Materials Links & Resources: Wood
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|
|Spruce-Pine-Fir (south)||Balsam Fir|
|Norway (Red) Pine|
|Southern Pine||Loblolly Pine|
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 Ci 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.
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(CM) - Will it be exposed to the environment?
- Load Duration(CD) - For what length of time will the load reside on the structure (see table W2 below)?
- Flat use(Cfu) - Will the piece of wood bear load on its wide face?
- Size factor(CF) - Tables in NDS for all species except the Southern Pines are based on a width of 12in.
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 (CM) should be applied.
Table W2. - Load Duration - NDS (1991)
|Load Duration||CD||Typical Design 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|
Must apply adjustment factor when load is applied to the wide face rather than the narrow face.
Must be applied when using a member that is not 12in. wide.
Calculation of the allowable bending stress (Fb' ) for a 2x4
- First get base design value Fb from Table 4A in NDS Supplement, p.20.
Douglas Fir-Larch No.2, Fb = 875 psi
- Apply size Factor (CF) from Table 4A in NDS Supplement, p. 18.
Fb x CF
875 psi x 1.5 = 1310 psi
- Apply Wet Service Factor (CM) from Table 4A in NDS Supplement, p. 18.
1310 x 0.85 = 1120 psi
- Apply Load Duration Factor (CD) 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, Fb' = 1790 psi.
- Apply the Flat Use Factor (Cfu) 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, Fb' = 1960 psi.
If the 2x4 is used by laying it on it's narrower face, then the allowable design value, Fb' = 1790 psi.
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.
Regular use Fb' = 995 psi
Flat use Fb' = 1090 psi
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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Pennsylvania State University
Department of Engineering Science and Mechanics
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