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

444 Chapter Sixteen
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Example 16-2
Determine the size, required strength, and safe spacing for steel joists
for a concrete bridge deck, given the following conditions:
Distance face to face between bridge piers, 25 ft 6 in.
Length of span for steel joists, 25 ft 0 in.
Unit weight of concrete, 150 lb per cu ft
Thickness of concrete slab, 14 in.
Assumed live load on slab, 50 lb per sq ft
Proposed decking, 7 ⁄8-in. Class I Plyform
Proposed spacing of wood joists, 24 in.
Deflection criterion, l/270, not to exceed ¼ in.
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The uniform pressure on the decking will be
Concrete (150 lb per cu ft)(14/12 ft) = 175 lb per sq ft
Live load = 50 lb per sq ft
Total pressure = 225 lb per sq ft
The size of the wood joists selected will depend on the weight
from the concrete and the strength and safe spacing of the steel joist,
one or more of which should be assumed initially. For this example,
the proposed 7 ⁄8-in. decking is proposed for placement on wood joists
spaced at 24 in. on centers.
Table 16-1 shows that, for a span of 25 ft 0 in., the allowable safe
total load on the listed steel joists, including the weight from the concrete,
the forms, and the joists, varies from 396 to 550 lb per lin ft, or
from 312 to 520 lb per lin ft for deflection limited to l/360 of the span.
Consider a 20K4 joist, for which the safe load is 312 lb per lin ft for a
deflection of l/360. Adjusting for the l/270 deflection criterion for this
example, the allowable load will be (360/270)(312), or 416 lb per lin ft.
However, this value exceeds the maximum permissible safe load of
396 lb per lin ft for this steel joist (see Table 16-1). Thus, the safe load is
limited to 396 lb per lin ft. Because this steel joist weighs 7.6 lb per lin
ft, the net safe load on a joist will be 396 lb per lin ft less the 7.6 lb per
lin ft dead load of the joist, or 388.4 lb per lin ft. Assuming that the
decking, wood joists, and bridging for the steel joists weigh 6 lb per sq
ft, the total weight on the joist will be 231 lb per sq ft. For this condition,
the maximum safe spacing of the joist will be 388.4 lb per lin ft
divided by the 231 lb per sq ft, or 1.68 ft, which is 20.2 in. Therefore, the
spacing of the steel joists must not exceed 20.2 in. For constructability,
choose a 20-in. center-to-center spacing of the steel joists.
A check must be made to determine the adequacy of the wood
formwork above the steel joists, the wood joists that will rest on the
steel joists, and the decking that will rest on the wood joists.
The wood joists will be placed in a direction perpendicular to the
steel joists. Thus, the wood joists that rest on top of the steel joists must
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