Different Diaphragms and distribution of forces

[ash060]

Working on a project that has a stepped elevation. Portion of the roof of the lower building piece is at the same elevation as the 2nd floor of the higher building piece. The roof diaphragm is just metal deck so assuming flexible, and the 2nd floor portion is concrete on metal deck with an aspect ratio around 1. Trying to figure out how to distribute the story shear to the different vertical elements of the LFRS. Not really sure about the way to distribute the story shear.

I have attached a sketch showing the layout of the building with the different diaphragm types highlighted. Any suggestions would be helpful.

 

[Once20036]

Where are your lateral force resisting elements?
What does the joint look like between the two deck types?

 

[Nor Cal SE]

Here are my default thoughts, though I'm open to other suggestions:

1) Have some type of lateral resisting element in the two wall lines that are farthest from the rigid diaphragm square: the far left line and the far bottom line on your sketch. Design those lines to resist load using a tributary area that extends halfway to the next adjacent resisting line, which should be the "interior edges" of the rigid diaphragm (such that your resulting tributary width to the outermost flexible diaphragm lines is approximately 1/4 of the whole structure width).
2) Analyze and design the rigid diaphragm area as you normally would, using relative stiffness of resisting elements, etc., and assume 100% of story load goes to the rigid diaphragm (conservative, because you have the resisting elements mentioned above on the outer lines, but the rigid concrete area probably accounts for most of the mass anyway). And at each junction of concrete diaphragm to flexible diaphragm, make sure you have elements to resist the collector force of the flexible-diaphragm-area load outside that junction.

Hopefully that makes sense in words.

 

[KootK]

Like Once20036, I think that more information would allow us to give much better advice. Specifically. What and where are your intended lateral resisting elements? What have you got going on with regard to drag struts and collectors?

My two cents without knowing much at this point:

- I don't like laterally supporting rigid diaphragms on flexible diaphragms. I think that invites stress concentrations and behavior that is difficult to predict. I do it from time to time but try to keep it to small stuff (mech platforms on roofs etc). My overwhelming preference where the rigid bit is "real building" would be to set up lateral such that the flexible parts of the diaphragm were independently supported or hanging off of the rigid diaphragm in a lateral sense.

- If the advice above isn't feasible for the project, and you've got the time and the tools, I'd recommend a semi-rigid diaphragm analysis. Under this path, it's hard to see how this wouldn't trigger some kind of irregularity in seismic country.





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.

 

[ash060]

The LRFS is special concentric braced frames. They are laid out so that no part of the flexible area is cantilevered. Haven't worked out the joint between the two diaphragms yet. There are some drag struts that are in the smaller flexible portion that continue back into the larger flexible portion since I can't get a frame in those areas. Each face of the rigid diaphragm has a frame, so torsion in that area is pretty small.

 

[KootK]

Haven't worked out the joint between the two diaphragms yet.

So is the intent to isolate one diaphragm from the other except for shared collector elements?

How tall is the portion of the building that continues up?

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.

 

[ash060]

Yes they would be isolated except for the shared collector elements. The taller portion continues up another 30' with additional floor and a roof level.