malformed existing flat slab
malformed existing flat slab
(OP)
As part of a Property Condition Assessment (under ASTM E2018) I looked at an 1980's office building (eastern US) with significant/noticeable deformation in the floors.
The building is a 5 story concrete structure with a flat slab with drop panels. The column bays are 25'-0" x 25'-0" and the slab is 9" thick. In one representative area with no live load, I measured slab elevation at the middle of the bay to be 2" (+/- 1/4") lower than near the column....slightly higher in the end bay.
I did a quick direct design method review and found that the reinforcing (as shown on the structural drawings) is what I'd expect it to be. I was able to look at the underside of this area and found no cracking, visible distress, or evidence of poor concrete.
After mulling this over, I believe that this building was built this way, likely cast on deflected formwork/shoring.
So, my question is: Can you share experience with something similar in an existing building that would support or refute my conclusion? Can you direct me to articles on flat plate shoring that discuss the issue of deflection or case studies?
The building is a 5 story concrete structure with a flat slab with drop panels. The column bays are 25'-0" x 25'-0" and the slab is 9" thick. In one representative area with no live load, I measured slab elevation at the middle of the bay to be 2" (+/- 1/4") lower than near the column....slightly higher in the end bay.
I did a quick direct design method review and found that the reinforcing (as shown on the structural drawings) is what I'd expect it to be. I was able to look at the underside of this area and found no cracking, visible distress, or evidence of poor concrete.
After mulling this over, I believe that this building was built this way, likely cast on deflected formwork/shoring.
So, my question is: Can you share experience with something similar in an existing building that would support or refute my conclusion? Can you direct me to articles on flat plate shoring that discuss the issue of deflection or case studies?






RE: malformed existing flat slab
They may have removed the form work to quickly, without any reshoring. Although, I think you would then see at least some tensile and shrinkage cracking on the underside of the slabs. Does the top of that slab show the same 2" settlement, or is it fairly level? Is there any top cracking around the drop panel areas? Can the owner live with this floor condition, maybe some light wt. leveling topping? Does the hurt the ultimate strength of the slab and structure.
RE: malformed existing flat slab
I took the measurements from the top surface which is covered with a thin carpet. The owner and tenants have lived with the perceptible slope in the floor, but they're now trying to sell the building. They're not trying to fix it
I just want to correctly identify the issue as 'just' a serviceability issue.
I'm also looking into the possibility of creep being a factor.
RE: malformed existing flat slab
More than likely, the problem is what dhengr says, and long term creep could also be an issue. I agree.
Mike McCann, PE, SE (WA)
RE: malformed existing flat slab
It's regular old cast in place. f'c = 4000 psi. As you suggest, L/33 puts it at the ACI table 9.5(c) minimum.
I just noticed this note which I find interesting. (I don't know how to embed the image):
https://res.cloudinary.com/engineering-com/image/upload/v1493234952/tips/note_skina1.bmp
RE: malformed existing flat slab
The floors had deflected about 3 to 4 inches (1970's to about early 2000's - a 30 year time span).
The case here was that the slab depth (10") was not up to the amount required by Table 9.5(c)...with almost 40 ft. spans. So the creep deflections had definitely kicked in.
In your case - not sure why 2" of deflection (L/150) would have occurred other than formwork sag or perhaps the particular concrete mix was more prone to creep?
I know that smaller aggregate sizes add to shrinkage and cracking - perhaps if you have a mix with small aggregate you'd have more long term creep?
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RE: malformed existing flat slab
BA
RE: malformed existing flat slab
RE: malformed existing flat slab
JAE: I did take note of small aggregate (1/2" maybe) at a chipped location.
BA: Thanks. I'll ask to have some carpet pulled up to have a look at the top of the slab. I would think that I'd see some cracking there if we had a moment distribution event. It's a typical condition throughout the building, so they would have had to have been pretty consistent in stepping on the rebar.
That said, I've seen consistency myself with mis-placed wall reinforcing. Once they start wrong, they just keep going.
hokie: I'd have to expand the scope to include non destructive testing, so I'll give some thought to how to best lay out the next steps.
RE: malformed existing flat slab
RE: malformed existing flat slab
Long-term (including creep) deflection under SW loading with some SDL?
RE: malformed existing flat slab
RE: malformed existing flat slab
Yes, that's correct. It's a less at the floor above, 1 1/2" at the end bay.
RE: malformed existing flat slab
BA
RE: malformed existing flat slab
Just because the slab currently has no live load, does not mean that it never had any. And it would have been stressed significantly during construction acting as a back-propping floor for the floors being poured above. That could increase my estimates even more.
So 2" +- 1/4" is not far out!
RE: malformed existing flat slab
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RE: malformed existing flat slab
JAE: yes, drawings called for 4,000 psi. (columns also 4,000 psi)
As I mentioned, I was brought in as part of a general assessment that was part of a potential buyer's due diligence and I didn't start with the scope (fee) to do a soup-to-nuts investigation. However, once I started asking about pulling up carpet and digging deeper I was awarded with a structural report that was done a year ago.
That engineer did some gpr investigation and concluded that the rebar is in the right place. They also performed rebound hammer testing (ASTM C805) to confirm that the concrete exceeds the specified strength.
I found that my yellowed Nilson Winter text was a helpful refresher in this process. Anyone who stumbles on this thread in the future might want to look at ACI 435.8R-85: Observed Deflections of Reinforced Concrete Slab Systems, and Causes of Large Deflections. I didn't buy it but even the abstract supports other stuff I've found about how applying load to early stage concrete in the process of shoring and reshoring can lead to more creep in the long term.
RE: malformed existing flat slab
That is interesting.
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RE: malformed existing flat slab
Yes, you get increased SW deflection due to the low concrete modulus at time of reshoring if it is not done properly, and early creep effects due to early applied compression stresses, but from my calculations it is not a huge amount.