ASD/LRFD Manual - American Wood Council

M3: Design Provisions and Equations
M3.1 General
This Chapter covers design of members for bending,
compression, tension, combined bending and axial loads,
and bearing.
M3.2 Bending Members - General
This section covers design of members stressed primarily
in flexure (bending). Examples of such members
include primary framing members (beams) and secondary
framing members (purlins, joists). Products commonly
used in these applications include glulam, solid sawn
lumber, structural composite lumber, and prefabricated
I‐joists.
Bending members are designed so that no design capacity
is exceeded under applied loads. Strength criteria
for bending members include bending moment, shear, local
buckling, lateral torsional buckling, and bearing.
See specific product chapters for moment and shear
capacities (joist and beam selection tables) and reference
bending and shear design values.
M3.3 Bending Members - Flexure
The basic equation for moment design of bending
members is:
M′ ≥ M (M3.3-1)
where:
M′ = adjusted moment capacity
M = bending moment
The equation for calculation of adjusted moment
capacity is:
M′ = F b ′ S (M3.3-2)
where:
S = section modulus, in. 3
F b ′ = adjusted bending design value, psi.
See product chapters for applicable
adjustment factors.
M3.4 Bending Members - Shear
The basic equation for shear design of bending members
is:
V′ ≥ V (M3.4-1)
where:
V′ = adjusted shear capacity parallel to
grain, lbs
V = shear force, lbs
The equation for calculation of shear capacity is:
V′ = F v ′ Ib/Q (M3.4-2)
which, for rectangular unnotched bending members,
reduces to:
V′ = 2/3 (F v ′) A (M3.4-3)
where:
I = moment of inertia, in. 4
A = area, in. 2
F v ′ = adjusted shear design value, psi.
See product chapters for applicable
adjustment factors.
American Wood Council