LFRS with Braced frames and moment frames in one direction
LFRS with Braced frames and moment frames in one direction
(OP)
I have read a lot of post regarding mixing stiff lateral force resisting systems and sway frames on the same column line but have not found an awnser to my particular question. My question is as follows, can you mix a Braced frame and moment frame in the same direction (not on the same column line) and still model your diaphram as flexible? Assuming you can and you distribute the loads based on trib area you would have different deflections in either side of the diaphram. Is this acceptable as long as the moment frame deflections are within acceptable limits? Since the diaphram is flexible and cannot transfer torsion is it ignored in this cause or do you need to transfer this load somehow to the chords. I have also read a lot of post in regards to three sided LFRS (I.e. a warehouse with shear walls and an expansion joint on one end. I have seen recommendations for that case to add a moment frame at the expansion joint to make it a foursided structure and i think this is similar to what I am asking. If the diaphram is not flexible how does the moment frame collect any load. Sorry if this post is long winded. Thanks in advanced for your responses.






RE: LFRS with Braced frames and moment frames in one direction
If the diaphragm is rigid, use the stiffness of the moment/braced frames like a rigid beam supported by springs. Unless eccentric, the moment and braced frames will have the same displacement, and the moment frame contribution will be small.
RE: LFRS with Braced frames and moment frames in one direction
It's possible but less likely. A diaphragm is generally classified, from a flexibility perspective, based on the ratio of diaphragm flexibility to frame flexibility. Replace braced frames with moment frames, and that ratio usually goes down. See the ASCE7 definition for flexible diaphragms if you're practicing in the U.S.
Yes.
Torsion becomes asymmetric reactions at the ends of the diaphragm.
It will collect load in proportion to its stiffness which usually means that it won't collect very much load in many cases. This is an inherent difficulty with hybrid systems and should be carefully cinsidered by designers as you are doing here.
RE: LFRS with Braced frames and moment frames in one direction
Kootk - I follow your replies except for the one regarding the torsion being resolved into asymetric loads at the ends of the diaphram. I am assuming the ends in this case are the Braced frame and moment frame in this case .this seems more like a semi rigid or rigid diaphram if the frames develop asymetric loading given that the trib areas are the same.
Sorry if I'm slow just want to make sure I'm getting it.
RE: LFRS with Braced frames and moment frames in one direction
No trouble at all. A flexible diaphragm is conceptually much like a simple span beam. And a simple span beam can, of course, have different (asymmetic) reactions at each end. That's all that I was getting at. It's not that flexible diaphragms can't handle applied torsion, they just handle it differently than rigid diaphragms do.
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: LFRS with Braced frames and moment frames in one direction
Rigid Concrete Slab Diaphragm: The rigid diaphragm displaces 1".
Moment frames displace 1" and resist force of 10 kips
Braced frames displace 1" and resist force of 100 kips
Flexible Roof Diaphragm: The force resisted is equal on all frames, 55 kips, because trib areas are equal.
Moment frames displace 5.5" and resist force of 55 kips
Braced frames displace 0.55" and resist force of 55 kips