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

346 Chapter Eleven
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Below is a summary of the allowable span lengths of the 2-in.-
thick beam bottom:
For bending, the maximum span length = 34.7 in.
For shear, the maximum span length = 50.8 in.
For deflection, the maximum span length = 28.2 in.
For this example, deflection governs the maximum span length of
the beam bottom. Therefore, the shores must be placed at a distance not
to exceed 28.2 in. to prevent excessive deflection of the beam bottoms.
For constructability, the shores will be placed at 24 in. on centers.
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Example 11-6
Determine the safe spacing of shores based on bending, shear, and
deflection of the beam bottom as illustrated in Figure 11-7(c). The
beam is 16 in. wide and the total depth from the top of the concrete
slab to the bottom of the beam is 24 in. Consider a 50 lb per sq ft live
load on 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 three 2 × 4 runners of dimension lumber placed in
the upright position. The runners will be No. 2 grade Hem-Fir in a
dry condition with a short load-duration of less than 7 days. The
applicable adjustment factors will be size C F
and load-duration C D
.
From Tables 4-3, 4-3a, and 4-4 the allowable stresses can be calculated
as follows:
Allowable bending stress, F b
= C F
(C D
) (reference bending stress)
= 1.5(1.25)(850 lb per sq in.)
= 1,593 lb per sq in.
Allowable shear stress, F v
= C D
(reference shear stress)
= 1.25(150 lb per sq in.)
= 187 lb per sq in.
Modulus of elasticity, E = 1,300,000 lb per sq in.
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 formwork = 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 runners, perpendicular to the direction of the concrete beam.
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