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

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Forms for Beams and Floor Slabs 325
For deflection not to exceed l/360, the maximum span length
from Table 11-1,
l d
= [1,743EI/360w d
] 1/3
= [1,743(1,650,000)(0.199)/360(150)] 1/3
= 21.9 in.
For deflection not to exceed ¹⁄16 in., the maximum span length
from Table 11-1,
l d
= [1,743EI/16w d
] 1/4
= [1,743(1,650,000)(0.199)/16(150)] 1/4
= 22.1 in.
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Summary for the ¾-in.-Thick Plyform Decking
For bending, the maximum span length of the Plyform = 26.5 in.
For shear, the maximum span length of the Plyform = 68.9 in.
For deflection, the maximum span length of the Plyform = 21.9 in.
In this example, deflection controls the maximum span length of
the Plyform decking. Therefore, the joists that support the Plyform
must be placed at a spacing that is no greater than 21.9 in. For constructability,
the spacing would likely be placed at 18 in. on centers.
Use of Tables to Determine Maximum Spacing of Joists
Tables have been developed by APA—The Engineered Wood Association
and by companies in the construction industry that supply formwork
products, which provide the allowable span length of plywood
for a given concrete pressure. When the design of formwork is based
on design tables of manufacturers or associations in the construction
industry, it is important to follow their recommendations.
Examples of these types of tables are presented in Chapter 4 for
Plyform, which is manufactured by the plywood industry specifically
for use in formwork for concrete. Tables 11-2 through 11-5 are
reproduced here from Chapter 4 for determining the support spacing
of Plyform for various panel thicknesses and uniform pressures.
Thus, the spacing of joists for formwork decking can be determined
based on the support spacing of the Plyform in the tables.
When computing the allowable pressure of concrete on plywood,
the center-to-center distances between supports should be used to
determine the pressure based on the allowable bending stress in the
fibers.
When computing the allowable pressure of concrete on plywood
as limited by the permissible deflection of the plywood, it is proper to
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