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

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Forms for Beams and Floor Slabs 345
distributed linear load of the beam bottom between supports may be
calculated as follows:
Dead load of concrete,
150 lb per cu ft (24/12)(16/12) = 400.0 lb per ft
Assumed dead load of formwork,
4 lb per sq ft (16/12) = 5.3 lb per ft
Live load on beam bottom,
50 lb per sq ft (16/12) = 66.7 lb per ft
Total load, w = 472.0 lb per ft
ww.EasyEngineering.n
The two 2 × 4 runners laid flat must safely transfer this 472 lb per
ft load between shores. Because there are two runners, each runner
must sustain one-half of this load, or 236 lb per ft. The physical properties
of section modulus and moment of inertia of the runners can be
obtained from Chapter 4 as follows:
From Table 4-1:
Section modulus, S = 1.313 in. 3
Moment of inertia, I = 0.984 in. 4
The allowable span length of the 2 × 4 runners between shores
based on bending, shear, and deflection may be calculated as
follows:
For bending, the maximum span length
from Eq. (5-34), l b
= [120F b
S/w] 1/2
= [120(1,804)(1.313)/236] 1/2
= 34.7 in.
For shear, the maximum span length
from Eq. (5-36), l v
= 192F v
bd/15w + 2d
= 192(168)(3.5)(1.5)/15(236) + 2(1.5)
= 50.8 in.
For deflection not to exceed l/360, the maximum span length
from Eq. (5-37a), l ∆
= [1,743EI/360w] 1/3
= [1,743(1,400,000)(0.984)/360(236)] 1/3
= 30.4 in.
For deflection not to exceed ¹⁄16 in., the maximum span length
from Eq. (5-37b), l ∆
= [1,743EI/16w] 1/4
= [1,743(1,400,000)(0.984)/16(236)] 1/4
= 28.2 in.
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