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

164 Chapter Six
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while concrete was being placed. The subject of failures of formwork
is discussed in Chapter 7.
Table 5-5 and Figure 5-12 show the relations between the capacities of
shores and their effective lengths. For example, the indicated allowable
load on a 4 × 4 S4S wood shore 6 ft long is 11,076 lb for wood with an
allowable unit stress in compression of 1,650 lb per sq in. and a modulus
of elasticity for column stability of 580,000 lb per sq in., whereas the indicated
allowable load on the same shore with an effective length of 12 ft is
only 3,318 lb. The former load is approximately three times the latter load.
In this instance, doubling the effective length of the shore reduces the
allowable load to 30% of the allowable load for the 6-ft length.
Table 6-1 gives the allowable loads on wood shores whose unsupported
lengths vary from 6 to 14 ft, expressed as percentages of the
Size of Shore, in.
F * , lb per sq in.
c
E min
, lb per sq in.
4 × 4
F ∗ = 1,650
c
E′ min
= 580,000
4 × 4
F ∗ = 1,552
c
E ′ min
= 580,000
4 × 4
F ∗ = 1,495
c
E ′ min
= 470,000
4 × 4
F ∗ = 1,322
c
E ′ min
= 510,000
4 × 6
F ∗ = 1,600
c
E ′ min
= 580,000
4 × 6
F ∗ c = 1,485
E ′ min
= 580,000
4 × 6
F ∗ c = 1,430
E ′ min
= 470,000
4 × 6
F ∗ c = 1,265
E ′ min
= 510,000
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Percent of the Allowable Load on a Shore 6 ft Long
Effective Length, ft
6 7 8 9 10 11 12 13 14
100 79.3 63.3 49.8 42.3 35.4 30.0 25.7 22.2
100 79.8 63.8 52.0 42.9 35.8 30.7 26.1 22.6
100 77.7 61.7 49.9 40.9 34.5 28.9 24.8 21.4
100 80.1 64.4 52.3 43.3 36.2 30.7 26.8 22.8
100 79.5 63.6 51.5 42.5 35.7 31.2 25.9 22.5
100 81.2 66.5 54.7 45.6 38.7 33.0 28.4 24.6
100 78.6 62.6 49.9 41.6 34.8 29.4 25.3 21.8
100 80.7 64.9 53.0 43.7 36.7 30.9 26.6 23.2
TABLE 6-1 Relations between Allowable Loads and Unsupported Lengths of Wood
Shores
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