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

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Forms for Beams and Floor Slabs 341
smooth surface for the beam bottom. As illustrated in Figure 11-7(c), the
beam bottom may also be plywood, backed with dimension lumber
placed in the vertical direction to provide more resistance to bending
and deflection.
The side forms are made from plywood panels, held together by
form ties. Side forms may also be made with boards, held together
with 2 × 4 cleats. The ledgers attached to the vertical 2 × 4 cleats assist
in supporting the ends of the joists.
Spacing of Shores under Beam Bottoms
The spacing of shores under beam bottoms will be limited by the
strength of the beam bottom in bending, shear, and deflection as well
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as by the capacities of the shores. Although the spacing of the shores
along the beam forms may be governed by the strength or deflection of
the beam bottoms, the shores must sustain the total vertical load from
the beam forms plus the dead and live loads from the slab to the midpoint
of the adjacent rows of shores on either side of the beam forms.
For the concrete beam form in Figure 11-7(a), the vertical load on
a 12-in. strip of beam bottom can be used to determine the allowable
span length of beam bottom using the equation presented in Chapter 5.
For the concrete beam form illustrated in Figure 11-7(b) and 11-7(c),
the plywood transfers the load in a direction perpendicular to the
direction of the beam, between the horizontal wood runners. The runners
must transfer the load between the shores. A strip of the beam
bottom, from the center points between spacings of the runners, may
be analyzed for bending, shear, and deflection. This procedure discards
any contribution of the plywood for transferring load between
shores, which is a conservative approach.
The pressure of concrete against beam forms acts in two directions,
one horizontally outward against the beam sides and one vertically
downward against the bottom of the beam forms. The horizontal
pressure begins at zero at the top of the beam sides and increases
linearly to a maximum at the bottom of the side. The vertical pressure
is uniformly distributed across the bottom of the beam form.
The maximum horizontal pressure on the beam sides, which acts
at the bottom of the beam, can be calculated by multiplying the unit
weight of the freshly placed concrete by the height of the beam, the
distance from the top of the concrete to the bottom of the beam. Thus,
the pressure on the beam sides is similar to the pressure on a column
form because the concrete is placed rapidly in beam forms.
Example 11-4
Determine the safe spacing of shores based on bending, shear, and
deflection of the beam bottom, as illustrated in Figure 11-7(a). The beam
is 16 in. wide and the total depth from the top of the concrete slab to
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