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

Downloaded From : www.EasyEngineering.net
Forms for Beams and Floor Slabs 323
FIGURE 11-2 Forms for concrete beams and slab with single-span joists.
ww.EasyEngineering.n
design of slab forms, the thickness and grade of the floor decking are
often selected based on the availability of materials. Plyform is commonly
used as decking for slab forms.
Figure 11-2 illustrates a system of wood forms for a beam and slab
type concrete floor with single-span joists; that is, there are no intermediate
stringers of support between the ends of the joists. This system is
satisfactory for joists with relatively short spans. However, for longer
spans the required joist sizes may be too large for single spans. For this
condition it may be necessary to place intermediate stringers under the
joists, perpendicular to the joists, to provide support to the joists. This
is further discussed in later sections of this chapter.
The slab decking must have adequate strength and rigidity to resist
bending stresses, shear stresses, and deflection between joists. The
equations for determining the allowable span length of plywood decking
for uniformly distributed pressures were presented in Chapter 5
and are reproduced in Table 11-1 for clarity. The allowable stresses for
Bending l b
, in.
[120F b
S e
/w b
] 1/2
Eq. (5-41)
Shear l s
, in.
20F s
(Ib/Q)/w s
Eq. (5-48)
l d
, in. for
∆ = l/360
[1,743EI/360w d
] 1/3
Eq. (5-57a)
Bending Deflection
l d
, in. for
∆ = 1 / 16 in.
[1,743EI/16w d
] 1/4
Eq. (5-57b)
Notes:
1. See Tables 4-9 and 4-11 for physical properties of plywood and Plyform,
respectively.
2. See Tables 4-10 and 4-12 for allowable stresses of plywood and Plyform,
respectively.
3. Units for stresses and physical properties are pounds and inches.
4. Units for w are lb per linear ft for a 12-in.-wide strip of plywood.
TABLE 11-1 Equations for Calculating Allowable Span Lengths for Floor
Decking with Three or More Spans, Based on Bending, Shear, and Deflection
Due to Uniformly Distributed Pressure, in.
Downloaded From : www.EasyEngineering.net