Factors of safety in NDS 1

[PostFrameSE]

Does anybody know what kind of factors of safety are used for the fasteners in the NDS? Thanks.

 

[JAE]

Some years ago I called some representatives associated with the NDS and they indicated it was somewhat nebulus but a 1.6 to 2.0 range was the approximate amount.

This was perhaps 10 years ago.

 

[MiketheEngineer]

I agree w/ JAE - that is what I was always told...

 

[SlideRuleEra]

I don't believe that NDS specifies a safety factor. However the USDA Forest Product's "Wood As An Engineering Material" does (for certain conditions), see page 7-5 at this link:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch07.pdf

The safety factor offered there is 6 to 1, for withdrawal.

The "Pre-1991" edition of "Wood As An Engineering Material" discussed this in more detail with withdrawal SF varying from 5 to 6, depending on the type fastener (nails, bolts, lag screws, etc). Lateral loading SF is in the 1.6 to 2.25 range that JAE & Mike recall.

Since the content of the NDS is largely based on experimental work by the Forest Products Lab, both the 6 to 1 figure and the 1.6 to 2 range are good bets - depending on the load direction.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[UcfSE]

Look in the introduction to the design values book. In it, we are told that most design values come from some ASTM spec, such as D 245 for visually graded timbers.

In D 245, there are quite a few adjustments made to values for clear wood ( from ASTM D 2550). Adjustments are made for shakes, checks, splits, knots, slope of grain and other quantities. The proper way to round the design values is given also. There are quite a few factors and things to go through before arriving at a final design value for a species and grade.

One of the tables in D 245 notes factors to be applied to clear wood values, and divides the different factors in to those for softwoods and those for hard woods. The bending strength factor for softwoods is 2.1 and for hardwoods is 2.3. I would imagine the grading agency starts with those factors, say 2.1, and apply adjustments for grading and size until a design value is met.

This is all for the wood material. It doesn't exactly apply to your specific question about fasteners. Try this link. It seems to have some insight into fastener safety factor, though not as well discussed as I would like to see.

 

[JAE]

Woops....I see the original post was for "fasteners". I was thinking stress values of wood itself.

Sorry.

I would think the fasteners have a higher SF than the wood itself.

 

[PostFrameSE]

Thank you all for your helpful responses. I appreciate it.