Concrete Cracks on Mezzanine 4

[SteelPE]

I am being asked to visit a project that was completed about 5 years ago. I designed the project. The owner called me today after getting my name from the architect. They said that late last week they heard a loud “bang” on the second floor “followed by what sounded like ice cracking. They are now concerned about the second floor of the building.

The existing structure is a single-story building with an interior mezzanine. A portion of the mezzanine is used for storage and the remaining portion of the mezzanine is used for general office. Construction is a 3” concrete slab on 9/16” form deck with open-web steel joists designed to be spaced at 2’-6” o.c. (this area actually ended up with open web joists at 2’-4” o.c.). The joists are supported by a system of WF steel beam and steel columns.

The owner provided me with a sketch of where they say the mezzanine has cracked (it’s terrible). From this sketch the cracks appear to be happening at the 1/3 points of the joists. Also according to the sketch the cracks are not in a distinct pattern (parallel to joists, perpendicular to joists and diagonal to joists).

It seems kind of odd that this would happen 4-5 years later. Does anyone have any insight to what may be happening before I make my trip to the site?

 

[KootK]

Accidental two span action of the slab across the girders. The non-structural bond between concrete and deck held until just recently and then gave.

Pencil me in as 5% confident.

 

[SteelPE]

If the sketches are correct, then the cracks are not occurring at girders as the floor members span across the entire room in a single span. There is an opening in the mezzanine with headers and carrying beams, there may or may not be cracks in this area (again, the sketch is not great).

 

[KootK]

Any reinforcing in the slab? What was the deck fastening type?

 

[SteelPE]

Reinforcing is WWM 6x6W2.9xW2.9. Deck fastening was puddle welds with welding washers.

Joists and beams in the area are 10" shorter than shown in the engineering models (I assume because of a changed that happened later on after the main model was completed). There is a brace adjacent to this area, but I don't think this has anything to do with the brace

 

[KootK]

Maybe some concrete shrinkage that just recently overcame restraint induced tension as a result of a humidity change?

 

[Tomfh]

Shrinkage cracking?

 

[SteelPE]

Shrinkage cracking 4 years later? I can't say that I have ever experienced that before.

 

[Ron]

Shrinkage cracking can continue for a long time....even longer than 4 years. I agree with the restraint finally giving way.

Why only a 3" slab...that's a bit weak.

 

[haynewp]

Can you post what sketches there are and show the joist and girder span dimensions?

 

[SlideRuleEra]

On the site visit pay particular attention to footprint and magnitude of load on the slab.
Chances are, loading will be quite different than the design assumption (uniform distributed load?).
Consider "real" loading on both the office and storage areas, not the nominal values from the building code.
If the centroid of the total load is off center (say, storage area heavily loaded while office area is lightly loaded) bending stress may be higher than the same load applied as a UDL... causing concrete cracking... crack location at about the 1/3 point of the span could be reasonable.

To see this concept graphically look at Case 5 of the "Beam Diagrams & Formulas" in the AISC Manual.

http://www.SlideRuleEra.net

 

[SteelPE]

All good information.

Attached is the plan for the area with a few dimensions. Storage area is highlighted.

They also mentioned something about file storage adjacent to this designated storage area. That area was not included in the design and I wonder how packed it is. Maybe this area is overloaded leading to the cracks they are seeing in the adjacent room.

 

[jayrod12]

My experience with file storage systems, especially if it's the rolling shelves type, is it's closer to 250 psf. So take from that what you will.

 

[r13]

I don't think it is due to shrinkage, as the process of such is gradual, hardly to image the energy will build up that much to cause a big bang. I guess it is more likely due to excessive deflection, which was caused by sudden change in support system. I suspect something has gave-in (failed), 3" thick concrete slab wouldn't be too difficult to break up.

SteelPE, please reload the plan (nothing was linked) when you are ready to update this thread.

 

[dik]

You should not be using rolling files on a slab on deck with OWSJ... same with forklift traffic. I couldn't download the linked file. Any photos of the cracking? What activity was happening when it went 'bang'.

Dik

 

[EdStainless]

Or more likely what had they moved in the month or so before this happened?
Just rearranging some file cabinets can change the load a lot.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[MIStructE_IRE]

I can’t seem to view the attachment?

What size is the slab on plan? Any movement joints?

 

[SteelPE]

I don't know what the attachment didn't work. This one appears to be working.

 

[JoelTXCive]

When you do the inspection, the Owner is obviously going to be concerned about safety and is going to be looking for assurances from you.

I would think ahead of responses. Something like:

"I do not see any visual evidence of deflections or movement. I feel safe in saying the region of building is safe to occupy, while we study and research further. Please don't schedule any big furniture/file cabinet moves until we determine exactly what is going on though. This could be a non-issue, or there could be legitimate problem. We are going to have to study further to figure out."

 

[SlideRuleEra]

SteelPE - When are you planning on making the site visit?

http://www.SlideRuleEra.net