10.5.12 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- creased if the chord is 2  4 in or smaller and has the narrow face braced by nailing to wood structural panel sheathing at least 3/8 in thick in accordance with good nailing practice. The increased stiffness may be accounted for by multiplying the design value of the modulus of elasticity E by the buckling stiffness factor CT in column stability calculations. When the effective column length Le, in, is 96 in or less, CT may be computed from K L C
1  M e (10.13) T K E T where KM
2300 for wood seasoned to a moisture content of 19% or less at time of sheathing attachment
1200 for unseasoned or partly seasoned wood at time of sheathing attachment KT
0.59 for visually graded lumber
0.75 for machine evaluated lumber (MEL)
0.82 for products with a coefficient of variation of 0.11 or less 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. 10.5.13 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

When the upper column is of such dimension that its finished end does not wholly bear on the lower column, one of two methods must be followed: In Fig. 7.56c, stresses in a portion of the upper column not bearing on the lower column are transferred by means of flange plates that are finished to bear on the lower column. These bearing plates must be attached with sufficient single-shear bolts to develop the load transmitted through bearing on the finished surface. When the difference in column size is pronounced, the practice is to use a horizontal bearing plate as shown in Fig. 7.56d. These plates, known as butt plates, FIGURE 7.56 Slip-critical bolted column splices. may be attached to either shaft with tack welds or clip angles. Usually it is attached to the upper shaft, because a plate on the lower shaft may interfere with erection of the beams that frame into the column web. Somewhat similar are welded column splices. In Fig. 7.57a, a common case, holes for erection purposes are generally supplied in the splice plates and column flanges as shown. Some fabricators, however, prefer to avoid drilling and punching of thick pieces, and use instead clip angles welded on the inside flanges of the columns, one pair at diagonally opposite corners, or some similar arrangement, Figure 7.57b and c corresponds to the bolted splices in Fig. 7.56c and d. The shop and field welds for the welded butt plate in Fig. 7.57c may be reversed, to provide erection clearance for beams seated just below the splice. The erection clip angles would then be shop welded to the underside of the butt plate, and the field holes would pierce the column web. The butt-weld splice in Fig. 7.57d is the most efficient from the standpoint of material saving. The depth of the bevel as given in the illustration is for the usual column splice, in which moment is unimportant. However, should the joint be subjected to considerable moment, the bevel may be deepened; but a 1/8-in minimum shoulder should remain for the purpose of landing and plumbing the column. For full moment capacity, a complete-penetration welded joint would be required. A clear understanding of what the fabricator furnishes or does not furnish to the erector, particularly on fabrication contracts that may call for delivery only, is all- FIGURE 7.57 Welded column splices. importantand in many instances fabricated steel is purchased on delivery basis

Mu  required flexural strength, kip-in calculated for primary bending and P- effects cPn  design compressive strength (Art. 7.19.3) bMn  design flexural strength (Art. 7.20.2) Mu may be determined for the factored loads from a second-order elastic analysis. The AISC LRFD specification, however, permits Mu to be determined from Eq. (7.60) with the variables in this equation determined from a first-order analysis. M  B M  B M (7.60) u 1 nt 2 lt where Mnt  required flexural strength, kip-in, with no relative displacement of the member ends; for example, for a column that is part of a rigid frame, drift is assumed prevented Mlt  required flexural strength, kip-in, for the effects only of drift as determined from a first-order analysis B1  magnification factor for Mnt to account for the P- effects  Cm 1  P /P u e Cm  reduction factor defined for Eq. (7.57) B2  magnification factor for Mlt to account for the P- effects B2 may be calculated from either Eq. (7.61) or (7.62), the former usually being the simpler to evaluate.