Key to Adjustment Factors:
Load Duration Factor (CD)
- CD, Load Duration Factor, applies when loads are other than the normal 10-year duration.
- Cr, Repetitive Member Factor, applies to bending members in assemblies with multiple members spaced at maximum 24 inches on center.
- CH, Horizontal Shear Factor, applies to individual or multiple members with regard to horizontal, parallel-to-grain splitting.
- CF, Size Factor, applies to member sizes/grades other than standard test specimens, but does not apply to southern yellow pine.
- CP, Column Stability Factor, applies to lateral support condition of compression members.
- CL, Beam Stability Factor, applies to bending members not subject to continuous lateral support on the compression edge.
- CM, Wet Service Factor, applies where the moisture content is expected to exceed 19% for extended periods.
- Cfu, Flat Use Factor, applies where dimensional lumber 2 to 4 inches thick is subject to a bending load in its weak axis direction.
- Cb, Bearing Area Factor, applies to members with bearing less than 6 inches and not nearer than 3 inches from the members’ ends.
- CT, Buckling Stiffness Factor, applies only to maximum 2x4 dimensional lumber in the top chord of wood trusses that are subjected to combined flexure and axial compression.
- CV, Volume Factor, applies to Glulam® bending members loaded perpendicular to the wide face of the laminations in strong axis bending.
- Ct, Temperature Factor, applies where temperatures exceed 100° F for long periods; not normally required when wood members are subjected to intermittent higher temperatures, such as in roof structures.
- Ci, Incising Factor, applies where structural-sawn lumber is incised to increase penetration of preservatives with small incisions cut parallel to the grain.
- Cc, Curvature Factor, applies only to curved portions of glued, laminated bending members.
- Cf, Form Factor, applies where bending members are either round or square with diagonal loading.
Lumber strength is affected by the cumulative duration of maximum variable loads experienced during the life of the structure. In other words, strength is affected by both the load intensity and its duration (i.e., the load history). Because of its natural composition, wood is better able to resist higher short-term loads (i.e., transient live loads or impact loads) than long-term loads (i.e., dead loads and sustained live loads). Under impact loading, wood can resist about twice as much stress as the standard 10-year load duration (i.e., normal duration) to which wood bending stress properties are normalized in the NDS.
When other loads with different duration characteristics are considered, it is necessary to modify certain tabulated stresses by a load duration factor (CD
) as shown in Table 5.3. Values of the load duration factor, CD
, for various load types are based on the total accumulated time effects of a given type of load during the useful life of a structure. CD
increases with decreasing load duration.
Where more than one load type is specified in a design analysis, the load duration factor associated with the shortest duration load is applied to the entire combination of loads.
For example, for the load combination, Dead Load + Snow Load + Wind Load, the load duration factor, CD
, is equal to 1.6.
TABLE 5.3 Recommended Load Duration Factors for ASD
Repetitive Member Factor (Cr)
When three or more parallel dimensional lumber members are spaced a maximum of 24 inches on center and connected with structural sheathing, they comprise a structural system with more bending capacity than the sum of the single members acting individually. Therefore, most elements in a house structure benefit from an adjustment for the system strength effects inherent in repetitive members.
The tabulated design values given in the NDS are based on single members; thus, an increase in allowable stress is permitted in order to account for repetitive members. While the NDS recommends a repetitive member factor of 1.15 or a 15% increase in bending strength, system assembly tests have demonstrated that the NDS repetitive member factor is conservative for certain conditions. In fact, test results from several studies support the range of repetitive member factors shown in Table 5.4 for certain design applications. As shown in Table 5.2, the adjustment factor applies only to extreme fiber in bending, Fb
TABLE 5.4 Recommended Repetitive Member Factors for Dimension Lumber Used in Framing Systems
With the exception of the 1.15 repetitive member factor, the NDS does not currently recognize the values in Table 5.4. Therefore, the values in Table 5.4 are provided for use by the inspector as an alternative method based on various sources of technical information, including certain standards, code recognized guidelines, and research studies.
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 should typically apply when repetitive framing or built-up members are used. For members with no splits, CH
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% 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 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 dimensional lumber only.
Size Factor (CF)
Tabulated design values in the NDS-S are based on testing conducted on members of certain sizes. The specified depth for dimensional 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
The size of a member affects unit strength because of the member’s relationship to the likelihood of naturally occurring defects in the material. 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.
Column Stability Factor (CP)
Tabulated compression design values in the NDS-S are based on the assumption that a compression member is continuously supported along its length to prevent lateral displacement in both the weak and strong axes. When a compression member is subject to continuous lateral support in at least two orthogonal directions, Euler buckling cannot occur. However, many compression members (e.g., interior columns or wall framing) do not have continuous lateral support in two directions.
The column stability factor, CP
, adjusts the tabulated compression stresses to account for the possibility of column buckling. For rectangular or non-symmetric columns, CP
must be determined for both the weak- and strong-axis bracing conditions. CP
is based on end-fixity, effective length of the member between lateral braces, and the cross-sectional dimensions of the member that affect the slenderness ratio used in calculating the critical buckling stress. Given that the Euler buckling effect is associated only with axial loads, the CP
factor applies to the allowable compressive stress parallel to grain, Fc
, as shown in Table 5.2.
Beam Stability Factor (CL)
The tabulated bending design values, Fb
, given in the NDS-S are applicable to bending members that are either braced against lateral-torsional buckling (i.e., twisting) or stable without bracing (i.e., the depth is no greater than the breadth of the member). Most bending members in residential construction are laterally supported on the compression edge by some type of sheathing product. The beam stability factor does, however, apply to conditions such as ceiling joists supporting unfinished attic space. When a member does not meet the lateral support requirements of NDS 3.3.3 or the stability requirements of NDS 4.4.1, the inspector should modify the tabulated bending design values by using the beam stability factor, CL
, to account for the possibility of lateral-torsional buckling. For glued laminated timber bending members, the volume factor (CV
) and beam stability factor (CL
) are not applied simultaneously; thus, the lesser of these factors applies. Refer to the NDS 3.3.3 for the equations used to calculate CL