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

386 Chapter Thirteen
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Example 13-2
Use the information in Table 13-1 to determine the values of y for a
shell roof for which the span is 60 ft, the rise is 10 ft, and the radius is
50 ft.
For x = l/10, x = 30 ft/10 = 3 ft,
2 2
y=R−
R −l/100
2
= 50 ft − (50ft) −(30ft)
2
/100
= 50 ft − 2,500 −900/100
= 50 ft − 2,
491
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= 50 ft – 49.9099
= 0.090 ft
For x = 2l/10, x = 6 ft,
2 2
y=R−
R −l/25
2
= 50 ft − (50ft) −(30ft)
2
/25
= 50 ft − 2,500 −900/25
= 50 ft − 2,
491
= 50 ft – 49.639
= 0.361 ft
Other values of y for increments of x are given in Table 13-2.
Forms for Circular Shell Roofs
The forms for circular shell roofs consist of decking plus the structural
members that support the decking, called centering, as illustrated in
Figure 13-3. The decking may be 5 ⁄8- or ¾-in. plywood supported by
joists, usually of at least 2-in.-thick lumber. The joists are supported by
lumber ribs, at least 2 in. thick that are sawed to provide the required
curvature. The ribs may bear directly on shores, but they preferably
bear on stringers that are supported by shores, as illustrated. It is
extremely important that ribs be cut to permit the full vertical bearing
of the ribs against the supporting stringers or shoring. A transverse
row of shores is installed under each rib. The shores may be wood,
patented, or metal scaffolding. The centering must include adequate
braces, both horizontal and diagonal, in the transverse and longitudinal
directions. Stringers should be securely fastened to shores with
scabs and nails, as illustrated.
The ribs are long enough to provide laps of approximately 12 in.
over the stringers. Also, the bottoms of the ribs are fabricated to
permit full bearing on the tops of the stringers.
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