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

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Forms for Footings 209
lumber that is lined with plywood or tempered hardboard. Table 4-21
gives the minimum bending radii for tempered hardboard. Figure 8-8
illustrates a form for a round footing.
For the job-built round form in Figure 8-8, determine the maximum
safe spacing of the ribs, based on filling the footing with concrete in one
operation. The pressure on the vertical surface will vary uniformly
from zero at the top to 375 lb per sq ft as shown in Figure 8-9.
If the vertical staves are assumed to be supported by the ribs at
the top and bottom ends, which is an exaggerated span on the safe
side, and if a vertical strip 12 in. wide is considered, the reaction at A
will be 156 lb, and at B it will be 312 lb. The depth y to the point of
maximum moment, which is the point of zero shear, is obtained from
the triangle whose area is
xy/2 = 156 lb
(a)
Also, from similar triangles,
x/375 = y/2.5
x = 375y/2.5
(b)
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Combining Eqs. (a) and (b) and solving for y:
y = 1.443 ft
This is the depth to the point of maximum moment. Considering
all forces acting above line x, determine the applied bending moment
at this depth:
M = 156 lb(1.443 ft) – [(216/2)(1.443) 2 /3)]
= 225.1 – 74.9
= 150.2 ft-lb
= 1,802.4 in.-lb
Consider No. 1 grade 1 × 4 S4S lumber for sheathing that has an
allowable bending stress of 1,625 lb per sq in. The resisting moment
of a strip of sheathing 12 in. wide will be:
M = F b
S
= F b
(bd 2 /6)
= 1,625[(12)(0.75) 2 /6]
= 1,828 in.-lb
Thus, the resisting moment is greater than the applied bending
moment. In order to limit the deflection, the ribs will be spaced not
more than 24 in. apart by placing them 3 in. below the top and 3 in.
above the bottom of the staves.
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