Vertical offset for a horizontal brace

[acwooten]

What are the current thoughts on calculations for horizontal braces that are offset some vertical distance below the main beam top of steel (say 3")?

AISC Steelwise says that the moment induced in the brace due to the eccentricity between the brace centroid and the beam centroid can usually be ignored. But then they say that you should consider the (typically) much smaller eccentricity between the brace centroid and the shear plane from brace to gusset.

Is there a time when you should or shouldn't consider these eccentricities?
Why would you ignore one but not the other?
Why would you ignore one at all?

Thanks!

 

[dhengr]

Acwooten:
You can reasonably ignore the eccentricity when it is insignificant to a safe and practical solution of the problem.

 

[acwooten]

Thanks for the response dhengr.

Why is that the case though? And is that documented anywhere other than the Steelwise article I mentioned?

What would you consider to be a practical solution? In my particular case, I am analyzing the brace for both axial and flexural loads and it fails in bending with the eccentricity, which I would consider to be impractical.

 

[KootK]

The key to making the judgement call lies in recognizing that eccentricity alone is not the parameter of interest. Rather, it’s the ratio eccentricity to flexural strength/stiffness. A 3” eccentricity is probably not a big deal for an 18” beam. Conversely, a 1” eccentricity may significantly impact the design of a 3” angle brace. Your findings regarding your brace would seem to be a fine example of this.

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.

 

[dhengr]

Acwooten:
Stop and think for a minute. Reread the entire thread, particularly your posts, and tell us if you’ve given the least clue of what you are actually doing. What does this whole thing look like, plan, sections, loads, dimensions, etc. Remember, we can’t see it from here, and there are only a few thousand possibilities which would fit your description. And, each might be designed and analyzed slightly differently. Every thing/detail in the universe is not yet codified (but, they are working on it) some of this just takes some engineering judgement, experience and some rational application of every day, well established, principles of engineering mechanics.

 

[rb1957]

why do you consider the eccentricity moment "impractical" ?

I think, and this isn't my subject matter, that the brace is providing lateral stability to the beam, ideally thru the beam centroid but (it sounds like) no big deal if there's an offset, the beam still feels the effect of the brace. But the load transfer into the beam is via the gusset and so this will impose a moment on the brace.

another day in paradise, or is paradise one day closer ?