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

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Design of Wood Members for Formwork 157
the spacing of the stringers and the span length of the stringers, 48 in.
by 60 in. The vertical load on a shore can be calculated as follows:
Area = 48 in. × 60 in.
= 2,880 sq in.
= 20.0 sq ft
Vertical load on a shore = 143 lb per sq ft (20.0 sq ft)
= 2,860 lb
Thus, a shore is required that will withstand a vertical load of
2,860 lb. There are numerous patented shores available that can sustain
this load. For this design, S4S wood shores will be installed with
a 10-ft unbraced length. Using a wood shore with an allowable compression
stress parallel to grain of 1,552 lb per sq in. and a modulus of
elasticity for column stability of 580,000 lb per sq in., Table 5-5 indicates
an allowable load of 4,672 lb for a 4 × 4 S4S wood shore.
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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⊥
= 2,860 lb/12.25 sq in.
= 233.5 lb per sq in.
From Table 4-3, the tabulated design value for unit compression
stress, perpendicular to grain, for No. 2 grade Douglas Fir-Larch is
625 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)(625 lb per sq in.)
= 418.7 lb per sq in.
This allowable unit stress of 418.7 lb per sq in. is greater than
the applied unit stress of 233.5 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. This topic is discussed
in the following sections.
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