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

360 Chapter Eleven
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Deflection in 4 ë 6 S4S Stringers
The permissible deflection is l/360, but not greater than ¹⁄16-in.
Using Eqs. (5-37a) and (5-37b), the allowable span lengths due to
deflection can be calculated as follows:
From Eq. (5-37a), l ∆
= [1,743EI/360w] 1/3 for ∆ = l/360
= [1,743(1,440,000)(48.53)/360(520)] 1/3
= 86.6 in.
From Eq. (5-37b), l ∆
= [1,743EI/16w] 1/4 for ∆ = ¹⁄16 in.
= [1,743(1,440,000)(48.53)/16(520)] 1/4
= 61.9 in.
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Summary for the 4 ë 6 S4S Stringers
For bending, the maximum span length of stringers = 73.5 in.
For shear, the maximum span length of stringers = 111.4 in.
For deflection, the maximum span length of stringers = 61.9 in.
For this design, deflection governs the maximum span length of
the stringers. Because the shores must support the stringers, the maximum
spacing of shores must be no greater than 61.9 in. For simplicity
in layout and constructability, a spacing of 60.0 in. center to center will
be used.
Shores for Support of Stringers
Based on the strength of the stringers in bending, shear, and deflection,
the stringers must be supported every 60 in. by a shore. A shore
must resist the concrete pressure that acts over an area consisting of
the spacing of the stringers and the span length of the stringers, 48 in.
by 60 in. The vertical load on a shore can be calculated as follows:
Area = 48 in. × 60 in.
= 2,880 sq in.
= 20.0 sq ft
Vertical load on a shore = 130 lb per sq ft (20.0 sq ft)
= 2,600 lb
Thus, a shore is required that will withstand a vertical load of
2,600 lb. There are numerous patented shores available that can sustain
this load. For this design, S4S wood shores will be installed with
a 10-ft unbraced length. Using a wood shore with an allowable compression
stress parallel to grain of 1,650 lb per sq in. and a modulus of
elasticity for column stability of 580,000 lb per sq in., Table 5-5 indicates
an allowable load of 4,688 lb for a 4 × 4 S4S wood shore.
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