Combining Rigid and Flexible Diaphragm Analysis

[sath]

I have a project, a one story 12,000 sq. ft commercial building that I am working on designing. I am facing a dilemma on the design. The problem is that when you divide the building into three parts (for the reason on explanation), the part on the left and right is a flexible diaphragm (metal trusses with OSB sheathing). The part in the middle is concrete over metal decking (supported by metal trusses) for supporting HVAC units. There is no discontinuity in the diaphragm between this flexible and rigid diaphragm. The width to depth ratio of the concrete roof is 1.5 (less than three, so I have to do rigid analysis).

Can somebody please guide me in combining the flexible and rigid analysis? I found no reference in this regard. Any help will be appreciated.

Thanks

 

[McEngr]

I have a project, a one story 12,000 sq. ft commercial building that I am working on designing. I am facing a dilemma on the design. The problem is that when you divide the building into three parts (for the reason on explanation), the part on the left and right is a flexible diaphragm (metal trusses with OSB sheathing). The part in the middle is concrete over metal decking (supported by metal trusses) for supporting HVAC units. There is no discontinuity in the diaphragm between this flexible and rigid diaphragm. The width to depth ratio of the concrete roof is 1.5 (less than three, so I have to do rigid analysis).
Can somebody please guide me in combining the flexible and rigid analysis? I found no reference in this regard. Any help will be appreciated.

Thanks

Hi Sath,

I'm sure you've come up with a solution already, but I'll shoot anyway. The one thing that you need to clarify from the very beginning is that you need a lateral system for each diaphragm: the 2 flexible ones and the 1 rigid one. You will also need them to perform in each direction. I know these seem like obvious things, but it's probably best for you to detail seismic separation joints around the rigid diaphragm so that it doesn't rip out the flexible diaphragm. There will be variations in stiffnesses/deflections.

One thing that I question is the OSB over your metal trusses. Unless the OSB is pretty substantial(thick), I would use plywood instead. OSB for roof diaphragms is "OK" when using it in residential construction, but there is a poor performance otherwise. This is, of course, just my opinion and is not covered in IBC/ASCE very well. AFPA has some articles on this on their website.