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

358 Chapter Eleven
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Stringers for Support of Joists
For this design consider using 4-in.-thick stringers. Thus the actual
size of a stringer will be 3½ in. Determine the unit stress in bearing
between joists and stringers.
Contact area in bearing, A = 3.5 in. × 3.5 in.
= 12.25 sq in.
The total load in bearing between a joist and a stringer will be the
unit pressure of the concrete acting on an area based on the joist and
stringer spacing. For this design, the area will be 20 in. wide by 48 in.
long. Therefore, the pressure acting between the joist and the stringer
will be:
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P = 130 lb per sq ft [20/12 × 48/12]
= 867 lb
The calculated applied unit stress in bearing, perpendicular to
grain, between a joist and a stringer will be
f c⊥
= 867 lb/12.25 sq in.
= 70.8 lb per sq in.
The allowable compression stress perpendicular to grain for No.2
grade Southern Pine can be obtained by multiplying the reference
value for shear perpendicular to grain in Table 4-2 by the adjustment
factor for a wet condition (C M
= 0.67) in Table 4-5. There is no
load-duration adjustment factor in the compression perpendicular to
grain.
Allowable compression stress perpendicular to grain is
F c⊥
= C M
× (reference value of compression perpendicular to grain)
= (0.67)(565 lb per sq in.)
= 378.5 lb per sq in.
The allowable compressive unit stress of 378.5 lb per sq in. is
greater than the applied unit stress of 70.8 lb per sq in. Therefore, the
unit stress is within the allowable value for the 4 × 4 No. 2 grade
Southern Pine joists.
Size of Stringer Based on Selected 48-in. Spacing
Although the loads transmitted from the joists to the stringers are
concentrated, it is generally sufficiently accurate to treat them as uniformly
distributed loads, having the same total value as the concentrated
loads, when designing forms for concrete slabs. With a 48-in.
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