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

344 Chapter Eleven
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the formwork. The unit weight of the concrete is 150 lb per cu ft and
the permissible deflection is l/360, not to exceed ¹⁄16 in.
Class I Plyform, which is ¾ in. thick, will be used for the beam
bottom, backed by two 2 × 4 runners of dimension lumber laid in the
flat position. The runners will be No. 2 grade Spruce-Pine-Fir laid flat
in a dry condition with short load-duration of less than 7 days. The
applicable adjustment factors will be size C F
, load-duration C D
, and
flat-use C fu
. From Tables 4-3, 4-3a, 4-4, and 4-7, the allowable stresses
can be calculated as follows:
Allowable bending stress, F b
= C F
(C D
)(C fu
) (reference bending stress)
= 1.5(1.25)(1.1)(875 lb per sq in.)
= 1,804 lb per sq in.
Allowable shear stress, F v
= C D
(reference shear stress)
= 1.25(135 lb per sq in.)
= 168 lb per sq in.
Modulus of elasticity, E = 1,400,000 lb per sq in.
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The uniformly distributed pressure acting vertically against
a 1-sq-ft area of the Plyform beam bottom may be calculated as
follows:
Concrete, 150 lb per cu ft (24/12 ft) = 300 lb per sq ft
Assumed dead load of form material = 4 lb per sq ft
Live load, 50 lb per sq ft
= 50 lb per sq ft
Total load, w = 354 lb per sq ft
The Plyform must transfer this 354 lb per sq ft pressure between
the flat runners, perpendicular to the direction of the concrete beam.
The unsupported length of Plyform will be the distance between the
runners, perpendicular to the direction of the beam. This distance is
calculated as follows:
Total width across the concrete beam bottom = 16 in.
Subtracting the width of 2 runners, 2(3.5 in.) = 7 in.
Unsupported length of plywood = 9 in.
An examination of Tables 11-2 and 11-3 for Class I Plyform indicates
that the ¾-in. Plyform can safely support the 354 lb per sq ft load
over an unsupported length that is greater than 9 in. Therefore, the
¾-in. thickness is satisfactory. The load transferred from the runners
must now be transferred between the shores.
Discarding the vertical support provided by the side members of
the beam forms, the 2 × 4 runners that are laid flat must transfer the
total load of the beam bottom between shores. The total uniformly
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