Aluminum allowable stress increase?

[FSS]

It's been a while since I did any detailed calculations with aluminum structures. Is the 1/3 allowable stress increase still used for wind loading? According to Aluminum Design Manual you can use the increase unless prohibited by code. And according to Florida code where I am you can only use allowable stress increase where demonstrated by material standards. So which way does it go?

 

[Lutfi]

If all you have is dead and wind load, I would not use the 33% increase even if it is alllowed.

 

[UcfSE]

I agree. I would stick with the load combinations listed in the FBC or in the ASCE 7-02 and forget the stress increase.

 

[FSS]

Although I agree with both of you I am looking for a more specific reference or clarification. My case in point is a review of someone else's design who feels that the allowable stress increase is OK per the Aluminum Design Manual. There has been so much discussion regarding this topic with steel but I cannot find much for aluminum.

 

[Ron]

FSS...I use the 1.33 factor on aluminum. The reason is that structural grades of aluminum (6061-T5,T6/6063-T6) have an excellent capacity to attenuate strain under temporary load. The allowable loads for aluminum are already reduced considerably (about 50% of yield in unwelded condition...about 25% of yield for the heat affected zone in welding), thus using the 1.33 factor puts you back into the range for other comparable elastic materials. Since the elastic modulus of aluminum is about 1/3 that of steel, the strain attenuation is considerably greater at the point of load (more flexibility).

In ASCE 7-02 the statement "Increases in allowable stress shall not be used with the loads or load combinations given in this standard unless it can be demonstrated that such an increase is justified by structural behavior caused by rate or duration of load."

Observed structural behavior in aluminum under wind loads supports the flexibility contention.

 

[epr]

FSS,
The one-third stress increase has gone by the wayside. The International Building Codes have specifically eliminated it. [Section 1605.3.1.1 "Increases in allowable stresses specofoed in the appropriate materials section of this code or referenced standard shall not be used with the load combinations of section 1605.3.1 except that a duration of loading increase shall be permitted in accordance with Chapter 23 {wood)"]

A good article explaining the situation appeared in the October 2004 "Modern Steel Construction".

Unfortunately, even though the specified load combinations take this into account in numerous situations, there are a few conditions where the elimination of this 4/3 factor has greatly affected the design.

The most extreme difference (from earlier designs) occurs in foundations that are subject to overturning from wind, such as sign and light standard foundations. It is readily apparent that the soils should be allowed to use this factor, since brief loads are attenuated considerably. In order to be technically correct, I try to compensate by requesting that the geotechnical engineer specify (if he deems appropriate) that the allowable soil stresses can be increased by 1.33 under wind and other brief forces.

Relative to aluminum, we do not appear to have this option.

 

[dozer]

Ron,

Can you cite a reference for your statement: "Observed structural behavior in aluminum under wind loads supports the flexibility contention."?