Design values obtained by the methods described in Art. 10.3 should be multiplied by adjustment factors based on conditions of use, geometry, and stability. The adjustments are cumulative, unless specifically indicated in the following.
The adjusted design value Fb for extreme-fiber bending is given by
Load Duration Factor
Wood has the capacity to carry substantially greater loads for short periods of time than for long periods. Design values described in Art. 10.3 apply to normal load duration, which is equivalent to application of full design load for a cumulative duration of about 10 years. The full design load is one that stresses a member to its allowable design value. When the cumulative duration of the full design load differs from 10 years, design values, except Fc for compression perpendicular to grain and modulus of elasticity E, should be multiplied by the appropriate load duration factor CD listed in Table 10.5.
When loads of different duration are applied to a member, CD for the load of shortest duration should be applied to the total load. In some cases, a larger-size member may be required when one or more of the shorter-duration loads are omitted.
Design of the member should be based on the critical load combination. If the permanent load is equal to or less than 90% of the total combined load, the normal load duration will control the design. Both CD and the modification permitted in design values for load combinations may be used in design.
The duration factor for impact does not apply to connections. Load duration factors greater than 1.6 shall not apply to structural members pressure-treated with fire retardants or with waterborne preservatives.
Wet Service Factor
Sawn-lumber design values apply to lumber that will be used under dry service conditions; that is, where moisture content (MC) of the wood will be a maximum of 19% of the oven-dry weight, regardless of MC at time of manufacture. When the MC of sawn lumber or timbers in service will exceed 19% for an extended period of time, design values published in the supplement to the ‘‘National Design Specification for Wood Construction’’ should be multiplied by the appropriate wet service factor listed in Table 10.6. This reduction factor for timber does not apply to southern pine.
MC of 19% of less is generally maintained in covered structures or in members protected from the weather, including windborne moisture. Wall and floor framing and attached sheathing are usually considered to be such dry applications. These dry conditions are generally associated with an average relative humidity of 80% or less. Framing and sheathing in properly ventilated roof systems are assumed to meet MC criteria for dry conditions of use, even though they are exposed periodically to relative humidities exceeding 80%.
Glulam-timber design values apply when the MC in service is less than 16%, as in most covered structures. When MC of glulam timber under service conditions is 16% or more, design values should be multiplied by the appropriate wet service factor CM in Table 10.6.
Design values apply to members used in ordinary temperature ranges. (Occasional heating to 150F is permissible.) Strength properties of wood, however, increase when it is cooled below normal temperatures and decrease when it is heated. Members heated in use to temperatures up to 150F return essentially to original strength when cooled. Prolonged exposure to temperatures above 150F, however, may result in permanent loss of strength. Design values for structural members that will experience sustained exposure to elevated temperatures up to 150F should be multiplied by the appropriate temperature factor Ct listed in Table 10.7.
For visually graded dimension lumber, design values Fb, Ft, and Fc published in the supplement to NDS for all species and species combinations, except southern pine should be multiplied by the appropriate size factor CF given in Table 10.8 to account for the effects of member size. This factor and the factors used to develop size-specific values for southern pine are based on the adjustment equation given in ASTM D1990. These factors, based on in-grade test data, account for differences in Fb, Ft, and Fc related to width and in Fb and Ft related to length (test span).
For visually graded timbers (5 5 in or larger), when the depth d of a stringer, beam, post, or timber exceeds 12 in, the design value for bending for all species should be adjusted by the size factor
Beam Stability Factor
Design values Fb for bending should be adjusted by multiplying by the beam stability factor CL specified in Art. 10.7.2. For glulam beams, the smaller value of CL and the volume factor CV should be used, not both. See also Art. 10.5.6.
Design values for sawn lumber beams adjusted by the size factor Cfu assume that load will be applied to the narrow face. When load is applied to the wide face (flatwise) of dimension lumber, design values should be multiplied by the appropriate flat-use factor given in Table 10.10. These factors are based on the sizeadjustment equation in ASTM D245. Available test results indicate that this equation yields conservative values of Cfu.
When a glulam member is loaded parallel to the wide face of the laminations and the member dimension parallel to the face is less than 12 in, the design value for bending for such loading should be multiplied by the appropriate flat-use factor in Table 10.11.
Repetitive Member Factor
Design values for bending Fb may be increased when three or more members are connected so that they act as a unit. The members may be in contact or spaced up to 24 in c to c if joined by transverse load-distributing elements that ensure action of the assembly as a unit. The members may be any piece of dimension lumber subjected to bending, including studs, rafters, truss chords, joists, and decking.
When the criteria are satisfied, the design value for bending of dimension lumber 2 to 4 in thick may be multiplied by the repetitive member factor Cr = 1.15.
This factor applies to three or more essentially parallel members of equal size and with the same orientation that are in direct contact with each other. Transverse connecting elements may be mechanical fasteners, such as through nails, nail gluing, tongue-and-groove joints, or bearing plates, that ensure that the members act together to resist applied bending moments.
For spaced members, the transverse distributing elements should be acceptable to the applicable regulatory agency and should be capable, as demonstrated by test, analysis, or experience, of transmitting design loads without unacceptable deflections or indications of structural weakness. The load may be uniform or concentrated, or both, applied on the surface of the distributing element.
A transverse element attached to the underside of framing members and supporting no uniform load other than its own weight and other incidental light loads, such as insulation, qualifies as a load-distributing element only for bending moment associated with its own weight and that of the framing members to which it is attached. Qualifying construction includes subflooring, finish flooring, exterior and interior wall finish, and cold-formed metal siding with or without backing. Such elements should be fastened to the framing members by approved means, such as nails, glue, staples, or snap-lock joints.
Individual members in a qualifying assembly made of different species or grades are each eligible for the repetitive-member increase in Fb if they satisfy all the preceding criteria.
Curvature Factor and Radial Stresses
For the curved portions of glulam beams, the design value for bending should be multiplied by the curvature factor
When M is in the direction tending to decrease curvature (increase the radius), tensile stresses occur across the grain. For this condition, the allowable tensile stress across the grain is limited to one-third the allowable unit stress in horizontal shear for southern pine for all load conditions, and for Douglas fir and larch for wind or earthquake loadings. The limit is 15 psi for Douglas fir and larch for other types of loading. These values are subject to modification for duration of load. If these values are exceeded, mechanical reinforcement sufficient to resist all radial tensile stresses is required.
When M is in the direction tending to increase curvature (decrease the radius), the stress is compressive across the grain. For this condition, the allowable stress is limited to that for compression perpendicular to grain for all species.
(K. F. Faherty and T. G. Williamson, ‘‘Wood Engineering and Construction Handbook,’’ and D. E. Breyer, ‘‘Design of Wood Structures,’’ 2d ed., McGraw-Hill Publishing Company, New York.)
Design values for bending Fb for beams with a circular cross section may be multiplied by a form factor Cƒ0= 1.18. For a flexural member with a square cross section loaded in the plane of the diagonal (diamond-shape cross section), Cƒ may be taken as 1.414.
These form factors ensure that a circular or diamond-shape flexural member has the same moment capacity as a square beam with the same cross-sectional area. If a circular member is taered, it should be treated as a beam with variable cross section.
Column Stability and Bearing Area Factors
Design values for compression parallel to the grain Fc should be multiplied by the column stability factor CP specified in Art. 10.8.1.
Design values for compression perpendicular to the grain Fc apply to bearing surfaces of any length at the ends of a member and to all bearings 6 in or more long at other locations. For bearings less than 6 in long and at least 3 in from the end of a member, Fc may be multiplied by the bearing area factor
Buckling Stiffness Factor
The buckling stiffness of a truss compression chord of sawn lumber subjected to combined flexure and axial compression under dry service conditions may be in
When Le is more than 96 in, CT should be calculated from Eq. (10.13) with Le 96 in. For additional information on wood trusses with metal-plate connections, see design standards of the Truss Plate Institute, Madison, Wis.
Shear Stress Factor
For dimension-lumber grades of most species or combinations of species, the design value for shear parallel to the grain FV is based on the assumption that a split, check, or shake that will reduce shear strength 50% is present (Art. 4.34). Reductions exceeding 50% are not required inasmuch as a beam split lengthwise at the neutral axis will still resist half the bending moment of a comparable unsplit beam.
Furthermore, each half of such a fully split beam will sustain half the shear load of the unsplit member. The design value FV may be increased, however, when the length of split or size of check or shake is known and is less than the maximum length assumed in determination of FV, if no increase in these dimensions is anticipated.
In such cases, FV may be multiplied by a shear stress factor CH greater than unity.
In most design situations, CH cannot be applied because information on length of split or size of check or shake is not available. The exceptions, when CH can be used, include structural components and assemblies manufactured fully seasoned with control of splits, checks, and shakes when the products, in service, will not
be exposed to the weather. CH also may be used in evaluation of the strength of members in service. The ‘‘National Design Specifications for Wood Construction,’’ American Forest & Paper Association, lists values of CH for lumber and timber of various species.