Building Design and Construction Handbook - Merritt - Ventech!

WOOD CONSTRUCTION 10.19
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 C T
in column stability calculations. When the effective column length L e, in, is 96 in
or less, C T may be computed from
KMLe CT � 1 � (10.13)
KE
where K M � 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
K T � 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 L e is more than 96 in, C T should be calculated from Eq. (10.13) with L e �
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 F V 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 F V 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 F V, if no increase in these dimensions is anticipated.
In such cases, F V may be multiplied by a shear stress factor C H greater than
unity.
In most design situations, C H cannot be applied because information on length
of split or size of check or shake is not available. The exceptions, when C H 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. C H 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 C H for lumber and timber of
various species.
10.6 PRESSURE-PRESERVATIVE TREATMENTS
FOR WOOD
Wood members are considered to be permanent without pressure treatment if located
in enclosed buildings where good roof coverage, proper roof maintenance,
T