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Braced Frame brace element question

Braced Frame brace element question

Braced Frame brace element question

I'm designing a four story steel framed building.  There will be a couple of concrete stair towers and a concrete elevator tower near the center of the building.  I't basically rectangular, about 135 feet x 90 feet.

The floors are concrete, so I've modelled them as rigid diaphragms.  I've placed a single braced frame at each of the four sides of the building.  The concrete towers (no surprise here) attract most of the lateral load (seismic governs over wind).  The braced frames attract relatively little load, and seem to basically resist the loads generated from rigid diaphragm torsion.

My question is about the braced frames.  Can I use a single brace at each level of each frame as both a compression and tension member?  I've used an "Ordinary Concentric Braced Frame" R factor in developing my seismic forces, and I can't find any code limitations that would prevent me from doing this.


RE: Braced Frame brace element question

I think I've answered my own question.  Bracing requirements are addressed in AISC Seismic Provisions .  Based on my review of requirements, I will be designing a Special Concentric Braced Frame (SPCF)rather than an Ordinaryt Concentric Braced Frame (OCBF).  13.2c of the Seismic Provisions requires that for any line of bracing, braces be deployed such that, for either direction of force parallel to the bracing, at least 30% but no more than 70% of the total horizontal force along that line is resisted by braces in tension, unless the compression brace is designed to resist amplified seismic load.

The "no more than 70%" seems to preclude using a single brace as I was planning.  

Thanks anyway folks!

RE: Braced Frame brace element question

are you sure you shouldnt be designing for R=3 'steel not specifically detailed with seismic provisions' at the bottom of the page?

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