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?

 

[SteelPE]

Tomorrow

 

[r13]

Have a close look on both top and bottom. Be careful, and good luck.

 

[Rabbit12]

Any bolted connections that maybe went from slip critical to bearing? It's been a few years since I've done steel frame buildings but it was really common for erectors to use twist off bolts everywhere. I could see sudden movement of the supporting girders causing some cracking of the slab.

 

[hokie66]

Is it still common in the US to use those thin slabs on corrugated deck, supported by bar joists? There never was a rational theory for why they worked, but there exist billions of square feet of that type construction.

 

[SteelPE]

Hokie66,

That's all I learned when first starting out. My old boss worked for a large joist supplier designing a portion of the billions of square feet. In fact, that's all my clients typically want. It's rare for my clients to want the alternate beam and composite deck.

 

[SlideRuleEra]

SteelPE - As you know, there is a lot riding on you solving this mystery. I spent a lot of time at our generating stations getting to the bottom of various structural "problems". FWIW, I'll offer some of the things I learned:

Site visits are best tool at your disposal. Think of the site as the perfect "full scale, 3-D model" of the problem... as I told my understudy, make full use of it. In addition to getting location of visible damage and determining actual floor loading, spend as much time as possible (preferably by yourself) slowly walking around... looking and interpreting what you are seeing. Keep an open mind, the clues are there, it's up to you to unravel them.

Don't expect to have an "ah-ha" moment tomorrow, that may happen but don't count on it.

Over the next day or so you will realize there is "something" else you want to check out. Return to the site, walk around, again. Don't be discouraged if many observations that seem important at first turn out to be dead ends.

Specifically, suggest looking at the areas of the floor I've circled in blue. Possible discontinuities in joist defection. Also, for the same all along the W18 & @ W21 beams on those column lines.

http://www.SlideRuleEra.net

 

[ldeem]

I am thinking an overload but that should be localized to the overloaded area. I get the impression from your original post the cracks are all over the storage area.

Secondly, maybe the cracks are not related to the noise they heard? I would suggest trying to establish a timeline of when the cracks were observed, hearing the noises and load changes on the floor.

I see a mix of 22" and 24" joists - might be worthwhile to verify they were all placed correctly.

 

[SteelPE]

Ideem,

Can't misplace the joists because they come fabricated to length. If they put them in the wrong spot then they won't fit.

 

[hokie66]

There are a couple of beams on the plan which seem to stop in midair, a W18x35 and a W16x45. Hopefully, that is not the case.

 

[ldeem]

SteelPE - I see now I jumped the gun - I did not notice the skewed beam at the bottom of the joists.

 

[r13]

Depends on the height of the mezzanine floor, you might need a hi-reach to perform inspection from below. Could it be a bad member in the joist? It will be too difficult to spot from below. But an unusual deflection could tell. Before you can pin point the problem, the access to the areas directly on and below the floor should be restricted.

 

[haynewp]

Maybe something very heavy fell in this area and that was the bang they heard that caused this cracking.

 

[MIStructE_IRE]

What’s happening at the top of the image where section mark S5 is? The w16x45 appears to stop?

 

[SteelPE]

What’s happening at the top of the image where section mark S5 is? The w16x45 appears to stop?

The W16x45 continues over to the column. What you are seeing are the joist shop drawings which gives the as-built joist spacing.

 

[bones206]

I think KootK's theory is plausible. If the area had historically been lightly loaded, the slab could have been spanning from girder to girder and bypassing the less stiff joists without a problem. Then along comes a heavy load and suddenly the slab cracks (big bang, cracks parallel to joists) and the load is now applied to the joists instead of spanning over them. So now the joists are deflecting under the live and and the slab cracks some more to conform to the new deflected shape of the joists (the ice breaking sound, cracks across joists at 1/3 points).

 

[dik]

Aren't the metal pans button welded to the OWSJ? then likely would deflect with them.


Dik

 

[bones206]

Yes, but the slab and metal deck don't act compositely. So if the slab alone could span girder to girder up to a certain level of superimposed live load, the deck wouldn't necessarily be picking up any load until the slab finally cracks in and load is redistributed into the smaller intermediate spans between joists. I picture it like a beam on elastic foundation, where the outer springs (girders) are stiff and the inner springs (joists) are soft. The beam in that analogy would have to deflect a certain amount before the inner springs see any load. But in order for a thin lightly reinforced slab to deflect that much, it would need to exceed it's elastic capacity and crack.

I could be way off the mark, but i think it's a plausible theory. Pencil me in as 6% confident...

 

[dik]

I doubt that a 3" slab on 9/16 form would span that far... specially over time.


Dik

 

[SteelPE]

Sorry this is so long.

So I made my visit to the site today. I spent about 3 hours onsite. I spent about 1/3 of that time doing a quick walk through and talking to the owners. I spent the other 2/3 looking at the floors and documenting just about everything I could.

The underside of the floor looks really good. I measured 3 separate joists. Two side by side where they heard the loud noise and another under the area where they keep their files. I measured from the distance from the bottom chord to the floor at each end and in the middle (I understand this isn’t fully accurate, but it’s the best I am going to do). 2 of the 3 joists appear to be flat all the way across, one joist I measured as being down 3/8” in the middle. SJI has the camber for these joists as being 3/8” so we are looking at a max ¾” deflection under total load. This equates to a total load of about 100 psf….. which is less than the 175 psf the floor was designed for.

I next looked at the cracks in the storage area. They varied from 0.025” to 0.04” in width. They basically mirrored the WF beam supports on the underside of slab. There were other cracks in the slab that ran perpendicular to the joists (roughly every 7’-0” o.c.) with a few cracks that just went off in random directions. Most of the wider cracks didn’t look new and the smaller cracks it was hard to tell.

Loading in the storage area was pretty light, if I had to guess (before running numbers) I would have placed a load of 20-30 psf on the slab.

Finally, we finished at the room they used for file storage (that was outside of where we had designated for allowable storage). Here they had 1’-8” wide x 3’-6” long 5 drawer high file cabinets. I couldn’t specifically measure the weight of the cabinets, but they did have bankers boxes filled with papers shoved against the wall. I estimated that there were 3 boxes per drawer. I measured 4 bankers boxes at once 3 separate times coming up with a weight of 32.5#/box. With this I estimated that their cabinets weighted about 550#.... this equals 94 psf…. However the cabinets were laid out in rows perpendicular to the joists….. in the end, I estimated the load in the room to be approximately 50 psf.

So from what I can tell, I have no clue on what was causing their issues. I saw nothing on the loading side and no evidence of damage do the building (no stuck doors, no cracked drywall…..even where the drywall was cut around the bottom chord of the joist at midspan of the joist…. There was no damage to the drywall). So I have no clue what they heard.

When I was leaving, I asked the owner how they wanted to proceed. They said they wanted a letter. I told them that I could write a report of my findings but I would need to be compensated for my time (we had briefly talked about that before but never really firmed it up…. Which I was OK with). That’s when their mood immediately changed. They said “you are going to charge us for this issue” and I said “yes”. Then they said “well, could you just send an email” and I said sure, but it’s going to cost you.

I gave them my business card and told them to let me know what they wanted to do.

I understand that it may be in my best interest to write a letter, but I am not going to give them the satisfaction of getting 2 hours of travel, 3 hours of a site visit and whatever time it takes to write a letter for free (obviously at least one of the owners thought that).

 

[ldeem]

Thank you for writing the follow up. I think you are well within reasonable bounds to be paid since they called you out and there was no problem found.