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

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Forms for Columns 287
clamps. Thus, the spacing of the column clamps is determined by the
allowable span length of the sheathing.
Example 10-1
Consider only S4S lumber as sheathing for a square concrete column
form. The column will be 6 ft high and the concrete unit weight is 150 lb
per cu ft. The pressure on the bottom of the form will be:
From Eq. (10-1), P m
= (150 lb per cu ft)(6 ft)
= 900 lb per sq ft
ww.EasyEngineering.n
The uniform load on a 12-in. (1-ft) strip of sheathing will be:
w = (900 lb per sq ft)(1 ft)
= 900 lb per lin ft
Consider using 2-in-thick No. 2 grade Douglas Fir-Larch lumber
laid flat for sheathing. Assume the lumber will be dry and a shortduration
loading, less than 7 days. The safe spacing of the column
clamps is l in. and can be calculated as follows. Using the actual thickness
of 1.5 in. for the S4S dimension lumber, the physical properties
for a 12-in.-wide strip of the lumber laid flat can be calculated:
From Eq. (5-9), S = bd 2 /6
= (12 in.)(1.5 in.) 2 /6
= 4.5 in. 3
From Eq. (5-8), I = bd 3 /12
= (12 in.)(1.5 in.) 3 /12
= 3.375 in. 4
The NDS permits an increase in tabulated design value when the
member is laid flat (refer to Table 4-7). However, the flat-use adjustment
factor will not be used in this design. Adjusting the tabulated
design value from Table 4-3 by 25% for short-duration loading, less
than 7 days, the indicated allowable stresses for a dry condition are
For bending, F b
= 1.25(900) = 1,125 lb per sq in.
For shear, F v
= 1.25(180) = 225 lb per sq in.
Modulus of elasticity, E = 1.0(1,600,000) = 1,600,000 lb per sq in.
The maximum allowable spacing of column clamps can be calculated
from Eqs. (10-3) to (10-6) as follows:
For bending, l = [120F b
S/w] 1/2
= [120(1,125)(4.5)/900] 1/2
= 25.9 in.
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