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

52 Chapter Four
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a column. The allowable load decreases rapidly as the slenderness
ratio increases. For this reason, long shores should be cross braced in
two directions with one or more rows of braces.
The following equation for calculating the column stability factor
C P
takes into consideration the modes of failure, combinations of
crushing and buckling of wood members subjected to axial compression
parallel to grain.
C = ⎡
+ ∗
c ⎡ +
∗
[ 1 ( F / F )]/2 – c⎤
P ⎣[ 1 ( F / F )]/2 2 ⎦ − (F F
∗ ⎤
/ )/
⎣⎢ ce c ce c
ce c
c ⎦ ⎥
where
F ∗ = compression stress parallel to grain, lb per sq in.,
c
obtained by multiplying the reference design value for
compression stress parallel to grain by all applicable
adjustment factors except C P
; F ∗ = F (C × C × C ×
c C// D M F
C i
× C t
)
F ce
= 0.822 E′ min
/(l e
/d) 2 , lb per sq in., representing the impact
of Euler buckling
E′ min
= modulus of elasticity for column stability, lb per sq in.,
obtained by multiplying the reference design value of
E min
by all applicable adjustment factors; E′ min
= E min
(C M • C T • C i • C t
)
l e
/d = slenderness ratio, ratio of effective length, in., to least
cross-sectional dimension, in. Note: For wood columns,
l e
/d should never exceed 50.
c = 0.8 for sawn lumber
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Adjustment Factors C fu
for Flat Use
Bending values in Tables 4-2 and 4-3 are based on loads applied to the
narrow face of dimension lumber. When the lumber is laid flat and
loaded perpendicular to the wide face, the reference design value for
bending may be increased by the flat use factor, C fu
. Table 4-7 gives
values of the flat use factor for dimension lumber when it is laid flat.
Adjustment Factors C b
for Bearing Area
In Tables 4-2 and 4-3 the reference design values for compression perpendicular
to grain (F C⊥ ) apply to bearing on a wood member. For
bearings less than 6 in. in length and not nearer than 3 in. from the
end of a member, the bearing area factor (C b ) can be used to account
for an effective increase in bearing length. The bearing length is the
dimension of the contact area measured parallel to the grain. The
bearing area factor can be calculated by the equation C b
= (bearing
length + ³⁄8 in.)/(bearing length). Values for C b
are always greater
than 1.0, as shown in Table 4-8.
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