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

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Design of Wood Members for Formwork 131
The allowable load on a column is equal to the allowable compression
stress multiplied by the cross-sectional area of the member.
When determining the maximum allowable load on a shore, the net
area of the cross section of the shore should be used:
P c
= F c
A (5-62)
where P c
= allowable compression load, lb
F c
= allowable compression stress, lb per sq in.
A= net cross-sectional area, bd, in. 2
Example 5-18
A 4 × 6 S4S rectangular shore of No. 2 Southern Pine has a reference
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compression stress parallel to grain of 1,600 lb per sq in. and a reference
modulus of elasticity for column stability of 580,000 lb per sq in.
It has an effective length of 6 ft about both the strong x-x axis and the
weak y-y axis. Calculate the allowable compression load.
The shore will be used for a condition that requires no adjustments
in modules of elasticity or the reference value of compression
stress parallel to grain, that is, dry condition, normal temperature and
load-duration, no incising, and so on. Therefore, the adjustment factors
for reference values of compression stress and modulus of elasticity
will be equal to 1.0: (F * = F and E′ = E ).
c c// min min
Because the shore is braced in both axes, the slenderness ratio
will be governed by the least cross-sectional dimension, 3½ in.
The slenderness ratio, l e
/d = 72/3.5
= 20.57
F ce
= 0.822 E′ min
/(l e
/d) 2
= 0.822(580,000)/(20.57) 2
= 1,126.6 lb per sq in.
The allowable compression stress can be calculated:
F = ∗⎡
+ ∗ − ⎡ +
∗
C
F F F ⎣ F F c⎤
c
[ 1 (
ce/ c
)]/2 [ 1 (
ce/ c
)]/2 2 ∗ ⎤
⎦ − (F
⎣⎢
ce/ Fc
)/c
⎦⎥
F C
= 1, 600⎡ ⎣ ⎢
[1+(1,126.6/ 1,600)] /(0.8)
− [ 1+
1,
126 6 1 600 2 0 8 2
⎤
⎡⎣ . / , ]/ ( . ) ⎤ ⎦ − ( 1, 126. 6) / 0.
8
⎦⎥
= 1 600 ⎡
⎣ 1 065 − 1 065 2
, . ( . ) − 0 . 704 / 0 . 8 ⎤ ⎦
= 1 , 600 ⎡
⎣ 1 . 065 − 1 . 134 − 00 . 880 ⎤ ⎦
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