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

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Forms for Beams and Floor Slabs 331
The allowable span lengths for the joists with single spans can be
calculated for bending, shear, and deflection as follows.
For bending in the joist, the maximum span length from
Table 11-2,
l b
= [96F b
S/w] 1/2
= [96(1,875)(7.15)/250] 1/2
= 71.7 in.
For shear stress in the joist, the maximum span length from
Table 11-2,
ww.EasyEngineering.n
l v
= 16F v
bd/w + 2d
= 16(225)(3.5)(3.5)/250 + 2(3.5)
= 183.4 in.
For deflection not to exceed l/360, the maximum span length
from Table 11-2,
l ∆
= [4,608EI/1,800w] 1/3
= [4,608(1,700,000)(12.51)/1,800(250)] 1/3
= 60.1 in.
For deflection not to exceed ¹⁄16 in., the maximum span length
from Table 11-2,
l ∆
= [4,608EI/80w] 1/4
= [4,608(1,700,000)(12.51)/80(250)] 1/4
= 47.0 in.
Summary for the 4 ë 4 Joist
For bending, the maximum span length of the joist = 71.7 in.
For shear, the maximum span length of the joist = 183.4 in.
For deflection, the maximum span length of the joist = 47.0 in.
For this example, deflection controls the maximum span length of
the joist. Therefore, the joist must be supported at a distance of not
greater than 47.0 in., or about 4 ft.
Example 11-3
The total load on a decking is 175 lb per sq ft. The decking will be
supported by 7-ft-long single-span joists spaced at 18 in. on centers.
Consider 6-in.-wide No. 1 grade Southern Pine and determine the
minimum section modulus required. The lumber will be used at a
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