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

328 Chapter Eleven
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Plywood
Thickness,
in.
¹ 5 ⁄32 3,560
(3,560)
½ 3,925
(3,925)
¹ 9 ⁄32 4,110
(4,110)
5
⁄8 4,305
(4,305)
23
⁄32 5,005
(5,005)
¾ 5,070
(5,070)
1 1 ⁄8 7,240
(7,240)
Support Spacing, in.
4 8 12 16 20 24 32
890
(890)
980
(980)
1,225
(1,225)
1,310
(1,310)
1,590
(1,590)
1,680
(1,680)
2,785
(2,785)
360
(395)
410
(435)
545
(545)
580
(580)
705
(705)
745
(745)
1,540
(1,540)
155
(205)
175
(235)
245
(305)
270
(330)
350
(400)
375
(420)
835
(865)
115
115
100
(135)
145
(190)
160
(215)
210
(275)
230
(290)
545
(600)
—
—
—
—
—
(100)
—
100
110
(150)
120
(160)
310
(385)
—
—
—
—
—
—
—
—
—
—
—
—
145
(190)
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Notes:
1. Courtesy APA—The Engineered Wood Association, “Concrete Forming,” 2004.
2. Deflection limited to l/360th of the span, l/270th for values in parentheses.
3. Plywood continuous across two or more spans.
TABLE 11-4 Recommended Maximum Pressures on Structural I Plyform—Values
in Pounds per Square Foot with Face Grain Across Supports
An analysis may be made to determine the economy of using
either grade No. 1 or No. 2 lumber for joists, considering the difference
in the cost of the lumber and the potential salvage value of each
grade. Joist sizes for concrete floor formwork are usually selected as
4-in.-thick members for stability, to allow workers to place joists
across stringers without the joists falling sideways.
The economy of member sizes should be investigated prior to making
the selection. The cost of joists includes the cost of the lumber plus
the labor cost of fabricating and erecting the joists, less the salvage
value of the lumber after the forms are stripped. The bending strength
of a joist of a given grade and species of lumber is directly related to its
section modulus. For example, a 4 × 6 S4S joist is 2.46 times stronger
than a 4 × 4 S4S joist, whereas the ratio for quantity of lumber for a
given length is 1.57. The labor cost of fabricating and erecting a 4 × 6
joist may be little, if any, more than that for a 4 × 4 joist. For these reasons,
it may be more economical to use 4 × 6 joists instead of 4 × 4 joists
if the additional strength of the former size can be fully utilized.
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