We have a client who's elevated slab on metal deck and bar joists is cracking. Thankfully it was not our design. It appears to be shrinkage cracks but I cannot find much information on control joint locations for elevated slabs. Any help or direction to authoritative source would be appreciated. Ken
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Concrete slab on metal deck cracking. Any insight? 7
- Thread starter MWPC
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Where are the cracks and how big are they?
If they're not very large and are over beams and girders at column lines, then they're not unusual. Assuming that it was screeded level, the slab's thinnest there. We tried everything at my old job and we could never get rid of these. They only caused major problems when we used sheet vinyl flooring. Even tiny cracks cause little poofed up areas that we called ant trails in that kind of flooring.
If they're not very large and are over beams and girders at column lines, then they're not unusual. Assuming that it was screeded level, the slab's thinnest there. We tried everything at my old job and we could never get rid of these. They only caused major problems when we used sheet vinyl flooring. Even tiny cracks cause little poofed up areas that we called ant trails in that kind of flooring.
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There was an article written titled "Let it Crack"
Written by J. Thomas Ryan, PE.
Could not find the direct link. I have pdf on disk, be glad to email it to you, or post it somewhere.
Here is some info to back up you assessment.
Bottom line, all slabs crack. The cracks in elevated slabs on steel deck usually do not compromise structural integrity.
Written by J. Thomas Ryan, PE.
Could not find the direct link. I have pdf on disk, be glad to email it to you, or post it somewhere.
Here is some info to back up you assessment.
Bottom line, all slabs crack. The cracks in elevated slabs on steel deck usually do not compromise structural integrity.
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I would add one caveat: Get the Owner's agreement and buy in before you just "let it crack". It has been my experience that Owners most often expect to have an "uncracked" slab in new buildings.
I agree that in commercial buildings the crack can be filled and be covered with finished floor material although if it a hard material, a working crack sometimes can mirror thru.
It is a trade off between adding more steel or accepting cracks. Get the Owner's direction on what he wants.
I agree that in commercial buildings the crack can be filled and be covered with finished floor material although if it a hard material, a working crack sometimes can mirror thru.
It is a trade off between adding more steel or accepting cracks. Get the Owner's direction on what he wants.
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Visited the site today. Most cracks appear to be due to negative moment over the beams where the joists butt. A few other random (shrinkage) cracks. Contractor did a poor job locating his construction joints and selecting the direction of placement. I think he is ready for some help on the next one. Thanks for the help. Ken
In years past I have discussed this very issue with the metal deck manfgs. I was told it was not necessary to provide control joints. Think about it, theres a solid layer of metal reinfg below the concrete slab (the deck) and typically the mesh will run thru the joinst anyway.
Don't count on them being from negative moment, not that it probably matters much. We had one slab that developed these cracks and the owner had us look at it.
The 3rd Floor was occupied and had ant trails (little ridges in the sheet vinyl) above beams and girders at every column line.
An upper floor was shell space, so had practically no load above its self weight, and it had cracks in the same locations, mostly small, but some pretty good size. We had seen these before, so provided #3@12 top, IIRC. That did a heck of a lot of good, LOL.
I figured these were from negative moment, but one of the older guys pointed out that there was no negative moment in the slab from the slab weight because it cured in that position--duh.
He said that the cracks simply form there because that's where the slab is thinnest and the studs provide stress risers, almost like the perforated edge of a paper designed to be torn out. There's also lots of restraint against shrinkage and the slab was probably 150' long. I had to think about it a while, but I'm sure that's right.
Unless you have a shored system or some serious superimposed loads, I have serious doubts if your cracks are from negative moment. Not sure that it matters now--they're there.
That being typed, I did cause negative moment cracks in a lab specimen once. It was a 3-span footbridge. When I put 20 psf on two adjacent spans, it cracked over an interior support. I think it did not crack earlier from shrinkage because it was so stiff that it wasn't much thinner over the supports and because it had no studs. It also wasn't as restrained against shrinking as a real slab would be.
The 3rd Floor was occupied and had ant trails (little ridges in the sheet vinyl) above beams and girders at every column line.
An upper floor was shell space, so had practically no load above its self weight, and it had cracks in the same locations, mostly small, but some pretty good size. We had seen these before, so provided #3@12 top, IIRC. That did a heck of a lot of good, LOL.
I figured these were from negative moment, but one of the older guys pointed out that there was no negative moment in the slab from the slab weight because it cured in that position--duh.
He said that the cracks simply form there because that's where the slab is thinnest and the studs provide stress risers, almost like the perforated edge of a paper designed to be torn out. There's also lots of restraint against shrinkage and the slab was probably 150' long. I had to think about it a while, but I'm sure that's right.
Unless you have a shored system or some serious superimposed loads, I have serious doubts if your cracks are from negative moment. Not sure that it matters now--they're there.
That being typed, I did cause negative moment cracks in a lab specimen once. It was a 3-span footbridge. When I put 20 psf on two adjacent spans, it cracked over an interior support. I think it did not crack earlier from shrinkage because it was so stiff that it wasn't much thinner over the supports and because it had no studs. It also wasn't as restrained against shrinking as a real slab would be.
Don't know if there are studs on these beams, but the rest of 271828's reasoning is spot on. Deflection of joist under wet concrete, but floor finished level, therefore floor thinnest at support. So they are shrinkage cracks of no structural import. I suppose construction joints over beams would at least tend to make the cracks straight, but have never seen it done.
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Boiler8019
Geotechnical
Dont have all the necessary info here to make a judgement, but there will be tension (negative moment) induced into the slab at the girders from the deflection of the bar joists. It's not negative moment caused by the deck span. If there's no bars or properly placed WWF in the slab then there's the possibility of pattern cracking....fibermesh doesnt count.
I would just caution that composite steel girder-beam construction, posted above, produces different issues than composite steel girder-bar joist construction.
I would just caution that composite steel girder-beam construction, posted above, produces different issues than composite steel girder-bar joist construction.
Boiler8019, I don't see how the joist floor is any different from a slab on beams for the current discussion.
If the joist deflections are due primarily to concrete weight, then these deflections would also occur before the concrete cures, so no negative moment. If there's a significant superimposed load AFTER the concrete cures (if this is the case, then how can he see the cracks?), then that could cause negative moment cracks.
But then again, this is also the case for a composite slab on beams. I think these cracks occur with or without any load being applied. Also, like I typed earlier, reinforcement won't prevent these cracks.
If the joist deflections are due primarily to concrete weight, then these deflections would also occur before the concrete cures, so no negative moment. If there's a significant superimposed load AFTER the concrete cures (if this is the case, then how can he see the cracks?), then that could cause negative moment cracks.
But then again, this is also the case for a composite slab on beams. I think these cracks occur with or without any load being applied. Also, like I typed earlier, reinforcement won't prevent these cracks.
Ken, since someone has flagged this thread as useful for posterity, it would be helpful if you would give the slab thickness, type of deck, and joist spacing. I took it as a thin slab on corrugated deck spanning about 2 feet, but others have thought it may be something different.
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haynwp, that is an excellent question, one that I've heard before. I did a quick literature search and didn't find any papers on the subject, so I doubt that anybody has ever done any tests using cracked slabs. Pushout tests are always with pristine little specimens.
Just from looking at the huge number of cracked slabs and the absence of problems, it seems that they probably don't do much.
Just from looking at the huge number of cracked slabs and the absence of problems, it seems that they probably don't do much.
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