Figure 3-4.Typical overhead slab form.
Sheathing. Shapes and holds the concrete.
Plywood (usually 3/4-inch thick) or solid sheet metal
(usually corrugated) is the best for use.
Joists. Supports the sheathing against deflection.
Performs the same function as STUDS in a wall form.
Normally, you should use 4-inch lumber; however, 2-inch
or 3-inch stock can be used if properly designed.
Stringers. Supports the joists against deflection.
Performs the same function as WALES in a wall form,
except stringers do not need to be doubled. Use
2-inch-thick lumber or larger.
Shores. Supports the stringers against deflection.
Performs the same functions as TIES in a wall form and
also supports the concrete at the desired elevation. The
lumber used for this must be as large as the stringers,
but never smaller than 4 by 4 inches in dimension.
Lateral bracing. May be required between
adjacent shores to keep the SHORES from bending
under load. Usually 1 by 6 inch or larger stock is used
for bracing. Bracing of some type will always be
required to support the formwork.
Wedges. Are normally use for two purposes: the
wedges are used for leveling of the forms and the forms
are easier to strip if wedges are used.
Mudsills. Continuous timber placed on the
ground that distributes a load and provides a level
surfaces for scaffolding and shoring.
Step 1. The engineer will design and specify the
materials you will need to construct the overhead
forms. Ensure that all the correct materials are on the
jobsite and your crew is familiar with the materials and
Step 2. Determine the maximum total load the
forms will have to support. The rule of thumb for
figuring the total load is live load (LL) plus dead load
(DL). The live load (materials, personnel, and
equipment) is estimated to be 50 pounds per square
foot unless the forms will support engine-powered
equipment. In this case, the LL would be 75 pounds
per square foot. The dead load (concrete/rebar) is
estimated at 150 pounds per cubic foot. However, you
cannot add dead load to live load until you convert the
dead load to square feet (SF). The formulas are as
For example, if the slab is 6 inches in thickness, the
formula would be as follows:
Step 3. Determine the maximum joist spacing. Use
table 3-1 and read the joist spacing based on the
sheathing material. Use the maximum TL in place of
the maximum concrete pressure. For example, the
sheathing is 3/4-inch plywood (strong way), the TL is
150 pounds per square foot, and the joist spacing
would be 22 inches.
Step 4. Calculate the uniform load on the joist. The
same procedure is used as for determining uniform
loads on the structural members in the wall form
Step 5. Determine the maximum stringer spacing.
Use table 3-2 and the uniform load on the joist is
calculated in Step 4. Round this load up to the next