Untopped precast plank diaphragms

[DETstru]

I found a few threads on this but nothing that exactly answered my question. I'm reviewing a design for a CMU building with an untopped precast plank roof.
See the sketch below for a representative diagram.

I often see documents (like the PCI manual) talk about using friction in the joints between the panels to create a 'contiguous' diaphgram when you don't have a topping. I don't understand how this works when you have suction on the walls.

The top of the south wall in my diagram is only connected to the south-most panel. As wind "sucks" it south, it will pull that edge panel away from the remaining panels unless some kind of positive connection is provided between every panel (grouted U-bar, weld plate, etc.). How is it that any situation can rely on friction when you always have a potential suction?

 

[KootK]

I don't understand how this works when you have suction on the walls.

It doesn't unless you make atypical connections like you've described. You basically have the lone plank nearest the edge acting as a wind girt. And the end connections probably don't work for that.

I do some work as a precaster's engineer and see the untopped system, very popular in some areas, from time to time. In my opinion, there are gaping holes in the system in all but the very simplest and most regular of structures. Take your same situation, stack it eight stories high, turn the roof, and consider the stability implications of the restraint, or lack thereof, of the wall. Barf.

The grout key based diaphragm business is a bit of a cheat. What you're getting is a shear only diaphragm that truly is not capable of resisting tension perpendicular to plank. When you truly consider the implications of that, as you have, it can get pretty ugly in a hurry. It also makes chords running perpendicular to the planks insanely important. No more of that BS where your chords are two #4's that follow all the jogs of the building.

I could go on all day about how much I hate untopped precast diaphragms...

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.

 

[DETstru]

Thanks, glad I'm not alone on this. It doesn't seem to make sense.

I've always used topping on plank if I need a diaphragm. The only time I've used untoppped is for small mezzanines.

 

[JLNJ]

For shorter spans I have used U-shaped rebar to tie the planks together to resolve the perpendicular forces and to engage more planks.

Several manufacturers have "standard" details for this, but you have to convince yourself that it works.

 

[DaveAtkins]

With an untopped plank system, don't they still grout the keys between planks?

DaveAtkins

 

[KootK]

Yup.

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.