LFRS with Braced frames and moment frames in one direction

[sadienerd]

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.

 

[RPMG]

If the diaphragm is flexible, use tributary values.

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.

 

[KootK]

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?

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.

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?

Yes.

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.

Torsion becomes asymmetric reactions at the ends of the diaphragm.

If the diaphram is not flexible how does the moment frame collect any load

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.

 

[sadienerd]

Thanks for the replies! I am probably being a bit dense here but I'm not sure I'm tracking your awnsers. RPMG you state that unless eccentric the Braced frame and moment frame will have the same displacement and the moment frames contribution will be small. I'm assuming that is for the flexible diaphram case. That makes sense to me but would indicate the load dilevered to the moment frame is less than that to the brace frame which is my hang. My understanding is that under the same load the moment frame would deflect more than the Braced frame. If the loading is not eccentric and the diaphram is modeled as flexible the two frames should take the same load if trib areas are the same.

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.

 

[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.

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.

 

[RPMG]

My response was not well stated. Here's an example of flexible vs rigid diaphragms. I have a building which alternates moment and braced frames, ...MBMBMB..., infinitely. The 'soft' moment frames have a stiffness of 10 kip/in. The 'hard' braced frames have 100 kip/in. The load applied is 110 kips.

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