Single Angle Compression

[SeanMD]

I am working on an existing structure with some single angle columns, built in 1955, and am working to make it meet the 2003 IBC code requirements. I am aware of the method used in the 1989 ASD specification and the article from 1991 AISC Engineering Journal showing how to calculate the compressive capacity of angles that are loaded at the legs. Using this method it shows the angles are seriously overstressed, although they have been in use for 50 years with no problems. A coworker commented and said the AISC ASD method for calculating capacity is a very conservative method. He mentioned that ASCE has a design guide for transmission towers that lists ASD capacity tables or a less conservative method for single angles in compression. Has anyone used this and does anyone know which publication this is?

Thanks.

 

[COEngineeer]

Whats your axial load, lenght and type of angles?

 

[aggman]

The new 13th edition has some updated information on single angle designs which may help some. The biggest difference being that it allows you to calculate the combined stresses based on the loads at a point. In the 9th edition you used the maximum moment, with the max axial load to check the section which more often these loads are not applied at the same point. The 13th edition AISC manual will also give you greater bending capacity in general. I would not be suprised to see the angles somewhat overstressed if they were sized in 1955. A lot has changed with column buckling since then.

 

[SeanMD]

Thanks I will check out the new manual and see. These are continuous columns with floor beams and bracing every 12', columns are 160' high. L6x6x7/16. I would never think they would calc by looking at it.

- One thing I just realized is that I applied the entire axial load at an eccentricity, and I should have only applied the incremental load at an eccentricity and the rest of the load should be concentric. That will change things quite a bit....

 

[AggieYank]

L6x6x7/16, concentrically loaded, with a KL of 12 feet, shows an allowable service load of 49.9 kips in the 13th edition of the AISC manual. Eccentric loading shows an allowable load of 25.7 kips.

Assuming there are 13 floors supported by this angle (160 feet @ 12 feet per floor), there is a DL of 50 psf and a LL of 70 psf (office), and the tributary area is 10 feet by 20 feet (edge column in small bay spacing?), the loading is around 310 kips.

Am I missing something here?

 

[CTW]

Is this really a building column???

 

[SeanMD]

Yes, this is not a typical building but an open structure, a stair tower.

Well re-running the #'s it almost works according to the 13th ed AISC manual, but not according to the 9th ed. And the 13th edition is not accepted in the 2003 IBC.