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

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Design of Wood Members for Formwork 103
The allowable shear stress of 168.8 lb per sq in. is greater than the
applied shear stress of 89.1 lb per sq in.; therefore, the beam is satisfactory
for shear stress.
Check Bending Capacity
For a uniformly distributed load on a single span 4 × 6 beam, the
bending moment and stress can be calculated as follows:
From Eq. (5-5), M = wl 2 /96 = [(450 lb per ft) × (72 in.) 2 ]/96
= 24,300 in.-lb
From Eq. (5-10), f b
= M/S = 6M/bd 2 = [6(24,300)]/[(3.5) × (5.5) 2 ]
= 1,376.8 lb per sq in.
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Determine the allowable bending stress for a 4 × 6 No. 2 grade
Spruce-Pine-Fir with a 7-day load-duration as follows:
Table 4-3, reference design value in bending = 875 lb per sq in.
Table 4-3a, size adjustment factor for 4 × 6, C F
= 1.3
Table 4-4, adjustment factor for 7-day load-duration, C D
= 1.25
Allowable bending stress, F b
= (875 lb per sq in.)(C F
)(C D
)
= (875 lb per sq in.)(1.3)(1.25)
= 1,421.9 lb per sq in.
The allowable bending stress of 1,421.9 lb per sq in. is greater than
the applied bending stress of 1,376.8 lb per sq in. Therefore, the 4 × 6
beam is adequate for bending. No lateral support is required because
the d/b ratio is less than 2.
Example 5-7
Consider the beam previously selected to resist the bending moment
in Example 5-3. It was determined that a 2 × 4 beam of No. 2 grade
Hem-Fir would be required for bending. Check the shear capacity of
this beam using the modified method of determining the unit shear
stress.
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