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

Downloaded From : www.EasyEngineering.net
Forms for Beams and Floor Slabs 361
Bearing between Stringer and Shore
For this design, 4 × 6 S4S stringers will bear on 4 × 4 S4S wood
shores. Determine the unit stress in bearing between stringers and
shores.
Contact area, A = 3.5 in. × 3.5 in.
= 12.25 sq in.
The calculated unit stress in bearing, perpendicular to grain,
between a stringer and a shore will be:
f c⊥
= 2,600 lb/12.25 sq in.
= 212.2 lb per sq in.
ww.EasyEngineering.n
From Table 4-2, the tabulated design value for unit compression
stress, perpendicular to grain, for No. 2 grade Southern Pine is 565 lb
per sq in. The allowable design stress can be calculated by multiplying
the reference design value by the wet condition adjustment factor
(C M
= 0.67) (refer to Table 4-5). There is no load-duration adjustment
factor in compression stress perpendicular to grain. The allowable
bearing stress will be:
F c⊥
= (0.67)(565 lb per sq in.)
= 378.5 lb per sq in.
This allowable unit stress of 378.5 lb per sq in. is greater than the
applied unit stress of 212.2 lb per sq in. for this grade and species of
wood. The formwork must also be adequately braced to resist lateral
forces on the slab-forming system as presented in Chapter 5.
Design Summary of Forms for Concrete Slab
The allowable stresses and deflection criteria used in the design are
based on the following values.
For ¾-in.-thick Plyform Class I:
Allowable bending stress = 1,930 lb per sq in.
Allowable rolling shear stress = 72 lb per sq in.
Modulus of elasticity = 1,650,000 lb per sq in.
For 4 × 4 S4S joists, No. 2 grade Southern Pine:
Allowable bending stress = 1,593 lb per sq in.
Allowable shear stress = 212 lb per sq in.
Modulus of elasticity = 1,440,000 lb per sq in.
Permissible deflection less than l/360, but not greater than
¹⁄16 in.
Downloaded From : www.EasyEngineering.net