RISA modeling ? - load going through diagonal bracing, not columns

[mikeCTE]

i'm running a spectrum analysis on a steel frame tower. the tower has 4 columns and supports a heavy load near the top.

when i run the analysis, some of the load cases show that the diagonal braces that span from column to intermediate beams are taking the bulk of the vertical load. meanwhile, the column for that section of bracing takes little load. in addition, the beam becomes overstressed in flexure due to the bracing forces even though the braces connect on the beam at a single node.

so, how do i correct this in my modeling? all of my members are pin-pin to one another. e.g (brace to column; column to beam, etc). columns and beams are continuous with fully fixed ends where there are joints.

i have observed that if i make my columns way bigger than the braces, that helps, but doesn't cure the issue.

thanks for any insight. it's been a frustrating few hours on this inherited design of mine.

first image is axial; second is flexure.

 

[KootK]

I think that your model may simply be telling you the truth. The way your bracing is configured, it provides a complete, stiff, axial, alternate load path for gravity loads. So it makes sense that the bracing draws load. If you're able, consider flipping the upper braces the other way.

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.

 

[Lomarandil]

Exactly as KootK said (and depending on the detailing), the braces may want to contribute to the vertical load path.

If the braces are meant to be slender tension-only braces (note, I haven't spent the time to think about whether that's a valid arrangement for your frame), you can set them as "Euler buckling" elements or "Tension-only" elements in RISA -- the model would then find that the braces buckle elastically, and redistribute the load back to the columns. If you take this approach, make sure that the details of the braces will physically allow them to buckle elastically without compromising their intended fuction.

 

[PicoStruc]

you are all aware that the load in the brace are negligible -> 0.01 seems ZERO to me !