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

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Design of Wood Members for Formwork 105
Most specifications for concrete structures limit the deflection of
formwork members in order to prevent excessive wave effects on the
surface of the concrete. For formwork members where appearance is
important, the permissible deflection may be limited to l/360, or as
¹⁄16 in. where l is the distance between the centers of supports. An
example of important appearance is sheathing for flooring where it is
desirable to have a smooth concrete surface without waves between
the supporting joints of the sheathing. For limitations in deflections,
the distance between supports for which the two will be the same is
obtained by equating the two values.
l/360 = ¹⁄16
l = 22.5 in.
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For formwork members where strength is the primary concern
and appearance is secondary, the allowable deflection may be specified
as l/270, or ¹⁄8 in., where l is the distance between the centers
of supports. For example, the inside surface of a parapet wall
around a roof will only be exposed to maintenance workers, rather
than the general public. Thus, appearance may not be important.
For limitations in deflections, the distance between supports for
which the two will be the same is obtained by equating the two
values.
l/270 = ¹⁄8 in.
l = 34 in.
For spans less than 34 in. the l/270 limit is more rigid, and for
spans greater than 34 in. the ¹⁄8 in. is more rigid. For long structural
members, some designers limit the deflection to ¼ in.
Thus, l/360 is a more rigid deflection requirement than l/270. The
selection of these two limiting deflection criteria depends on the relative
importance of appearance and strength. If appearance is not
important, the less stringent requirement will allow economy in the
construction costs of formwork.
Deflection of Beams with Concentrated Loads
A beam may have one or more concentrated loads applied at various
locations on the beam. The loads cause the beam to deflect downward.
The amount of resistance to deflection is called rigidity. For a
beam the amount of rigidity is determined by the physical properties
of the beam, EI. The amount of deflection depends on the magnitude
and location of the applied loads, length and size of beam, grade and
species of the beam, and the number and location of supports underneath
the beam.
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