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

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Forms for Concrete Bridge Decks 445
have adequate strength and rigidity to span the 20-in. distance between
the supporting steel joists. Assume a 5 lb per sq ft dead load to include
the weight of the Plyform decking and wood joists. Then the design
load on the wood joists will be 225 + 5 = 230 lb per sq ft. Since the proposed
spacing of the wood joists is 24 in., the uniformly distributed
load on the wood joists will be (230 lb per sq ft) (24/12 ft) = 460 lb per
lin ft. Table 4-17 indicates a 2 × 4 S4S wood joist, which is continuous
over four or more supports and has a maximum span length of 32 in.
for an equivalent uniform load of 600 lb per lin ft. Because the design
load of 460 lb per lin ft is less than the 600 lb per lin ft, and the 32 in. is
greater than 22 in., the 24-in. spacing of the wood joists on centers with
a 20-in. span length will be satisfactory.
Now the decking must be checked for adequate strength and
rigidity to sustain the 225 lb per sq ft pressure between the supporting
wood joists. The 7 ⁄8-in. Class I Plyform decking will rest on the
wood joists, which are proposed at a 24-in. spacing on centers. Therefore,
the decking must have adequate strength and rigidity to sustain
the 225 lb per sq ft pressure over the 24-in. span length, the spacing of
the supporting wood joists.
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From Table 4-11, the physical properties of 7 ⁄8-in. Class I Plyform for
stress applied parallel to grain are
Section modulus, S e
= 0.584 in. 3
Moment of inertia, I = 0.296 in. 4
Rolling shear constant (Ib/Q) = 8.555 in. 2
From Table 4-12, the allowable stresses for Class I Plyform are
Allowable bending stress, F b
= 1,930 lb per sq in.
Allowable shear stress, F s
= 72 lb per sq in.
Modulus of elasticity, E = 1,650,000 lb per sq in.
The Plyform decking will be continuous over multiple wood
joists. Therefore, the equations for stress and deflection for span conditions
of three or more spans will apply (see equations in Table 5-4).
Determine the spacing of wood joists based on bending, shear, and
deflection of the Plyform decking joists as follows:
For bending in the 7 ⁄8-in. Plyform decking, from Eq. (5-41),
l b
= [120F b
S e
/w b
] 1/2
= [120(1,930)(0.584)/225] 1/2
= 24.5 in.
For shear in the 7 ⁄8-in. Plyform decking, Eq. (5-48),
l s
= 20F s
(Ib/Q)/w s
= (20)(72)(8.555)/(225)
= 54.7 in.
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