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

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Forms for Beams and Floor Slabs 351
support the stringers. The stringers are supported by slotted wedgetype
form clamps.
The coil-type hanger illustrated in Figure 11-10(b) consists of two
bent rods with two coil nuts welded to the ends. The lag screws,
which engage these nuts, support stringers, as illustrated. A flat steel
washer should be used with each screw.
The frame-type hanger, illustrated in Figure 11-10(c), is a type used
to support forms for concrete slabs where it is not necessary to encase
the steel beams in concrete. Lag screws, inserted from below, engage
the coil nuts, which are welded to the hanger frames. A flat steel
washer should be used with each screw.
The wire beam saddle hanger, illustrated in Figure 11-10(d), is a type
used to support joists where the total loads from concrete are relatively
small. Because there is no means of adjusting the lengths of these
hangers, they should be ordered for the exact sizes required. A steel
shim should be placed under the bottom of the joist, as illustrated, to
increase the area in bearing between the joist and the hanger.
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Designing Forms for Concrete Slabs
There are several approaches to the design for slab forms. One approach
is to select the material to be used for formwork, then use the equations
in Tables 5-3 and 5-4 in Chapter 5 to determine the spacing of the materials;
including decking, joists, stringers, and shores. Another approach
is to use tables that have been developed by a company that furnishes
formwork systems and accessories. Still another approach is to use
patented forming systems that have been developed by companies
that rent or sell formwork for concrete slabs. The latter approach is
discussed in Chapter 12. In the latter two approaches, the materials
and recommendations of the company that supplies the formwork and
its accessories must be followed.
Forms for concrete 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. The designer should know the magnitudes of the forces that
act on the component parts and the strengths of these parts in resisting
the forces. Chapter 3 presented pressures and loads that act on
formwork.
The strength of lumber varies with the species and grade used for
formwork. The allowable unit stresses in bending, horizontal shear,
compression perpendicular to the grain, and compression parallel to
the grain are presented in Chapter 4. Each design should be based on
information that applies to the allowable unit stresses for the species
and grade of lumber used for the formwork.
As presented in Chapter 5, many calculations are required to
determine the adequate strength of formwork members. It requires
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