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

320 Chapter Eleven
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Safety of Slab-Forming Systems
Forms for concrete beams and slabs should provide sufficient strength
and rigidity at the lowest practical cost, considering materials, labor,
and any construction equipment used in making, erecting, and removing
them. Consideration must be given to both the static dead load
and any impact loads that may be applied to the forming system. The
forming system must provide adequate resistance to lateral forces that
may be imposed, in addition to the vertical loads from concrete, workers,
tools, and equipment.
In many instances, the failure of formwork is a result of improper
or inadequate shoring for slabs. The shores that support slab-forming
systems must have sufficient load capacities, and they must be
securely fastened at the bottom and top ends to prevent movement or
displacement while they are in use. It is especially important to attach
both ends of the shores to the slab form because it is possible for the
slab form to lift off the top of a shore due to unbalanced loading during
placement of concrete. Also, a shore system may shift due to inadequate
support at the bottom of a shore. Two-way horizontal and
diagonal braces should be installed to brace the shores adequately for
slab formwork.
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Loads on Concrete Slabs
Prior to designing the forms to support a concrete slab, it is necessary
to know the magnitude of the loads that the forms must support.
The loads that will be applied to the slab forms include the
dead weight of the reinforcing steel and the freshly placed concrete,
the dead load of the formwork materials, and the live load of
workers, tools, and equipment. Chapter 3 presents loads on concrete
formwork.
The effect of impact loads should also be included in designing
forms, as that caused by motor-driven concrete buggies, concrete falling
from a bucket, or pumping of concrete. The effect of concrete falling
from a bucket is discussed in Chapter 7.
The unit weight of concrete for most structures is 145 to 150 lb per
cu ft. It is common to refer to normal concrete as having a unit weight
of 150 lb per cu ft. The load from the concrete on a sq ft of the floor
decking will be:
p = w c
(h)
where p = load, lb per sq ft
w c
= unit weight of concrete, lb per cu ft
h = thickness of concrete, ft
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