Building Design and Construction Handbook - Merritt - Ventech!

where F* b �
WOOD CONSTRUCTION 10.23
design value for bending multiplied by all applicable adjustment factors
except C ƒu, C V, and C L (Art. 10.5)
F bE � K bEE�/ 2
RB
K bE � 0.438 for visually graded lumber
� 0.561 for machine evaluated lumber (MEL)
� 0.609 for products with a coefficient of variation of 0.11 or less
E� � design modulus of elasticity multiplied by applicable adjustment factors
(Art. 10.5)
The effective length L e for Eq. (10.17) is given in Table 10.14 in terms of the
unsupported length of beam. 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 displacement are also prevented at intermediate 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.
10.7.3 Deflection of Wood Beams
The design of many structural systems, particularly those with long span, may be
governed by deflection. Verifying structural adequacy based on allowable stresses
alone may not be sufficient to prevent excessive deflection. Limitations on deflection
may increase member stiffness.
TABLE 10.14 Effective Length L c for Lateral Stability of Beams*
Loading L u/d � 7† L u/d � 7‡
Simple beam§
Uniformly distributed load 1.63L u � 3d 2.06L u
Load concentrated at midspan 1.37L u � 3d 1.80L u
Equal end moments 1.84L u 1.84L u
Equal concentrated loads at third points 1.68L u 1.68L u
Equal concentrated loads at quarter points 1.54L u 1.54L u
Equal concentrated loads at fifth points 1.68L u 1.68L u
Cantilever§
Uniformly distributed load 0.90L u � 3d 1.33L u
Concentrated load on the end 1.44L u � 3d 1.87L u
* As specified in the ‘‘National Design Specification for Wood Construction,’’
American Forest & Paper Association.
† L u � clear span when depth d exceeds width b and lateral support is provided to
prevent rotational and lateral displacement at bearing points in a plane normal to the
beam longitudinal axis and no lateral support is provided elsewhere.
‡ L u � maximum spacing of secondary framing, such as purlins, when lateral support
is provided at bearing points and the framing members prevent lateral displacement
of the compression edge of the beam at the connections.
§ For a conservative value of L e for any loading on simple beams or cantilevers,
use 2.06 L u when L u / d � 7, and 1.63 L u � 3d when 7 � L u / d � 14.3, and 1.84 L u
when L u / d � 14.3.