Construction Joints in a Mezzanine Slab

[hopcoin]

Structural Metal Decking, bar joists 5' on center, 4" thick slab on deck.
50,000 sq ft so we are required to make sepearate pours. Where and how should we make construction joints at pour stops?

 

[JAE]

Parallel to your joists - place them either on top of the joist or at midspan.

Perpendicular to the joists - anywhere as this is parallel to the span of the deck/slab system.

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[hopcoin]

Any advice on sawcut joint? Given the construction joints, are sawcut joints necessary, or not recomended?

 

[KootK]

Usually not recommended. The deck creates too much in plane restraint for it to be effective. Link

Two additional bits of advice:

1) Most people keep construction joints away from welded studs if present. Probably not an issue with joists.

2) You may want to stitch the slab together across the joint with rebar to restore diaphragm continuity.

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[JAE]

What KootK said...agree.

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[hopcoin]

Thanks for the advice guys, we appreciate it.

The Architect changed from floor covering to stained and sealed exposed concrete, therefore we're trying to elminiate as much of the random cracking as possible. From everything you've helped us with we're going to use slab boosters in the thick areas of the decking to keep the wire mesh at the top to help with cracking, and eliminate as many construction joints in the stained concrete as possible.

Thanks again!

 

[JAE]

You may get some random cracks in any case.
For slabs-on-grade (which you don't have) you get the drag from the subgrade resisting the shrinkage and thus shrinkage cracks.

For elevated slabs on metal deck - there is still some degree of shrinkage resistance due to the deck flutes, deck-concrete bond, etc. but you also have a much higher variability in vertical stiffness across the slab due to the change in flexibility of members (more flexible at midspans ans with lighter members - less flexible near heavier girders or columns).

These flexibilities will cause cracking as well.

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[KootK]

I had one very bad experience with exposed composite deck that you might benefit from hearing about. The contractor drove a genie lift over the slab to install some items in the ceiling above. This caused the slab to crack terribly over the supports. It was/is hideous. What’s worse, I approved this because the genie loads were well within the bounds of the composite deck capacity. Of course, that capacity is based on simple spanning deck. So, while from a strength perspective everything was/is fine, it was a serviceability nightmare from my client’s point of view. And I had WWM in the slab per SDI recommendations.

Nowadays, if I have composite deck systems that are exposed or overlain with sensitive material, I specify 15M@400 EW everywhere at minimum. I know, it sounds pretty extreme. This is commensurate with the extreme displeasure that my client communicated to me when I messed this up. I expect that a heavy safe or filing system could produce a similar result.

If you place your construction joints at the supports, I’d recommend this at minimum:

1) Either construct a hard joint with no rebar or concrete crossing it or;
2) Install some real top steel across the joint and be generous with it.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I only specify 10m@400 to avoid precisely kootk's situation

 

[Lion06]

There is a difference between a construction joint and a control joint. A construction joint merely separates two pours. A control joint is meant to keep cracking under control. I can't think of any reason that you would sawcut a construction joint because this would already have a through-the-thickness cold-joint.

koot - Do you typically provide top bars perpendicular to, and over top of, the beams for slab on deck? That's a typical detail we use to control the exact cracking you described.

 

[KootK]

My firm's typical detail shows some isolated top steel placed over all girders. The idea is that, at those locations, slab curvature discontinuity is rather extreme so some crack control is prudent. I think that's pretty standard logic. I had this in the offending deck mentioned above and wound up with cracking issues over my infill beams. So now it's 15M@400 everywhere and in both directions. Depending on the girder frequency, I guess that I'd be willing to relax this some in the perpendicular to deck direction.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.