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

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Forms for Beams and Floor Slabs 337
The allowable stresses are based on a load-duration of 7 days
maximum, and not more than 19% moisture content. The deflections
are limited to l/360 with maxima not to exceed ¼ in. The values
are for lumber framing directly supporting the plywood. Although
Tables 11-7 and 11-8 are for single members, these tables can be
adapted for use with multiple members.
Stringers
Figure 11-2 illustrated a system of forms using joists supported at the
ends only. Although this system may be satisfactory for joists with
relatively short spans, the sizes of joists required for longer spans are
so large that it is economical to install one or more rows of stringers
at intermediate points under the joists, as illustrated in Figure 11-3.
Also, stringers must be used to support joists for flat slabs. The spacings,
sizes, and spans of the stringers should be selected to obtain the
maximum economy for the form system, considering the cost of
materials and labor and the salvage value of the lumber.
As illustrated in Figure 11-3, the strip of decking whose load is
supported by a joist is one whose width extends from midway
between supports on opposite sides of a stringer. This width is designated
by the dimension a. Because the loads acting on a stringer are
transmitted from the joists, they are concentrated. For this reason, the
calculation for the bending moment in a stringer is somewhat complicated,
and the magnitude of the moment will vary with the relative
position of the joists with respect to the shores that support the
stringers. If the reduced bending moment resulting from the continuous
spans of a stringer over several shores is neglected, this should
provide a safety factor large enough to offset the effect of considering
the loads from the joists to be uniformly distributed along the stringer
and will simplify the design of a stringer. This allows the designer to
use the equations previously presented for uniformly distributed
loads for calculating the bending, shear, and deflection of stringers.
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FIGURE 11-3 Forms for concrete beams and slab with intermediate stringer.
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