional area. The adjusted compression strength is computed by multiplying the reference strength by a series of adjust— ment factors. The reference values for Fc are tabulated in Chapter 3 of this supplement for all species and grades listed in the 1 99 1 NDS. Fc' : F,((.‘Mr:,r:,,,c,,ch:p ) and CM is 1.0 for 19% MC and less, or as shown in Table 4.1 otherwise ...
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