Page 234 - Civil Engineering Formulas

168 CHAPTER SIX

K 0.3 for visually graded lumber and machine-evaluated lumber
cE
0.418 for products with a coefficient of variation less than 0.11
c 0.80 for solid-sawn lumber
0.85 for round timber piles
0.90 for glued-laminated timber
For a compression member braced in all directions throughout its length to pre-
vent lateral displacement, C 1.0.
p
The buckling stiffness of a truss compression chord of sawn lumber subjected
to combined flexure and axial compression under dry service conditions may
be increased if the chord is 2 4 in (50.8 101.6 mm) or smaller and has the
3
narrow face braced by nailing to plywood sheathing at least 8 in (9.5 mm)
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 in column stability calculations. When the
T
effective column length L , in (mm), is 96 in (2.38 m) or less, C may be com-
e
T
puted from
K M L e
C T 1 (6.32)
K T E
where K 2300 for wood seasoned to a moisture content of 19 percent or less
M
at time of sheathing attachment
1200 for unseasoned or partly seasoned wood at time of sheathing
attachment
K 0.59 for visually graded lumber and machine-evaluated lumber
T
0.82 for products with a coefficient of variation of 0.11 or less
When L is more than 96 in (2.38 m), C should be calculated from Eq. (6.32)
T
e
with L 96 in (2.38 m). For additional information on wood trusses with metal-
e
plate connections, see design standards of the Truss Plate Institute, Madison,
Wisconsin.
The slenderness ratio R for beams is defined by
B
L e d
R B (6.33)
B b 2
The slenderness ratio should not exceed 50.
The effective length L for Eq. (6.33) is given in terms of unsupported length
e
of beam in reference data. Unsupported length is the distance between supports
or the length of a cantilever when the beam is laterally braced at the supports to
prevent rotation and adequate bracing is not installed elsewhere in the span.
When both rotational and lateral displacements are also prevented at intermedi-
ate points, the unsupported length may be taken as the distance between points
of lateral support. If the compression edge is supported throughout the length of
the beam and adequate bracing is installed at the supports, the unsupported
length is zero.