AISC Allowable Stress-Gaylord's SE Hdbk

[Binary]

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.

 

[JedClampett]

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

 

[steve1]

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

 

[Binary]

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.

 

[JedClampett]

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.

 

[Lutfi]

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?Se...ry&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.