2012 ASD/LRFD Manual for Engineered Wood Construction

30 M7: PREFABRICATED WOOD I-JOISTSM7.3 Adjustment of Reference Design ValuesGeneralMember design capacity is the product of referencedesign values and adjustment factors. Reference designvalues for I-joists are discussed in M7.2.The design values listed in the evaluation reports aregenerally applicable to dry use conditions. Less typicalconditions, such as high moisture, high temperatures, orpressure impregnated chemical treatments, typically resultin strength and stiffness adjustments different from thoseused for sawn lumber. NDS 7.3 outlines adjustments toreference design values for I-joists; however, individualwood I-joist manufacturers should be consulted to verifyappropriate adjustments. Table M7.3-1 shows the applicabilityof adjustment factors for prefabricated wood I-joistsin a slightly different format for the designer.The user is cautioned that manufacturers may notpermit the use of some applications and/or treatments.Unauthorized treatments can void a manufacturer’s warrantyand may result in structural deficiencies.Table M7.3-1Applicability of Adjustment Factors for Prefabricated WoodI-JoistsAllowable Stress DesignM r ′ = M r C D C M C t C L C rV r ′ = V r C D C M C tR r ′ = R r C D C M C tLoad and Resistance Factor DesignM r ′ = M r C M C t C L C r K F (0.85) λV r ′ = V r C D C M C t K F (0.75) λR r ′ = R r C M C t K F (0.75) λEI′ = EI C M C tEI′ = EI C M C tEI min ′ = EI min C M C t EI min ′ = EI min C M C t K F (0.85)K′ = K C M C t K′ = K C M C tBending Member ExampleFor fully laterally supported bending members loadedin strong axis bending and used in a normal building environment(meeting the reference conditions of NDS 2.3and 7.3), the adjusted design values reduce to:For ASD:M r ′ = M r C DV r ′ = V r C DR r ′ = R r C DE I ′ = E IK′ = KFor LRFD:M r ′ = M r K F (0.85) λV r ′ = V r K F (0.75) λR r ′ = R r K F (0.75) λEI′ = EIK′ = KLateral StabilityThe design values contained in the evaluation reportsfor prefabricated wood I-joists assume the compressionflange is supported throughout its length to prevent lateraldisplacement, and the ends at points of bearing have lateralsupport to prevent rotation (twisting). Lateral restraint isgenerally provided by diaphragm sheathing or bracingspaced at 16" on center or less (based on 1½" width joistflanges) nailed to the joist’s compression flange.Applications without continuous lateral bracing willgenerally have reduced moment design capacities. Thereduced capacity results from the increased potential forlateral buckling of the joist’s compression flange. NDS7.3.5.3 provides guidance for design of an unbraced I-joistcompression flange.Special Loads or ApplicationsWood I-joists are configured and optimized to actprimarily as joists to resist bending loads supported atthe bearing by the bottom flange. Applications that resultin significant axial tension or compression loads, requireweb holes, special connections, or other unusual conditionsshould be evaluated only with the assistance of theindividual wood I-joist manufacturers.AMERICAN WOOD COUNCIL