×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

NDS factor of safety

NDS factor of safety

NDS factor of safety

(OP)
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.

RE: NDS factor of safety

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.

RE: NDS factor of safety

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).

RE: NDS factor of safety

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.

RE: NDS factor of safety

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close