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RE: Joints in Slabs on Ground

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The original post on the subject was: "expansion joints requirements for a Slab-on-Grade footing for a multiple houses.
 
What do you think a REQUIREMENT should encompass?
 
In my opinion a REQUIREMENT should first address the question of its NEED; implying if it is needed at all. Secondly, if it is needed, then it should address the location and ofcourse its SPACING. Without SPACING (and location), how can you address the question of the requirement?
 
It is correct, for a covered (interior) SOG you don't need EXPANSION joints. But, you do need ISOLATION, CONTRACTION (saw cut filled with sealants) and CONSTRUCTION joints.
 
In your last post you just focussed on ISOLATION joint; in detail of course with a little mention (in passing) of CONTRACTION & CONSTRUCTION joints around columns only and of course none related to SOG. This is a good piece of lecture, as I mentioned in my last post; probably for an undergrad school. This lecture with NO mention of "recommended spacing" (meaning its location) is of no use to a "professional Engineer" who is waiting to put that on the shop drawing. This is exactly what is my "observation" on your last post; thats all.
 
If you allow me to proceed further ISOLATION joints are, as you also touched base on, intended to completely separate the concrete SOG from any adjacent slab or from any restraints like columns, footings, or adjacent walls. They in fact transfer NOTHING vertically nor horizontally.
 
CONTRACTION or Control joints are essential to SOG for prevention of cracking due to drying shrinkage of the concrete. They may be produced in several ways, like by placement of a flexible (sometimes removable) strip, or by saw-cutting the hardened concrete. They are intended to relieve the tensile stresses produced by restraint to slab motion. And, their SPACING is extremely important to this function. All contraction joints are required to be filled with sealant, for obvious reason and I haven't seen any contraction joint without sealant. If any body does not place sealants (in the saw-cut), I can simply say this is NOT a standard practice.
 
There is no need, in my opinion, to discuss the CONSTRUCTION joint which is self-explanatory.
 
As for the SPACING of the CONTRACTION joints simple thumb rules still exist, but it depends on designer discretion, which obviously stems out of his/her experience. As per PCA (as mentioned in my last post also), joint spacing (in feet) is to be 2 to 3 times the SOG thickness in inches for PLAIN concrete. For REINFORCED concrete, you can get away with larger spacing. A reasonable spacing from experience is 15 to 25 ft. But, it is important to mention here that the centre-line-to-centre-line spacings of columns, and other structural features may, somtimes, become a CONTROLLING factor. For example, the Column spacing may  be 30 ft but the SOG contraction requirement could be 18 ft (3 * 6-in Th.). If the CONSTRUCTION joints are placed at the column centre lines, then the CONTRACTION joints will NOT be possible at a 18 ft spacing. They will most probably be palced at 15 ft.
 
I hope I am able to make my points thru. Best regards,
 

Syed Faiz Ahmad; MEngg, M.ASCE
Senior Structural Engineer
Saudi Oger Ltd
Riyadh, Saudi Arabia

 
-----Original Message-----
From: GSKWY(--nospam--at)aol.com [mailto:GSKWY(--nospam--at)aol.com]
Sent: Saturday, November 15, 2003 5:35 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Joints in Slabs on Ground

As I remember,  the original post asked about expansion joint requirements for a  slab on ground footing,  not recommended practice for joint spacing.

The simple answer to the original post is that expansion joints are not required for slabs on ground.  Since I had something written that touched on the issue, and specifically mentioned confusion in terminology, I thought I'd share.

Expansion joints are actually not a bad idea on things like golf cart paths and sidewalks since typically the contraction joints aren't sealed.  If the contraction joints get filled with dirt and debris, they will not be able to accommodate volume changes in the slab.  

I would also note that I don't believe in simply passing on information I have found in a book.  Although what is found in books is often good information,  useful information is really only gained by experience.  This is particularly true for things like repair work and slab on ground work, where much of the "standard practice" is based on experience.

Even if the information is found in several references,  one sometimes finds that they are all using the same source, and that there is no documentation of any research to back up the recommendations.  

To cite a specific reason for my reluctance to rely on book learning,  the author of the current edition of the PCA Concrete Floors on Ground book is not, to my knowledge, an engineer.  I am fairly sure he has never designed anything and I know he has never designed a slab on ground.  

Why exactly do you think I would want to take his recommendations?  The book has come under a lot of criticism because it makes a number of questionable statements,  perhaps because the author did not really understand what he was saying.  Or maybe they are just typos that weren't caught because the book was not adequately reviewed.

Gail Kelley