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Mixed Rigid & Flexible Diaphragm

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siuceric21

siuceric21

Civil/Environmental
Joined
Apr 2, 2011
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I have a structure that has 2 bays of rigid diaphragm (concrete over metal deck) & 5 bays of flexible diaphragm (metal roof deck). This is in Seismic Design Category D.

My concern that the lateral force-resisting system (N-S Special braced frame, E-W Moment Frame ) located under the rigid diaphragm will attract all the lateral load for the whole structure.

One solution I thought of was to create a horizontal lateral bracing system to create a full rigid diaphragm to evenly distribute the lateral load. Any concerns with this approach or a better way to achieve this?

I am trying to avoid expansion joints everywhere and also wanted to avoid putting a concrete deck over the whole structure.

Any help is appreciated.
 
 https://files.engineering.com/getfile.aspx?folder=c7660df2-1cac-489f-861a-9df70d99fdb4&file=DIAPHRAGM_20201130.pdf
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Apparently , SFRS is not orthogonal ..(i.e. N-S Special braced frame is not orthogonal to E-W Moment Frame ). Will you pls post the structural plan ( showing the roof beams, trusses, columns with strong axis location etc ..)
 
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Your lateral force will be carried by the moment frames and braced frames. The differences are how the diaphragms (flexible and rigid) distribute loads to the frames, and in-plane rigidity. I don't quite follow your concerns.
 
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HTURKAK - Yes I do have a Nonparallel system irregularity I am designing towards.
Attached shows most of the general structural system I am thinking, sorry if there is information missing just got the building this week, but should give you an idea.

R13 - Thinking on this more and I see your point, maybe I am over complicating. Rigid will distribute on frame rigidity with will be equal & flexible will be based on the trip area as typical, life does on as normal...
 
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op said:
..based on the trip area as typical...
Click to expand...

I agree with that for the transverse direction and your particular bracing arrangement. The mixing of rigid and flexible can make for some analytically tricky situations when there are not stiff braces at the edges of the rigid bits but, set out as you have it, the force distribution should be simple and reliably predictable. Kudos to you for a rational bracing layout.
 
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As pointed out before, the column will take it's share of gravity load and lateral load based on diaphragm theory. When excited by seismic/inertia force, the load remains, with the excitation/lateral force distributed through frame action, for which, the rigidity of the frames determines how the force is distributed. You should concerned with and make sure the continuity of the load path.
 
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I looked to second picture that you have posted , i will try to respond your questions based on available info ;


If you do not provide HB at roof level , The rigid diaphragm will NOT attract all the lateral load for the whole structure.. Moreover, the VB 's provided at axis 1,4,6 and 8 will not attract the total forces from the axis 2,3, an 7..and without HB at roof level, you should not classify the whole SFRS Special braced frame in N-S direction.

You may perform preliminary 3 D analysis to see how the braced frames will attract the seismic loads.


IMO, if i were in your shoes, i would try first flexible Diaphragm Condition with distributing the mass with tributary area assumption and complying the requirements of Nonparallel System Irregularity (Type 5)..



 
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