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

434 Chapter Sixteen
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Deflection less than l/360, from Eq. (5-37a),
l ∆
= [1,743EI/360w] 1/3
= [1,743(1,600,000)(20.8)/360(292)] 1/3
= 82.0 in.
Below is a summary for the allowable span lengths of the joists:
For bending, the maximum span = 69.6 in.
For shear, the maximum span = 89.8 in.
For deflection, the maximum span = 82.0 in.
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In this example, bending governs the allowable span length of
the joists. For constructability, the double stringers must be spaced at
60 in., or 5 ft on centers.
Design of Stringers to Support Joists
The double stringers must support the joists. The loads applied to the
stringers from the joists will be every 20 in. along the stringer. The
magnitude of these loads can be calculated by multiplying the design
pressure by the spacing and span length of the joists as follows:
Point load on stringers, P = (175 lb per sq ft)(20/12 in.)(60/12 in.)
= 1,458 lb
Figure 16-2 shows the point loads from the joists on the stringers.
The length of the stringers will be less than the distance between the
steel beams because the stringers must fit between the steel beams.
For this example, the length of the stringers is 66 in., or the 5 ft 6 in.,
the distance between the coil bolts of the hangers. The load on each
hanger will be the 2,916 lb as shown in Figure 16-2.
The maximum bending moment will occur at the center of the
stringer and can be calculated by summing the moments of all forces
as follows:
Coil bolt hanger
2,916 lb
Joist
1,458 lb
Joist
1,458 lb
Joist
1,458 lb
Coil bolt hanger
2,916 lb
Joist
1,458 lb
3'' 20'' 20''
20''
3''
66''
FIGURE 16-2 Concentrated loads from joists on stringer.
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