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

322 Chapter Eleven
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of wood material, usually 4 in. thick with a depth that depends
on the loads applied to the form, the species and grade of
lumber, and span length of the joist. Joists may also be metal
beams or trusses.
3. Stringers are members under the joists that provide support for
the joists. Stringers are usually single members of wood material,
usually 4 in. thick with a depth that depends on the loads applied
from the joists, the species and grade of lumber, and span length
of the stringer. Stringers may also be metal beams or trusses.
4. Shores are members that support the joists and stringers and
beam bottoms for beam-slab forming systems. Shores may be
single wood posts, single steel joists, or shoring frames.
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Design of Forms for Concrete Slabs
The steps in the design of forms to support concrete slabs include the
following:
1. Determine the total unit load on the floor decking, including
the effect of impact, if any.
2. Select the kind and net thickness of the floor decking.
3. Determine the safe spacing of floor joists, based on the
strength and permissible deflection of the decking.
4. Select the floor joists, considering the load, kind, size, and
length of the joist.
5. Select the kind, size, and length of the stringers to support the
joist.
6. Select the kind, size, and safe spacing of the shores, considering
the load, the strength of the stringer, and the safe capacity
of the shores.
Usually the most economical forms result when the joists are spaced
for the maximum safe span of the decking. Similarly, reasonably large
joists, which permit long spans, thus requiring fewer stringers, will be
economical in the cost of materials and in the cost of labor for erecting
and removing the forms. The use of reasonably large stringers will permit
the shores to be spaced greater distances apart, subject to the safe
capacities of the shores, thus requiring fewer shores and reducing the
labor cost for erecting and removing them.
Spacing of Joists
The joists in a slab-forming system provide support for the floor
decking. Therefore the maximum spacing of joists is determined by
the allowable span length of the decking that rests on the joists. In the
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