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

46 Chapter Four
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dimension lumber incised to facilitate preservative treatment, the
incising factor C i
should be used. Incising adjustment factors apply to
dimension lumber that is incised parallel to grain a maximum depth
of 0.4 in., maximum length of ³⁄8 in., and density of incisions up to
1,100 per sq ft. The repetitive member adjustment factor permits a
15% increase in bending stress under specific conditions for joists,
truss chords, rafters, planks, or similar members that are spaced less
than 24 in. on centers. The adjustment factors for temperature, incising,
and repetitive-member can be found in ref. [4].
Tables 4-2 and 4-3 give basic reference design values for lumber
that is commonly used in construction of temporary structures and
formwork. Values in these tables must be adjusted based on the particular
condition in which the lumber will be used. Table 4-2 gives
reference design values for Southern Pine lumber. The values in the
table for Southern Pine are adjusted for size; therefore, C F
= 1.0.
Table 4-3 gives the reference design values for several other species of
lumber that are commonly used in construction. The values in Table 4-3
are not adjusted for size. Table 4-3a gives size adjustment factors C F
for the species of lumber shown in Table 4-3. The adjustment factors
provided in Tables 4-4 through 4-8 apply to all of the grades and
species of lumber given in Tables 4-2 and 4-3.
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Adjustment Factor C D
for Load-Duration
The reference design values for lumber tabulated in Tables 4-2 and
4-3 are based on normal load-duration of 10 years. For loads that are
applied for durations other than normal load-duration, the values
should be multiplied by the appropriate adjustment factor shown in
Table 4-4.
For many construction operations, particularly for formwork, the
duration of the loads is usually less than 7 days. Thus, the tabulated
design values shown in Tables 4-2 and 4-3 may be increased by 25%,
provided the load-duration will not be more than 7 days. However, it
should be noted that the modulus of elasticity and compression stress
perpendicular to grain are not adjusted for load-duration.
Adjustment Factors C M
for Moisture Content
The reference design values shown in Tables 4-2 and 4-3 are based on
lumber that with a moisture content of 19% or less. When dimension
lumber is used where the moisture will exceed 19%, the values in
Tables 4-2 and 4-3 should be reduced by the appropriate adjustment
factors given in Table 4-5.
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