Although forms are generally constructed from wood, fiber, earth, or
metal, the TO carpenter usually constructs wood or fiber forms.
Wood forms are the most common in building construction. They are economical,
easy to handle, and easy to produce, and they adapt to many shapes. Form
lumber can be reused for roofing, bracing, or similar purposes.
Lumber should be straight, strong, and only partially seasoned. Kiln-dried
lumber tends to swell
when soaked with water. Swelling may cause bulging and distortion. If
green lumber is used, allow for shrinkage, or keep it wet until the concrete
is in place. Softwoods (pine, fir, and spruce) are the most economical;
they are light, easy to work with, and generally available.
Wood coming in contact with concrete should be surfaced (smooth) on the
side towards the concrete and on both edges. The edges may be square,
shiplap, or tongue-and-groove. Tongue-and-groove lumber makes a more watertight
joint, which reduces warping.
Plywood is economical to use for wall and floor forms; however, plywood
used for this purpose should be made with waterproof glue and marked for
use in concrete forms. Plywood is warp-resistant and can be used more
often than other lumber.
An advantage of using plywood for forms is the great number of sizes
available. It is made in thicknesses of 1/4, 3/8, 1/2, 5/8, and 3/4 inch,
and in widths up to 48 inches. The 8-foot lengths are most commonly used.
The 6/8- and 3/4-inch thicknesses are most economical. Thinner plywood
requires solid backing to prevent deflection. The 1/4-inch thickness is
useful for curved surfaces.
· Watch for protruding nails. They are the
principal cause of accidents on form work.
· Inspect tools frequently.
· Place mud sills under shoring that rests on
· Protect all men on scaffolds and on the
· Do not raise large form panels in heavy gusts
· Brace all shoring securely to prevent
collapse of form work.
Waterproof cardboard and other fiber materials are used for round concrete
columns and other preformed shapes. Forms are made by gluing layers of
fiber together and molding them to the right shape. The advantage is that
fabrication at the job site is not necessary.
FOUNDATION AND FOOTING FORMS
When possible, earth is excavated to form a mold
for concrete-wall footings. If wood forms are
needed, the four sides are built in panels.
Panels for two opposite sides are made at exact
footing width; the other pair has two end cleats on
the inside spaced the length of the footing plus
twice the sheathing thickness. One-inch-thick
sheathing is nailed to vertical cleats spaced on 2-
foot centers. Two-inch dressed lumber should be
used for the cleats.
NOTE: Panels are held in place with form
nails until the tie wire is installed. Nails
should be driven only part way from the
outside so that they can be easily removed.
Figure 5-1. Typical large footing form
Tie wires are wrapped around the center cleats.
Wire holes on each side of the cleat should be less
than 1 inch in diameter to prevent mortar leaks.
Reinforcing bars must be placed before the wire is
For forms 4 feet square or larger, stakes are driven
as shown in Figure 5-1. These stakes, and 1 x 6
boards nailed across the top of the form, prevent
spreading. Panels may be higher than the required
depth of footing, since they can be marked on the
inside to show the top of the footing. If the footings
are less than 1 foot deep and 2 feet square, forms
can be constructed of 1-inch sheathing without cleats (Figure 5-2). When
placing a footing and a small pier at the same time, the form is built
as shown in Figure 5-3. To ensure that support for the upper form does
not interfere with the placement of concrete in the lower form, 2 x 4
or 4 x 4 pieces are nailed to the top of the lower form (as shown). The
top form is then nailed to these pieces.
Construction and bracing of forms for wall
footings are shown in Figure 5-4. The sides are
2-inch lumber held in place by stakes and held
apart by spreaders. The short brace shown at
each stake holds the form in line.
Wall forms are made of wall "panels" and other parts shown
in Figure 5-5 page 5-4. These Darts are described as follows:
Wall panels are made by nailing the
sheathing to the studs and can be built in
place or prefabricated elsewhere.
Prefabricated wall panels should be no
more than 10 feet long so that they can
be easily handled. Figure 5-6, page 5-4,
shows how wall panels are connected and
how wall corners are constructed.
Sheathing forms the surface of the
concrete. It should be smooth, especially
if the finished surface is to be exposed. It
is normally 1-inch (3/4-inch dressed)
tongue- and-groove lumber or 3/4-inch
plywood. Concrete is plastic when placed
in the form, so sheathing should be
watertight. tongue-and-groove lumber or
plywood gives a watertight surface.
Reinforce sheathing to prevent bulging
from the weight of the concrete.
Vertical studs make the sheathing rigid. These studs are generally made
from 2 x 4 lumber. Studs also
require reinforcing when they extend more than 4 feet.
Double wales reinforce the wall form. They also tie wall panels together
and keep them in a straight line. They run horizontally and are lapped
at the corners.
Braces give the forms stability. The
most common brace uses a
horizontal member and a diagonal
member nailed to a stake and to the
stud or wale. The diagonal member
of the brace should make a 30° angle
with the horizontal member.
Additional bracing may be provided
by strongbacks (vertical members)
behind the wales or in the corner
formed by intersecting wales.
(Braces are not part of the form
design and are not considered as
providing additional strength.)
Shoe plates are nailed into the
foundation or footing and must be
carefully placed to maintain the wall
dimensions and alignment. Studs
are tied into the shoe plate.
Spreaders must be placed near each
tie wire. Spreaders are cut to the
same length as the thickness of the
wall and placed between the two
sheathing surfaces of the forms.
They are not nailed, but are held in
place by friction.
Spreaders are removed as the forms
are filled (Figure 5-7), so that they
will not become embedded as the
concrete hardens. A wire is attached
to the spreaders to allow them to be pulled out of the form after the
concrete has put enough pressure on
the walls to allow easy removal. The wire fastened to the bottom spreader
passes through a hole drilled off
center in each spreader above it. Pulling on the wire will remove the
spreaders one after another as the concrete level rises in the forms.
Tie wires hold the forms secure against
the lateral pressures of unhardened concrete. Double strands are always
used. Ties keep wall forms together as the concrete is positioned; Figure
5-8 shows two ways of doing this. The wire should be No. 8 or 9 gauge,
black, annealed iron wire. Barbed wire may be used in an emergency. Tie
spacing should be the
same as the stud placing, but never more than 3 feet. Each tie is formed
by looping the wire around a wale, bringing it through the form, and looping
it around the wale on the opposite side. The tie wire is made taut by
twisting it with a smooth metal rod or
NOTE: Wire ties should be used only
for low walls or when tie rods are not available.
An alternate to tie wires and spreaders, the tie rod and spreader combination
is shown in Figure 5-9, page 5-6. After the form is removed, each rod
is broken off at the notch. If appearance is important, the holes should
be filled with a mortar mix. The use of a wood strip as a wedge when curtain
walls and columns are placed at the same
time is shown in Figure 5-10, page 5-6. In removing the forms, the wedge
is removed first.
Sheathing runs vertically in column forms to save saw cuts. Corner joints
are firmly nailed to ensure watertight construction. Battens or narrow
strips of boards (cleats) are placed directly over the joints to fasten
the several pieces of vertical heathing together.
A column and footing form is shown in Figure 5-11. The
column form is erected
after the steel reinforcement is assembled and is tied to dowels in the
footing. The form should have a cleanout hole in the bottom to help remove
debris. The lumber removed to make the cleanout holes should be nailed
to the form so that it can be replaced before the concrete is positioned.
BEAM AND GIRDER FORMS
Figure 5-12 shows both beam and girder forms. The type of construction
of these forms depends on whether the form is to be removed in one piece
or whether the bottom is to be left until the concrete is strong enough
for shoring to be removed. Beam forms receive little bursting pressure
but must be shored at close intervals to prevent sagging.
The bottom of the form is the same width as the beam; it is in one piece
for its full width. Form sides are 1- inch tongue-and-groove material;
they lap over the bottom (as shown). The sheath is nailed to 2 x 4 struts
placed on 3-foot centers. A 1 x 4 piece is nailed along the struts to
support the joists for the floor panel. The sides of the
form are not nailed to the bottom but are held in position by continuous
strips. Crosspieces nailed on top serve as spreaders. After erection,
the slab panel joints hold the beam in place.
A beam and girder assembly is shown in Figure 5-13, page 5-8. The beam
bottom butts tightly against the side of the girder and rests on a 2 x
4 nailed to the girder side. Details in Figure 5-13 show the clearances
for stripping and the allowances for movement caused by the concrete's
weight. The 4 x 4 posts are spaced to support the concrete and are wedged
at the bottom or top for easy removal.
Floor panels are built as shown in Figure 5-14, page 5-9. The 1-inch
tongue-and-groove sheathing or 3/4- inch plywood is nailed to 1 x 4 cleats
on 3-foot centers. These panels are supported by 2 x 6 joists. Spacing
of joists depends on the thickness of the concrete slab and the span of
the beams. If the slab spans the distance between
two walls, the panels are used in the same manner as when beams support
the floor slab.
A method for building stair forms up to 3 feet in width
is shown in Figure 5-15, page 5-9. The underside of the steps should be
1-inch tongue-and-groove sheathing. This platform should extend 12 inches
beyond each side of the stairs to support stringer bracing blocks. The
back of the panel is shored with 4 x 4 pieces (as shown). The
2 x 6 cleats nailed to the shoring should rest on wedges to make both
adjustments and removal of the posts
easy. The side stringers are 2 x 12 pieces cut as required for the treads
and risers. The face of the riser should be 2-inch material, beveled (as