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]

I appreciate the theories, but how do you go about proving something like that?

I don't necessarily know that you have to prove it unless someone's asking you to and is willing to compensate you for what that effort might entail. As long as you're up front about what you do and do not know with certainty, I see no problem with spewing some theories that are poorly vetted in this context. That, particularly, if they might enhance your reputation. I'd be inclined to present things in this fashion:

1) Based on my review, I see no reason to question the safety of the as built system and believe it fit for continued use.

2) I believe the observed slab cracking to be a service issue only and offer the following, possible explanations for it:

a) blah

b) blah

c) blah

Each of these explanations has a rational basis but none can be said to be the cause with great certainty based on the investigative work undertaken to date.

As with many things, I think that the key is to attempt to answer precisely the question that has been asked. I take your assignment to date to be something akin to "We saw and heard some stuff. Can you check it out, let us know if it's safe, and give us some idea what we might expect in the future".

 

[r13]

I agree with Bones206 that you shall make report to conclude your trip, no matter will be paid or not. In the report, simply states the reason for the visit; what you have seen (include inspection time and method); the evaluation you have done (for the change of use), and the conclusion. However, in the end you shall states "due to time and method constraints, the inspection may not uncover the hidden problems, and the problems at locations were not accessible", and follow up with recommendation that "a comprehensive inspection is required shall the problem (noise) persists". The report shall be sent to the architect with the bill, and copy to the client for his information.

Note, although it is not very likely that the metal deck has delaminated from the slab, but future inspection (if) shall at least consider hammering the deck in a few strategic areas to ensure no such separations, the sounds will tell the difference. A third party structural inspection company is recommended.

 

[bones206]

All great points. Didn't think of hammering the bottom side of the deck. That's a great idea. And hiring a 3rd party is a good way to erase doubts or appearance of a conflict of interest.

 

[r13]

bones206,

Thanks for your support. Actually the "pull chain" method (performed on top of slab) may reveal separation/cracks too, if any. If needs to go to the bottom side of slab, a steel rod sliding across, would be easier to handle than hammer. These are after thought for information.

 

[bones206]

I found a very similar post from 2007 that's worth a look: thread507-185763

Towards the end of the thread someone said this:

A very interesting post. I have seen a great number of cracked slabs with composite steel deck slabs. Some were office floors and some 125 psf mezzanines.

In two separate cases, one office and one storage mezzanine slab cracked a couple of years after the initial pour with a very load noise that alarmed the occupants and called for our investigation. It appeared that the cracks were negative moment stresses that belatedly let go.