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RE: Joints in structural floor slab

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Rich,

Your case is going to be different to Kevin's.  Kevin's columns could bend and he would have had the worst shrinkage problems on the first elevated slab and reducing for the floors above.

With a slab on drilled piers, the horizontal restraint of the piers on slab shortening is going to be critical. The slab is going to be basically fully restrained. You will need a lot more than normal minimum shrinkage and temperature reinforcement if you want reasonable levels of crack control.

Leaving pour strips could help with shrinkage depending on the degree of restraint you get from the piers but does nothing for temperature movement. If you get full restraint even this will not help.

The Australian code would suggest a minimum area of reinforcement at any cross section of between .0035BD and .006BD depending on the level of crack control you want.

And remember that the extra restraint cracking will make your deflections worse than you expect.


At 02:40 PM 16/12/2005, you wrote:
Rich, I just finished a 7-story building 84 metres (about 260 ft) long.  Each floor was done in 3 pours over a 2-week period.  So there were construction joints, and you would do well to specify how you want them done, because otherwise the contractor will decide what's easiest for him. 
 
We have shrinkage cracks in the floors.  What I don't understand is how a shrinkage crack can open up on the top and the bottom surfaces at the same time, and still transmit shear and bending across the crack.  But the floors are standing up well, in spite of that.
 
To minimise shrinkage, apart from choosing a good mix design and curing properly, you could consider leaving a strip in the middle of the 180 ft length, to be poured later.  While this sounds like a good idea, it is not so good in practice.  Pouring that strip on the first floor a month later is not easy when the 2nd and 3rd floors have been poured in the meanwhile.  However it can and has been done with a certain degree of success.
 
We used a camber in the floors, and I am very happy about that.  Deflections after 12 months are very acceptable as a result, despite predictions of doom from one particular competitor because of my flat plate design with 8.4 metre spans.  (Why are engineers so free with criticism of their fellows - or is it just a local phenomenon ?  One of my new clients told me yesterday that he has little esteem for engineers because of that.  Some seem to think they can build themselves up by pushing others down.)
 
With only 15 ft spans, camber will not be necessary.
 
 
 

Kevin Below
-----Original Message-----
From: Rich Lewis [ mailto:seaint03(--nospam--at)lewisengineering.com]
Sent: 15-Dec-05 20:41
To: seaint(--nospam--at)seaint.org
Subject: Joints in structural floor slab

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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