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

132 Chapter Five
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= 1,600 (1.065 − 0.504)
= 1,600 (0.561)
= 897.6 lb per sq in.
Using Eq. (5-62), the allowable compression load can be calculated
by multiplying the allowable compression stress times the
actual cross-sectional area of the member:
P c
= F c
A
= (897.6 lb per sq in.) × [(3½ in.) × (5½ in.)]
= (897.6 lb per sq in.) × (19.25 sq in.)
= 17,279 lb
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Table for Allowable Loads on Wood Shores
Table 5-5 shows allowable compression loads in rectangular solid
wood shores for the specified condition. The loads in this table are
calculated using Eqs. (5-61) and (5-62) for dry condition wood having
a moisture content of less than 19%. The calculated values are based
on sawn wood; therefore, the value of c = 0.8 is used. If shores are
used that have design values different than those shown in Table 5-5,
Eqs. (5-61) and (5-62) must be used to determine the allowable load
on the shore. For a particular job condition, it may be necessary to
adjust the values used in calculating the allowable loads shown in
Table 5-5.
Shores of 4 × 4 S4S lumber are commonly used because screw
jacks are readily available for this size lumber. Therefore, Table 5-5
shows values for 4 × 4 lumber. Screw jacks are not commonly available
for 4 × 6 or 6 × 6 lumber. However, the 4 × 6 and 4 × 4 sizes of
lumber are sometimes used for reshoring. The most common method
of shoring is patented shores or scaffolding. Manufacturers of shoring
and scaffolding have devoted considerable effort to developing
shoring systems that are reliable and easily erected and removed.
Towers can be erected of scaffolding to the desired height and the
required strength, provided the manufacturer’s recommendations
are followed. Chapter 6 provides additional information on shoring
and scaffolding.
Bearing Stresses Perpendicular to Grain
When a wood member is placed on another wood member, bearing
stresses are created that tend to crush the fibers of the wood. For
example, joists rest, or bear, on stringers. Similarly, stringers rest on
posts or shores, and studs bear against wales. When a joist is placed
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