Building Design and Construction Handbook - Merritt - Ventech!

WOOD CONSTRUCTION 10.21
structural glued-laminated timber pressure treated with fire-retardant chemicals
should be obtained from the company providing the treatment and redrying service.
Load duration factors greater than 1.6 (Table 10.5) should not be applied to structural
members pressure-treated with fire-retardant chemicals.
10.7 DESIGN PROVISIONS FOR FLEXURAL
MEMBERS
Design of flexural members requires consideration primarily of bending and shear
strength, deflection, and end bearing.
10.7.1 Strength of Flexural Members
The stress induced in a beam (or other flexural member) when subjected to design
loads should not exceed the strength of the member. The maximum bending stress
ƒ b at any section of a beam is given by the flexural formula
ƒ � M/S (10.14)
b
where M is the bending moment and S the section modulus. For a rectangular beam,
the section modulus is bd 2 /6 and Eq. (10.14) transforms into
2
ƒb � 6M/bd (10.15)
where b is the beam width and d the depth. At every section of the beam, ƒ b should
be equal to or less than the design value for bending F b adjusted for all end-use
modification factors (Art. 10.5).
Shear stress induced by design loads in a member should not exceed the allowable
design value for shear F V. For wood beams, the shear parallel to the grain,
that is, the horizontal shear, controls the design for shear. Checking the shear stress
perpendicular to the grain is not necessary inasmuch as the vertical shear will never
be a primary failure mode.
The maximum horizontal shear stress ƒ V in a rectangular wood beam is given
by
ƒ � 3V/2bd (10.16)
V
where V is the vertical shear. In calculation of V for a beam, all loads occurring
within a distance d from the supports may be ignored. This is based on the assumption
that loads causing the shear will be transmitted at a 45� angle through
the beam to the supports.
(K. F. Faherty and T. G. Williamson, ‘‘Wood Engineering and Construction
Handbook.’’ McGraw-Hill Publishing Company, New York.)
10.7.2 Beam Stability
Beams may require lateral support to prevent lateral buckling. Need for such bracing
depends on the unsupported length and cross-sectional dimensions of the members.
When buckling occurs, a member deflects in the direction of its least dimension b.
In a beam, b usually is taken as the width. If bracing precludes buckling in that