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RE: Joints in structural floor slab

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Rich,

Generally, structural floors are not jointed.

You have to assume that the concrete will shrink and that you will have thermal cycles. The concrete will shrink to the center. If the piers are relatively short, you may be constraining the slab and causing constraint issues and crack potential.

There is detailing that can be employed to avoid constraint cracks. Longer columns allow the slab to move by the flexing of the columns. I have seen this done by over drilling the piers dropping in a cardboard form tube and filling the interstitial space with bentonite.

You could do this like you would do precast. Make the columns gravity only except near the center. Cast the piers, put a bearing pad on top and cast the slab so that the slab can move laterally and differentially with the piers.

You can also employ the shrinkage steel in ACI 350 which will keep the shrinkage cracks much smaller. The area of the pour that can be done in a single shot is a function of the ability of the contractor. I have worked with contractors that can do this in one pour. I have also worked with contractors that couldn't do that much area at gun point.

Regards,
Harold Sprague





From: "Rich Lewis" <seaint03(--nospam--at)lewisengineering.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Joints in structural floor slab
Date: Thu, 15 Dec 2005 19:41:29 -0600

I am designing structural slab 80 ft. x 180 ft.  It is a suspended slab on
drilled piers.  Supports are typically about 15 ft. on center.  I am
investigating the need for jointing.  I have a 6 inch slab with #4 top and
bottom reinforcing each way.  Since this is a structural slab I don't see a
need for shrinkage joints. I was wondering what precautions should be taken
to minimize shrinkage cracking of the concrete.  Also, would the 180 ft.
span be too long for a single pour?  Would a construction joint be
recommended?



Thanks for any insight



Rich




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