Materials Links & Resources: Wood

Wood Species

Woods for structural applications are typically chosen as a matter of economics. The most commonly used woods, grouped in species combination, are the Southern Pines, Spruce-Pine-Fir (SPF) and the Douglas Fir-Larch (see table W1 below).

Table W1. - Most Common Species Combinations - NDS (1991)

Species Combination	Species that may be included in combinations
Douglas Fir-Larch	Douglas Fir
Douglas Fir-Larch	Western Larch
Spruce-Pine-Fir (south)	Balsam Fir
	Black Spruce
	Jack Pine
	Norway (Red) Pine
	White Spruce
	Engelmann Spruce
	Lodgepole Pine
	Sitka Spruce
Southern Pine	Loblolly Pine
	Longleaf Pine
	Shortleaf Pine
	Slash Pine

Wood Properties

There are two values of properties given in the literature for wood: maximum and base design values. Maximum properties found in references like Tsoumis (1991) and the Wood Handbook (1990) are primarily for scientific purposes or for comparisons. Base design values are greatly reduced from these maximum values because of the statistical nature of wood and can be found in the National Design Specification (1991). These values must be adjusted by applying service adjustment factors C_i where i denotes the type of factor, e.g., M = moisture, L = column stability, D = load duration, F = size factor, fu = flat use, etc. Details on these and other factors including examples can be found in the NDS manuals.

Example 1:

In the design of a footbridge, you decide to use a Douglas Fir-Larch No. 2 as your material. Things you have to consider:

Moisture(C_M) - Will it be exposed to the environment?
Load Duration(C_D) - For what length of time will the load reside on the structure (see table W2 below)?
Flat use(C_fu) - Will the piece of wood bear load on its wide face?
Size factor(C_F) - Tables in NDS for all species except the Southern Pines are based on a width of 12in.

Moisture:

Must apply adjustment factor when expected moisture content exceeds 19%. So for the example, it is expected that the bridge will be exposed to rain and other types of moisture. Therefore, a wet service factor (C_M) should be applied.

Load Duration:

Table W2. - Load Duration - NDS (1991)

Load Duration	C_D	Typical Design Load
Permanent	0.9	Dead Load
Ten years	1.0	Occupancy Live Load
Two months	1.15	Snow Load
Seven days	1.25	Construction Load
Ten minutes	1.6	Wind/Earthquake Load
Impact	2	Impact Load

Flat Use:

Must apply adjustment factor when load is applied to the wide face rather than the narrow face.

Size Factor:

Must be applied when using a member that is not 12in. wide.

Calculations:

Calculation of the allowable bending stress (F_b' ) for a 2x4

First get base design value F_b from Table 4A in NDS Supplement, p.20.
Douglas Fir-Larch No.2, F_b = 875 psi
Apply size Factor (C_F) from Table 4A in NDS Supplement, p. 18.
F_b x C_F
875 psi x 1.5 = 1310 psi
Apply Wet Service Factor (C_M) from Table 4A in NDS Supplement, p. 18.
1310 x 0.85 = 1120 psi
Apply Load Duration Factor (C_D) from Table 2.3.2 in NDS, p. 6.
Load - Ten Minutes
1120 x 1.6 = 1790 psi

All factors except flat use have been applied. If these are the only conditions, then the allowable design value, F_b' = 1790 psi.
Apply the Flat Use Factor (C_fu) from Table 4A in NDS Supplement, p. 18.
1790 psi x 1.1 = 1960 psi

If the 2x4 is used by laying it on it's wider face, then the allowable design value, F_b' = 1960 psi.

If the 2x4 is used by laying it on it's narrower face, then the allowable design value, F_b' = 1790 psi.

Example 2:

Same design as example 1 only this time use a Southern Pine No. 2 (Table 4B) as your material and a permanent load duration factor.

Answers:

Regular use F_b' = 995 psi
Flat use F_b' = 1090 psi

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This material is based upon work supported by the National Science Foundation under Grant No. 0633602. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).

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