Chopped up Diaphragm?

[UTvoler]

Hi all,

I am getting pretty heavily involved in hotel structural design, both as EOR and as delegated designer for a cold formed steel (CFS) supplier. I am starting to see, and being asked to design with CFS. The current approach is utilizing CFS floor framing, CFS load bearing (and sometimes shear) walls, and 9/16 steel deck. My concern is with the continuity of the steel deck diaphragm; I can't find any design values for web crippling for two flange loading that would even let me begin to think about running the deck continuously at the floor level and stack the load bearing walls on top of the untopped deck. This means that the diaphragm is broken at every single load bearing wall. With the current project that I'm looking at as a delegated designer, the EOR is not showing any detailing at the diaphragm edges. I suppose the edges of the deck are fastened to the rim track, which is fastened to the wall studs (and maybe the top track), and so diaphragm continuity is maintained much like at side laps. I have no idea if the numbers work yet, but I don't like it. My thought as EOR is to require a continuous strap wider than the walls to ensure continuity in the horizontal plane, which I think is an easy calc and I can hang my hat on. See attached details.

Any thoughts on this? Seems to be a little bit of uncharted waters to me....

 

[DaveAtkins]

Why is the concrete topping not continuous? This seems like an unusual way to build a multistory building. If the interior walls are already in place, how does the contractor make the concrete flow down hallways, into rooms, etc.?

DaveAtkins

 

[UTvoler]

DaveAtkins, the concrete topping is a gypsum concrete that is poured several floors at a time, via pump and hose. The issue with running the deck and gypcrete continuous (platform framing) and stacking the walls on top is sequencing the construction and fastening. The CFS installers don't want to stop and wait on the gypcrete to be poured to set the next level of framing, and want to use their typical self tapping screws says the CFS supplier/installer. Also, I'm told some jurisdictions want gypcrete poured into the bottom track of the walls to somewhat maintain a rated floor assembly. I haven't pushed back on this yet.

 

[KootK]

- for taller buildings, I agree, balloon framing is often the way to go.

- I think that the flat strap is a good idea. I've had it worse where they want double party walls and no deck across. They use intermittent diamond plate connectors over the top tracks which give me the heebeejeebees.

- If your building is as gloriously jog free as your sketch would suggest then, yeah, rim track and/or wall track is a logical choice for your chords as long as they are spliced for axial continuity.

- If you've got rampant jogging, as my buildings always seem to, I see no practical way to achieve a proper diaphragm chord without having a real concrete topping into which to place some rebar. It would take oodles of strapping and blocking which is never popular.

- With gypcrete topping, I don't love taller incarnations of these buildings. They basically have all the load path gaps of light frame wood buildings, just executed at a much larger -- and more terrifying -- scale.

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.