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

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Forms for Footings 205
FIGURE 8-5 Forms for concrete footings.
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The studs and plates can be made from 2 × 4, or 2 × 6 lumber,
depending on the size and species of lumber used for the forms and
the pressure from the concrete. If the footing is 18 in. deep, the maximum
footing width will be less than 5 ft, using 2 × 4 plates for the
species of lumber commonly used in formwork.
Where the size and depth of a footing produce excessive compression
stresses between the pin and the wood in the bottom plate,
1- by 4-in. vertical planks can be nailed to the plates, as shown in
Figure 8-5, to increase the strength of the joints, or nails can be used
to supplement the strength of the pins.
The top view of Figure 8-5 shows that, by starting with long top
and bottom plates on opposite sides of the forms, the length of a footing
may be increased while the width remains constant. By adding
strips of plywood, adequately attached to the plates, the forms can
quickly be converted to larger sizes. A similar method can be used to
increase both dimensions of a footing. In general, it is better to use the
forms for the smallest footings first, and then go to the next larger
sizes. However, the forms may also be used in the reverse order.
Additional Forms for Concrete Footings
There are many types of forms that can be used for concrete footings,
depending on the footing shape, as described in the following sections
and illustrated in Figures 8-6 through 8-10.
Figure 8-6 shows a plan and elevation view that is sometimes used
for square or rectangular footings whose depths vary from 12 to 36 in.
For footing depths up to 24 in., ¾-in. Plyform sheathing is supported
by 2 × 4 studs, spaced at 12 in. center to center. For 36-in.-deep footings,
7 ⁄8-in. Plyform with 2 × 4 size lumber is used. The procedure for
determining the required size of lumber for a particular job application
is illustrated in previous sections of this chapter and in Chapter 5
of this book. The forms should be adequately braced.
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