Thank you for the reply. I agree about the stress riser, I now have a clearer picture of the pin-wheel detail (since I have now seen the ACI detail).
In a message dated 5/4/2004 8:43:08 AM Pacific Standard Time, spragueho(--nospam--at)bv.com writes:
What you have stated is not correct from what I do in practice.
I like to eliminate as many constraints to the slab on grade as possible. I prefer running the control joints (CJ) from column corner to column corner. I will generally run the spacing 30’ from CJ to CJ. I also use reinforcing steel, and a prepared FLAT subgrade to allow the slab to shrink in a controlled manner.
Properly designed EJ’s will not constrain an adjacent slab in the longitudinal direction. Concrete shrinks. Constraints cause uncontrolled shrinkage cracks. All I try to do is allow the concrete slab on grade to shrink in a controlled manner.
I will lay out a CJ and EJ grid so that I will have 4 30’ x 30’ sections before I put in an EJ that will be spaced roughly in a 60’ x 60’ pattern. The reason is that CJ’s will constrain an adjacent slab in the longitudinal direction. EJ’s will not constrain an adjacent slab in the longitudinal direction.
The other way of laying out CJ’s and EJ’s is to lay it out as a 30’ strip bounded by expansion joints on each long side, and then cut up the strip with CJ’s at 30’ o.c.
Kalman flooring does not use CJ’s. They only use EJ’s in a 100’ x 100’ grid. The column bases are prepared so that they are only penetrations detailed so that the slab will NOT constrain the slab in any lateral direction. Kalman can do it that way, because they have total control over the details, preparation, concrete mix, and curing. I do not have that degree of control.
Regardless, the pinwheel at the column bases at the CJ or EJ extend from column corner to column corner. Any joint that just goes out a few feet will just create a stress riser.
Harold O. Sprague
From: Mlcse(--nospam--at)aol.com [mailto:Mlcse(--nospam--at)aol.com]
Sent: Monday, May 03, 2004 11:02 PM
Subject: Re: Steel Terminology
Harold, Gail, Scott,
The pin-wheel isolation joint seems simple enough to use, so I guess its time to modify the block-out detail for steel columns. It makes sense, basically converting the wide flange shape to a rectangular shape. I haven't seen any one doing this for commercial office buildings, retail buildings, or other buildings with steel columns. Then again, the slab-on-grade is generally just 4" thick and does't get the same attention as industrial concrete slabs-on-grade where slabs are 6" thicker and often have re-bar instead of gauge wire mesh.
Is there a preference between using the pin-wheel detail or just adding the diagonal rebar at the re-entrant corners. For the pin-wheel detail, since I don't have the ACI detail, I am assuming the crack control joints from the corners of the column only extend out a few feet and not from column to column. Is that correct?.