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

348 Chapter Eleven
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The physical properties of section modulus and moment of inertia of
the 2 × 4 runners can be obtained from Table 4-1 in Chapter 4 as follows:
Section modulus of two members, S = 3.06 in. 3
Moment of inertia of two member, I = 5.36 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(1,593)(3.06)/214] 1/2
= 52.2 in.
ww.EasyEngineering.n
For shear stress, the maximum span length
from Eq. (5-37), l v
= 192F v
bd/15w + 2d
= 192(187)(1.5)(3.5)/15(214) + 2(3.5)
= 65.7 in.
For deflection not to exceed l/360, the maximum span length
from Eq. (5-37a), l ∆
= [1,743EI/360w] 1/3
= [1,743(1,300,000)(5.36)/360(214)] 1/3
= 54.0 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(1,300,000)(5.36)/16(214)] 1/4
= 43.3 in.
Below is a summary of the allowable span lengths of the 2-in.-
thick beam bottom:
For bending, the maximum span length = 52.2 in.
For shear, the maximum span length = 65.7 in.
For deflection, the maximum span length = 43.3 in.
For this example, deflection governs the maximum span length of
the beam bottom. Therefore, the shores must be placed at a distance
not to exceed 43.3 in. to prevent excessive deflection of the beam bottoms.
For constructability the spacing would likely be at 40 in. on
centers. The 2 × 4 runners have a d/b < 2; therefore, beam stability is
satisfied (refer to Table 4-6).
Forms for Exterior Beams
Figure 11-8 illustrates a method of constructing forms for an exterior
beam. A brace should be installed from the exterior end of each shore
head to the top of a stud. The 2 × 4 wale along the tops of the studs
ensures better alignment of the forms between braces.
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