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

150 Chapter Five
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The design will include the decking, joists, and stringers. Requirements
for shoring of formwork are discussed in the following section.
Similar to wall forms, in the design of slabs the thickness and grade of
the plywood are often selected based on the availability of materials.
The following design criteria will apply:
1. Thickness of concrete slab, 7 in.
2. Concrete unit weight, 150 lb per cu ft.
3. Deflection limited to l/360, but not greater than ¹⁄16 in.
4. All form lumber will be S4S No. 2 grade Douglas Fir-Larch
with no splits, assuming a short-term load-duration and
wet condition.
5. Decking for the slab form will be 7 ⁄8 in. plywood sheathing,
from the Group II species with a S-2 stress rating.
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The design will be performed in the following sequence: determine
the dead and live loads on the slab forms; determine the
spacing of joists based on the strength and deflection of plywood
decking; determine the spacing of stringers based on the strength
and deflection of joists; check the bearing strength between the
joists and stringers; determine the spacing of shores based on the
strength and deflection of stringers; and check the bearing strength
between stringers and shores.
Loads on Slab Forms
The loads that will be applied to the slab forms include the dead
weight of the freshly placed concrete and reinforcing steel, the dead
load of the formwork materials, and the live load of workers, tools,
and equipment. If the live load will produce impact, as that caused by
motor-driven concrete buggies or by concrete falling from a bucket,
the effect of the impact should be considered in designing the forms.
For this design it is assumed that there will be no impact. The effect of
the impact of concrete falling from a bucket is discussed in Chapter 7.
The load from the concrete slab on a square foot of plywood
decking can be calculated as follows:
Uniform load per sq ft = 150 lb per cu ft (7/12 ft)
= 87.5 lb per sq ft
The dead load due to the weight of formwork material is assumed
as 5.5 lb per sq ft. The recommended minimum live load according to
ACI Committee 347 is 50 lb per sq ft. Following is a summary of the
loads on the slab form that will be used for the design:
Dead load of freshly placed concrete = 87.5 lb per sq ft
Assume dead load of formwork materials = 5.5 lb per sq ft
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