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

AISC Allowable Stress-Gaylord's SE Hdbk

AISC Allowable Stress-Gaylord's SE Hdbk

AISC Allowable Stress-Gaylord's SE Hdbk

I'm looking at the 2nd edition of Gaylord's Structural Engineering Handbook in the Towers and Transmission Pole Structures section.

In working through an example (pg. 24-9) that looks at a leg of a tower, after determining the loads, he says

"Using the AISC allowable stress, Fc=15.90 x 1.33 = 21.2 ksi" He then uses this with the area to show that the calc'd load is within allowable limits. My question is where did the 1.33 factor in allowable stress come from?

I find the 15.90 in the AISC Allowable Stress table for the Kl/r of the leg but I'm at a complete loss to explain the 1.33 factor, especially because it INCREASES the allowable stress.

RE: AISC Allowable Stress-Gaylord's SE Hdbk

AISC allows a 1/3 increase for wind or seismic loading.  See section A5.2.

RE: AISC Allowable Stress-Gaylord's SE Hdbk

Increase is from AISC. See for example Specification for Structural Steel Buildings, Allowable Stress Design, 1989, paragraph A5.2.

RE: AISC Allowable Stress-Gaylord's SE Hdbk

Thank you both, Steve & Jed.

Question, though...Why does the structure care where the load is coming from? It doesn't make sense to me that the allowable stress is dependent on the source of the load.

Does this have something to do with the failure theory being used?

It seems to imply that a factor of safety is built in to the AISC codes.

RE: AISC Allowable Stress-Gaylord's SE Hdbk

There are several reasons that AISC (and many other codes) allowed a 1/3 increase in allowable stresses for wind and seismic.  Some of the reasons are the small chance of full wind load being applied with other loads being at maximum, the limited probability of maximum wind applied to a whole member or structure at once and that this is what the factor of safety is for.
Note that this is quite controversial.  1997 UBC modified this provision such that the 1/3 increase could only be used with full live load cases.  Previously, you could use the 1/3 increase for any case where wind was applied.  
At a seminar a couple of years ago, one of the lecturer's asked if anyone knew where the 1/3 increase started.  His point was why not 1/4 or 1/2?  
Anyway, as codes all approach a limit (ultimate load) type design, the 1/3 increase will go away.  Future arguments will be be based on the applicable load factors.

RE: AISC Allowable Stress-Gaylord's SE Hdbk

I would like to add that the 1/3 increase was justified due to the transient nature of the loads. Dr. Duane Ellifritt published a paper and was published in the AISC Engineering Journal. This article can be downloaded from the following AISC link http://www.aisc.org/Template.cfm?Section=Browse_by_Category&template=/Ecommerce/EjournalSearchForm.cfm  articles can be downloaded for $10.00 per copy.

Keep in mind that AISC LRFD load combinations do not allow for the use of 1/3 increase. I think it is not prudent to use it if the only load combinations are wind plus dead.

As Jed mentioned, this subject is very controversial. Personally, I like to err on the side of safety.

Good luck.

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


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