Precast Planks Failure at Bearing Wall 1

[Torok123]

We are getting called out to inspect a hotel with precast slabs. We're looking for some ideas of what could be causing the slab to crack before we go out and inspect it. See picture attached. Our initial thoughts are shear failure, however there's not really much load being applied. The second thought is that the concrete walls are bearing on the slab and causing the concrete to crush from the weight. The buildings are two stories, the planks are roughly 8', the bearing walls are poured concrete, and we believe the slab is sandwiched in between the walls. Right now they added a beam to the wall to catch the slab before the crack, we don't know if that was really engineered. We're suggesting shoring it until it's figured out.

Any thoughts?

 

[jayrod12]

Do you know if the planks were grouted prior to erecting the next wall? Generally speaking around here, pre-cast hollow-core planks have the first couple of feet grouted solid from end of plank in all scenarios to prevent crushing and improve the shear capacity. That certainly looks like a shear failure.

 

[Torok123]

jayrod12. That's a good point. We would have to see if they grouted the ends or not. It almost looks like they didn't from the picture they gave us. Which would explain the shear failure there.

 

[KootK]

I design miles of plank each year and have the following to add.

- There's no way that bearing wall load from a two story building are crushing ungrouted cores. I've run the numbers on this kind of thing and you need to be carrying 8+ stories of concrete before you get to the point where that's a concern.

- Shear failure is unlikely both because the loads appears light and because the presentation of shear failure simply looks much different. It would present as shallow angle flexural/shear crack that would be wide at the bottom and, probably, not present at the top.

- My money is on the phenomenon shown below which they seem to be concerned with in Europe but folks typically ignore in north america.

What's the back story? New cracks in an old building?

 

[MIStructE_IRE]

To ask the obvious question - are the planks definitely spanning onto this wall? Ie, do we know that this crack is not running parallel to a plank?

I saw something similar before but it turned out to be a lack of a movement joint in a 120m long concrete building - so the building formed its own.. Something else to keep an open mind about. It looked very similar though.

I’m inclined to agree with Koot’s diagram. Particularly if this happens to be a long PC Unit continuously spanning over an internal wall. Who knows how this thing was built!

 

[oldestguy]

Looks like a shrinkage crack, possibly caused by the topping layer.

 

[jayrod12]

I change my answer from shear crack to Koot's diagrams. That appears to be exactly what it is.

 

[Rabbit12]

If this is a flexural restraint crack like suggested above is there potential for complete collapse? Seems those failure modes presented by Koot could indeed end in collapse.

 

[Ingenuity]

A typical hotel room has 12 to 14 foot span module (unless they went 2 x rooms for 24-28 foot, which is unlikely) so these precast planks for a span of <14' are possibly solid planks and not hollow-core. Do you know the make-up of the planks, topping, prestressed etc?

A 8' wide plank is unusual for hollow core.

I am going to guess it is an old crack, been there for years, and only discovered during recent room modifications/renovations.

 

[Torok123]

Some more info. These are framing into the wall, you can see the 8' planks running into the wall in the last picture. The building was built in the 70's and they found this crack doing renovations. So if this is what Koots is talking about, the restraint is causing tension in top of the plank and there wasn't any reinforcement there to prevent cracking. It looks like this would fall under the favorable condition in the figure, where the shear could transfer over to the supporting wall. Right now, they added that mounted beam to act as extended seat for the plank. I guess that would decrease the distance of the crack even more from the supporting face (or eliminate it) which would be okay? The way I'm looking at it, after the cracking, the plank went back to acting more like a simply supported beam, and you would just be worried about the spalling in the figure A. If I'm mistaken, does anyone have any recommendation for a fix?

 

[KootK]

Additional thoughts:

1) I actually do very similar, heavily restrained joints like this on a regular basis. It's usually 6+ story hotels with ICF walls. On paper, the joint couldn't look much more pinched. That said, no problems to date. I suspect that some reasons for that include:

a) I don't take the plank into the wall more than 4".

b) Wall themselves are pretty slender.

c) I don't ever let anybody two-span plank without top reinforcing in the plank or topping. You'd be surprised how often this is suggested.

d) I put the plank on a think Korolath bearing strip. It's no real movement joint but, I suspect, at least allows for a little end rotation.

e) I make sure that plank span to depth ratios are not excessive as this limits plank end slope.

f) I mostly don't let folks drive 30 kip genie lifts around on 8", untopped plank.

So, when you're on site reviewing this, I'd look for the absence of these measures where access permits.

2) I struggle to think of a non-replacement fix for this that would make me comfortable having my kids sleep beneath these planks. Your kids... sure. I could probably get behind bearing angles welded to the top of the walls if the walls could handle it. And, even at that, I might want to sawcut the plank somehow so that I could feel confident about knowing where the next crack was likely to open up. Given that this is a long standing thing, there probably isn't much real and present danger. I wouldn't be willing to stake my professional reputation or anybody's life on that though.

 

[PEinc]

"Right now they added a beam to the wall to catch the slab before the crack."

Do you know this for sure? The crack looked to be fairly far from the wall. If the bolted-on beam is after the crack (i.e. closer to the wall), the beam may not support the slab.

http://www.PeirceEngineering.com

 

[Torok123]

No, I'm just going off of the picture as of right now. We will verify when we go out there and inspect.

 

[BadgerPE]

Can you tell if the crack goes all the way through the plank? In the first picture, I don't see any plank joints so there is a topping there. Could this just be shrinkage restraint cracking of the topping due to the wall? Maybe not if the topping was correctly poured so it bonded to the PC plank, but if this was 70s construction, who knows.

Edit: on second thought, that is probably way too big for shrinkage restraint cracking.

 

[MIStructE_IRE]

I don’t believe its too big to be shrinkage. I’ve put my hand into a very similar crack before - which did turn out to be shrinkage.

 

[Ingenuity]

Torok123:

When you are on site for your inspection check other rooms for the inclusion of this bolted-on wood header concealed behind the drywall enclosure.

It may be that this was how the small-span precast panels were supported during construction (with some midspan shores too). I am involved with a project right now (mid 70's build, but not in the mid-West) where they ran 2x12 wood bearers parallel with, and on each side of the bearing walls to provide support during plank erection. The bearing walls on my project are only 6" thick, so presumably they did not have enough bearing width, and relied upon overlapping plank rebar with a wet-joint to 'stitch' the joint together. In my case the wood bearer has rotted causing other issues not related to your crack issue, but the point is the wood bearer on your project may have been there since original construction.

Is this an end-span (end-wall) condition where the crack is present?

You have to be diligent and also check if other rooms have similar cracking issues.

 

[civeng80]

Could well be shrinkinge and restraint in the form of (b) Kootk.

But what about the solution ?

Grout the crack, keep an eye on it and move on ?