Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

44 Chapter Four
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d = depth or height of beam face parallel to the direction in which
the load or force is applied, in.
I= area moment of inertia of the cross section of a beam
= bd 3 /12, in. 4 for a rectangular beam
c = distance from neutral axis to most distant, or extreme, fiber of
the beam, in.
S= section modulus of the cross section of a beam
= bd 2 /6, in. 3 for a rectangular beam
r= radius of gyration of the cross section of a beam
= I/ A, in.
E= modulus of elasticity, lb/in. 2
Q= static moment of area, a measure of resistance to shear, in. 3
X-X = the location of the neutral axis of the cross section of a beam for
edgewise bending (load applied to narrow face). The X-X axis
is often referred to as the strong axis of the beam
Y-Y = the location of the neutral axis of the cross section of a beam for
flatwise bending (load applied to wide face); the Y-Y axis is
often referred to as the weak axis of the beam
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Allowable Stresses of Lumber
The loads applied to wood members create applied bending moments,
shear forces, and compression forces in the member. The lumber used
must have sufficient allowable stresses to resist the applied stresses.
The magnitudes of the allowable stresses of lumber depend on the
grade and species of wood, size of lumber, duration of load, moisture
content, and other factors.
Organizations involved in the lumber industry have published
design values for various grades and species of lumber based on normal
loading conditions. Because the strength of wood varies with the
conditions under which it is used, these design values should only be
applied in conjunction with appropriate design and service recommendations
from the 2005 National Design Specification (NDS) for
Wood Construction published by the American Forest & Paper
Association [1, 2].
The Supplement to the 2005 NDS provides reference design
values for various grades and species of wood. It should be noted
that reference design values for dimension lumber are different than
for post and timber members. Allowable design stresses are determined
by multiplying the reference design values from the NDS Supplement
by adjustment factors that are presented in the NDS National
Design Specification. The adjustment factors for sawn lumber are
1. Size adjustment factor, C F
2. Load-duration factor, C D
3. Wet service factor, C M
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