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Precast Topping - Diaphragm Design

Precast Topping - Diaphragm Design

Precast Topping - Diaphragm Design

(OP)
This is regarding the design of a diaphragm for a podium building which will utilize precast plans for the podium level. However the question applies to any situation.

When meeting with the precaster they indicated that they would prefer the topping handle the diaphragm forces and to be designed by the EOR. The topping would be structural.
My question: Are there any guidelines for minimum thickness of the topping based on aspect ratio or chord forces. I am concerned that a 2" topping is too thin and why would we want to utilize an 8, 10 or 12" thick concrete plank? I would like to use the topping to transfer chord forces, etc. but what is the load path to transfer shear to the shear wall? through the slab? How is this analyzed?

Has anyone had to do this?

I did just find this, which may have some answers:
http://c.ymcdn.com/sites/www.nibs.org/resource/res...

Also I looked in the PCI manual but this mainly deals with designing the actual planks for transferring diaphragm shears.

EIT
www.HowToEngineer.com

RE: Precast Topping - Diaphragm Design

Quote (RF)

Has anyone had to do this?

I suspect that most of us have had to do this at some point. It's a bit like getting diddled by a clergy member however. Not something that you necessarily want to discuss in public.

Quote (RFreund)

Are there any guidelines for minimum thickness of the topping based on aspect ratio or chord forces. I am concerned that a 2" topping is too thin

I don't know of any explicit code limits on this. I've done it myself with a 2" minimum thickness topping (camber) and welded wire fabric. In my case the precast floor was the main floor and the topping downturned into perimeter beams of a sort which served as chords and allowed for tie-in to the basement walls below (which also could have been the chords in some instances I suppose). I agree though, 2" seems sketchy. I like to be able to accommodate at least two crossing layers of #4 with cover and 1/4" for tolerance. That takes you to 3" which can have cost consultants crying bloody murder.

Quote (RF)

why would we want to utilize an 8, 10 or 12" thick concrete plank?

1) It's got everything to do with the shear connection between planks and almost nothing to do with the actual planks themselves.

2) If the EOR handles the diaphragm design, it saves the precaster a bunch of liability and work. Can't blame them for wanting to shed that.

Quote (RF)

but what is the load path to transfer shear to the shear wall? through the slab? How is this analyzed?

Depends on the nature of the shear wall. If the shear wall is a basement wall or a bearing wall, something like the downturned topping detail that I mentioned may work. If the shear wall just runs along side the precast planks, then you need a joint that can transfer shear but allow vertical displacement. EOR solution seems to usually be slotted clip angles. Precasters to it more elegantly with PSA inserts etc. Most of these solutions will involve the diaphragm shear being passed through a plank adjacent to the shear wall but not necessarily passed between adjacent planks.

My advice would be to carefully study all of your diaphragm to VLFRS connections in detail before committing to a structural topping or a structural topping thickness. Most of this stuff is resolvable but, at the same time, it's pretty easy to get caught with you pants down if you're not diligent about resolving issues in the DD phase.

When considering this scenario, a movie always plays in my head whereby my precious topping is cracked in half as a result of it trying to span two planks that deflect differentially. Stripped off like an orange peel... That seems to not be a valid failure mode but, nonetheless, I worry.

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.

RE: Precast Topping - Diaphragm Design

KootK says.... “If the EOR handles the diaphragm design, it saves the precaster a bunch of liability and work. Can't blame them for wanting to shed that.” Why not just shed the precast and do a post tensioned slab. That will help them shed any other effort too.

RE: Precast Topping - Diaphragm Design

(OP)
KootK - I've had many of the same thoughts.

Quote (Kootk)

It's a bit like getting diddled by a clergy member however. Not something that you necessarily want to discuss in public.
Awesome rofl2

Quote (Kootk)

If the shear wall just runs along side the precast planks, then you need a joint that can transfer shear but allow vertical displacement
I spoke with a precaster about this and they said they'd be ok setting the plank in a 4" notch in the wall. The same way they do the bearing end. They weren't concerned with allowing for vertical deflection. I thought this was a bit odd.
However another thought is - The precast will be simple spanning between interior beams. However the topping is continuous across these beams. I imagine the slab cracking over the beam due to the induced tension from negative bending, no?

In my case I have a fair bit of diaphragm load because it is also acting to transfer earth pressure loads (walkout basement type of a situation).

Looking into the example I posted above - for structural toppings:
For chord forces the example uses the gross section of the topping (although they note that with grouted joints the planks could be used also) and assumes an elastic stress distribution. fc = 6*M_u/(t_topping*d_topping^2)
They compare this stress to the requirements of ACI 21.11.7.5 i.e. Fc=0.2f'c (or 0.5f'c if using omega).
This seems to go slightly against the grain of what the ACI 21.22.8 and commentary is saying, which is - Diaphragms in flexure maybe analyzed the same as a beam/wall/column (without the deep beam requirement of non-linear stress distribution).
ACI does state that the slab must be 2" for composite and 2.5" for non-composite. However I still imagine this thin slab wanting to buckle, the only relief here is ACI R21.11.4 which says that the bonded slab is required to resist buckling.

EIT
www.HowToEngineer.com

RE: Precast Topping - Diaphragm Design

Quote (RF)

I spoke with a precaster about this and they said they'd be ok setting the plank in a 4" notch in the wall. The same way they do the bearing end. They weren't concerned with allowing for vertical deflection. I thought this was a bit odd.

I first encountered these kinds of precast connections when I was a freshly minted EIT in WI. I found them shocking but learned to accept them and do as the Romans were doing. Then, a few years later, I did my first serious, all precast parking structure. I toured some facilities to get a feel for things and, low and behold, they had all manner of slick connections to address differential deflection etc. I couldn't help but feel betrayed. When it's an all precast building, the sexy hardware comes out; when it's KootK's building, it's all "smash this with a sledge hammer and fill it with coat-hangers and goo!" End of rant.

Quote (RF)

The precast will be simple spanning between interior beams. However the topping is continuous across these beams. I imagine the slab cracking over the beam due to the induced tension from negative bending, no?

1) You want top steel over the interior support. The Spancrete detail that I used to use showed waste pre-stressing strand as the top steel. In retrospect, I wonder how that was chaired.

2) Keep in mind that, within reason, cracked concrete still retains Vc shear capacity. Were this not true, we couldn't use Vc + Vs.

Quote (RF)

However I still imagine this thin slab wanting to buckle,

In general, the 0.5 MPa or so lift of stress capacity between plank and topping should be more than adequate for this. I guess it would be a little more questionable at concentrated chord elements if present.

I find ACI a bit confusing with regard to the difference between composite and non-composite diaphragms. Is a composite diaphragm one that:

1) is composite for out of plane bending and able to keep the topping slab from buckling during in-plane diaphragm bending or;

2) #1+ the precast actually participates in resisting in-plane diaphragm bending?

I feel as though even non-composite diaphragms would require the composite action implied by #1. Surely, even a 2.5" topping would buckle at the compression edge if not restrained by connection to the underlying precast.

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.

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