Building Design and Construction Handbook - Merritt - Ventech!

WOOD CONSTRUCTION 10.17
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 F b if they satisfy all the
preceding criteria.
10.5.9. Curvature Factor and Radial Stresses
For the curved portions of glulam beams, the design value for bending should be
multiplied by the curvature factor
2
Cc � 1 � 2000(t/R) (10.10)
where t � lamination thickness, in, and R � radius of curvature, in, of inside face
of lamination. t/R should not exceed 0.01 for hardwoods and southern pine or 0.008
for other softwoods. The curvature factor does not apply to design values of F b for
the straight portions of a member, regardless of curvature elsewhere.
Radial Tension or Compression. The radial stress induced by a bending moment
in a member of constant cross section may be computed from
3M
ƒr � (10.11)
2Rbd
where M � bending moment, in-lb
R � radius of curvature at centerline of member, in
b � width of cross section, in
d � depth of cross section, in
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