NDS factor of safety

In various lumber mechanical "design" strength tables, and in National Design Specification (NDS) for Wood Construction, I assume they're referring to factored strength values, i.e., design allowables (correct me if I'm wrong). But they never seem to come out and say what factor of safety they're using. If the strength values are not ultimate, what approximate factor of safety, w.r.t. the lumber ultimate or rupture strength, is built into those tables or code? Thanks.

 

[DaveVikingPE]

My understanding is that the values in the NDS are some sort of weighted average based on a lot of tests.

Now, let me ask you this: isn't it strange that there's a dearth of mechnical/engineering properties info pertaining to wood? - Except the NDS?

The reason seems to be that the properties of wood are variable, so much so that NO ONE (except maybe the Southern Pine Association) wants to put out anything that remotely resembles a guarantee of one sort or another regarding wood strengths. That is, wood is not an engineered material, unless one is speaking of glulam.

To your question... I wouldn't assume there's a factor of safety built-in to the NDS values at all. There is a factor "reliability" in that, I think, one may assume a value of, say 600 psi should be used as the maximum allowable tensile stress for spruce/pine/fir.

 

[dougantholz]

I agree with Dave, the design value fo the strength of the lumber represents the 95% confidence from 2 std deviations of the strength of the material. The APA has a lot of test publications showing the actual strength of the material and what the design strength is.
When you pile onto that multipliers for the moisture content, load duration, etc...you get the multi-faced factor of safety built into the design. Of course to know for sure you could design in the LRFD for wood and compare that to the raw values from NDS...Of course when we all go to the LRFD for wood, I'm gonna retire (and I'm not 30).

 

[RiBeneke]

In our older South African codes of practice on timber, the basic 'Grade Stress' rating was equivalent to the 'Allowable Design Stress' that would be used for normal structures.
For short-term loading (falsework, some live loads etc) the allowed design stress was approx 1.5 x Grade Stress.
The 95% confidence limit was about 1.9 x Grade Stress.
The average failure load was approx 3.1 x Grade stress.
Although our timber is probably poorer (quicker growing) than yours, these figures may give you some idea of the factors involved.

 

[JDPE]

The Commentary to the 1997 NDS(Page 8)states the following:

"Permanent Loading Factor. By the late 1930's, a factor of 9/16 the short term strength had become established as the safe working stress level for long term loads... these recommendations were based on the results of load duration tests on small, clear specimens..."

This "safe working stress level" is then modified by load duration factors and adjustment factors.

This is the only mention I have ever come across relating to a factor of safety.