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

Downloaded From : www.EasyEngineering.net
Forms for Concrete Bridge Decks 433
For this example, deflection governs the spacing of the Plyform
decking. For constructability, the joists will be spaced at 20 in. on
centers.
Design of Joist to Support Decking
If the joists and the decking are to be prefabricated, 2 × 6 S4S joists
may be desirable. However, if the joists are to be placed across stringers
followed by placement of the decking on the joists, it may be
desirable to use 4 × 4 or 4 × 6 S4S joists because the 4-in. width is more
easily laid across the stringers and will stand in place without falling
sideways. Also, the larger size joists will allow longer spans for the
joist that will reduce the cost for the labor of erecting and removing
the forms.
For this example assume that the joists and decking will be
prefabricated in panels of convenient lengths. With joists spaced at
20 in. on centers, the uniform load on a joist can be calculated as
follows:
ww.EasyEngineering.n
w = (175 lb per sq ft)(20/12 ft)
= 292 lb per lin ft
Consider 2 × 6 joists. The physical properties for S4S lumber are
presented in Table 4-1. For dry condition and load-duration less than
7 days, the allowable stresses for No. 2 grade Southern Pine can be
obtained from Tables 4-2 and 4-4 as follows:
Allowable bending stress, F b
= C D
× (reference bending stress)
= 1.25(1,250)
= 1,562 lb per sq in.
Allowable shear stress, F v
= C D
× (reference shear stress)
= 1.25(175)
= 218 lb per sq in.
Modulus of elasticity, E = 1,600,000 lb per sq in.
Determine the maximum permissible span length of 2 × 6 S4S
joists (Table 5-3) as follows:
For bending, from Eq. (5-34),
For shear, from Eq. (5-36),
l b
= [120F b
S/w] 1/2
= [120(1,562)(7.56)/292] 1/2
= 69.6 in.
l v
= 192F v
bd/15w + 2d
= 192(218)(1.5)(5.5)/15(292) + 2(5.5)
= 89.8 in.
Downloaded From : www.EasyEngineering.net