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Thoughts on multistory X bracing with zipper column

Thoughts on multistory X bracing with zipper column

Thoughts on multistory X bracing with zipper column

(OP)
Hello everyone,

What are your thoughts on multistory X bracing with zipper columns (attached) for seismic design?
I ask this because in textbooks and references I haven't seen this type of bracing, though for me it behaves the same as the inverted V bracing with zipper columns.

I know Bruneau's book has some pros and cons for each type of bracing but this one in particular is not there. So I was also wondering if you know references for this type of bracing.

Best regards.

RE: Thoughts on multistory X bracing with zipper column

If what you're showing is the full extent of your bracing system, then I'm not sure that I see much of a benefit to the zipper strut. The goal is usually to distribute damage over the height of a multi tier frame. Grabbing on to some more stuff usually helps stability, ductility, and redundancy to some degree but, here, you're just grabbing on to a beam in flexure. Not much gain in it I'd think. In high seismic work, I've also noticed and understandable trend towards keeping things as simple and predictable as possible.

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.

RE: Thoughts on multistory X bracing with zipper column

(OP)
I forgot to mention the mass is heavily concentrated in the upper beam. In the lower beam there is practically no mass.

At the beginning I put that zipper column to distribute the weight at 3 points instead of 2, but I'm not sure if it will have detrimental effects over the lower braces, apart from the obvious additional axial force.

RE: Thoughts on multistory X bracing with zipper column

Then I'd say that your column has benefits for gravity load considerations but not really seismic.

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.

RE: Thoughts on multistory X bracing with zipper column

When your brace(s) buckle you won't have much axial capacity left in them.

The beam should be designed for the full gravity load (spanning main column to main column) and the additional load generated from a buckled brace and one in tension. Also the axial load in the beam if present.

If you add the zipper column, you need to determine how much gravity load gets distributed to each beam and design both accordingly along with what I stated above.

RE: Thoughts on multistory X bracing with zipper column

This is an interesting concept. I wonder if that zipper column will do more harm than good. Increasing the axial force in the Chevron braces at the lower level. It might make for a better gravity system. But, my guess is that the V bracing at the upper story do a better job of increasing ductility than the zipper column.

Just my 0.02$, this is certainly an interesting discussion.

RE: Thoughts on multistory X bracing with zipper column

Why are you putting a zipper column on a two story X? The zipper column is for V-brace configurations with the difference in tension to compression capacity is very large. Braces above and below reduce the loading considerably. The column addition is likely to be more costly than an increase in the size of the brace beam.

RE: Thoughts on multistory X bracing with zipper column

I have been investigating typologies for this particular problem and this appears to be the best so far. I have a 450 m3 ton silo mounted at the upper beam and the zipper column helps for gravity both for the structure and for the silo itself (which is very thin and according to my analyses it needs further supports), without heavily stressing the lower braces (because they need to be big anyway).

My only concern was that mentioned by DETstru, but my analyses so far tell me that if one lower brace buckles, the upper truss resists the loads from the dead and the tension brace.

Another concern is that I haven't seen this type of typology in similar structures, and I was simply wondering why.

(MonsieurR)

RE: Thoughts on multistory X bracing with zipper column

It's a pretty unique scenario. You're probably the world expert now.

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.

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