Vertical Braces Taking Compression Under Gravity Loads

[tavery]

I saw a thread on this from 2012 and had a followup question dealing with the same sort of behavior. Here is the previous post.

http://www.eng-tips.com/viewthread.cfm?qid=328629

If we model this where the braces see some portion of the axial load in the column (which was established as being accurate behavior)from gravity load only, what happens when you introduce a lateral load that would put your brace in tension? The tension value that you would see would be reduced by the amount of compression that is there from the gravity case and can sometimes depending on the relative magnitude of the lateral force compared to the axial force in the column the brace would still be in net compression. Essentially without exception, the design of a two-way brace member itself will be compression controlled, but if you have a smaller tension force than what might actually occur you would get a smaller compbined uplift/shear reaction at the base of the column where the brace attaches. If you have designed your anchorage for the worst shear which was created by a compression from axial in the brace combined with a compression from lateral force then the shear portion of the anchorage design is probably ok. But what about the tension design of your anchorage and what about the sizing of your foundation for uplift? Perhaps the reduced tension values from residual compression are reliable, but if some of these methods of construction are followed where the dead load of the superstructure is already present prior to installing the brace therefore reducing the compression from gravity load, I begin to question if enough tension was considered in the anchorage/foundation design.

Thanks.
Tyler

 

[KootK]

For many common bracing scenarios, the vertical compression component coming into the anchorage joint would make it's way to the joint via the columns if not through the brace. In those cases, I'm comfortable taking advantage of the reduced uplift. For configurations where that is not the case, I'd be inclined to design for an uplift value discounting the compression effect.

I also believe that bi-directionality could create rare situations where relying on gravity induced brace compression could be unconservative for shear anchorage. As always, designers need to keep their wits about them.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.