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

204 Chapter Eight
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FIGURE 8-4 Use of void forms under a grade beam.
8. As the forms are aligned, attach a brace near the top of each
strongback and to a stake set in the ground.
Erecting and stripping the forms can proceed at a very rapid
rate after the form crew becomes familiar with this procedure.
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Forms for Grade Beams
The side forms for grade beams can be fabricated and erected in a
manner similar to those previously described for foundation walls.
The bottom of a beam may rest on the ground with no required bottom
form. However, designs frequently require a void space between
the bottom of a beam and the ground under it. The void space is
required for clay-type soils that are sensitive to expanding when the
moisture of the soil increases, causing extensive uplift pressures on
the bottom of concrete grade beams or slabs on grade. For this purpose
fiber void units, such as carton forms made from cardboard, are installed
under the bottom of a beam between its supporting piers, columns,
or footings, as illustrated in Figure 8-4. The units are initially strong
enough to support the fresh concrete, but they will soon soften and
decay to leave a void space under the beam.
Forms for Concrete Footings
The selection of forms for concrete footings will depend on the size and
shape of the footings and the number of times the forms can be reused
without modification. Rectangular footings with constant cross sections
are easily formed, but stepped footings are more complicated.
Figure 8-5 illustrates a form for a rectangular footing that can be
erected and stripped rapidly and reused many times if other footings
have the same dimensions.
The sheathing may be made from plywood attached to wood
frames made of 2 × 4 S4S or larger lumber. The use of a steel rod at
each corner permits quick assembly and removal, and it also ensures
adequate anchorage. When the four rods are removed, the panels can
be reassembled in another location within a few minutes.
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