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

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Properties of Form Material 53
Application of Adjustment Factors
The designer must assess the condition the lumber will be used and
then determine applicable adjustment factors that should be applied
to the reference design values. Knowledge of the specifications of the
NDS, ref. [4], is necessary to properly design wood members. The following
examples illustrate applying adjustment factors to reference
design values to determine allowable stresses. Additional examples
are given in Chapter 5.
Example 4-1
A 3 × 8 beam of No. 2 grade Hem-Fir will be used to temporarily support
a load at a job site during construction. The beam will be dry, less
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than 19% moisture content, and the applied loads will be less than
7 days. The adjusted stresses for bending and shear stresses for this
beam can be calculated as follows:
From Table 4-3, the reference design value for bending of a 3 × 8
No. 2 grade Hem-Fir for bending is 850 lb per sq in. The allowable
bending stress is obtained by multiplying the reference design value
by the adjustment factors for size and load-duration. Table 4-3a shows
a size factor for a 3 × 8 beam in bending as C F
= 1.2 and Table 4-4
shows a load-duration factor as C D
= 1.25. Therefore, the adjusted
bending stress can be calculated as:
F′ b
= (reference bending design value) × C F
× C D
= (850 lb per sq in.)(1.2)(1.25)
= 1,275 lb per sq in.
Before determining the bending adequacy of this member to resist
the applied loads, the support constraints related to the d/b ratio
must be satisfied as discussed in Chapter 5. Also, the beam stability
factor must be in accordance with the NDS.
For shear Table 4-3 shows the reference design value is 150 lb per
sq in. and Table 4-4 shows a load-duration adjustment factor C D
=
1.25. Therefore, the adjusted shear stress can be calculated as:
F′ v
= (reference shear design value) × C D
= (150 lb per sq in.)(1.25)
= 187.5 lb per sq in.
Example 4-2
Lumber of 2 × 8 of No. 1 grade Southern Pine will be laid flat as formwork
for underside of a concrete beam. The load-duration will be less
than 7 days and the lumber will be used in a wet condition. Determine
the adjusted stress for bending.
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