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Horizontal Shear Factor (CH) Given that lumber does not dry uniformly, it is subject to warping, checking, and splitting, all of which reduce the strength of a member. The horizontal stress values in the NDS-S conservatively account for any checks and splits that may form during the seasoning process and, as in the worst-case values, assume substantial horizontal splits in all wood members. Although a horizontal split may occur in some members, all members in a repetitive member system rarely experience such splits. Therefore, a CH of greater than 1.0 should typically apply when repetitive framing or built-up members are used. For members with no splits CH equals 2.0. In addition, future allowable horizontal shear values will be increased by a factor of 2 or more because of a recent change in the applicable standard regarding assignment of strength properties. The change is a result of removing a conservative adjustment to the test data whereby a 50 percent reduction for checks and splits was applied in addition to a 4/9 stress concentration factor as described in Section 5.2.3. As an interim solution, a shear adjustment factor, CH, of 2.0 should therefore apply to all designs that use horizontal shear values in 1997 and earlier editions of the NDS. As shown in Table 5.2, the CH factor applies only to the allowable horizontal shear stress, Fv. As an interim consideration regarding horizontal shear at notches and connections in members, a CH value of 1.5 is recommended for use with provisions in NDS•3.4.4 and 3.4.5 for dimension lumber only. 5.2.4.4 Size Factor (CF) Tabulated design values in the NDS-S are based on testing conducted on members of certain sizes. The specified depth for dimension lumber members subjected to testing is 12 inches for No. 3 or better, 6 inches for stud-grade members, and 4 inches for construction-, standard- or utility-grade members (i.e., CF=1.0). The size of a member affects unit strength because of the member’s relationship to the likelihood of naturally occurring defects in the material. 5-14 Residential Structural Design Guide Chapter 5 – Design of Light-Wood Framing Therefore, an adjustment to certain tabulated values is appropriate for sizes other than those tested; however, the tabulated values for Southern Yellow Pine have already been adjusted for size and do not require application of CF. Table 5.2 indicates the tabulated values that should be adjusted to account for size differences. The adjustment applies when visually graded lumber is 2 to 4 inches thick or when a minimum 5-inch-thick rectangular bending member exceeds 12 inches in depth. Refer to NDS-S for the appropriate size adjustment factor.