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RE: Joints in Slabs on Ground
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Joints in Slabs on Ground
- From: "Syed Faiz" <sfaiz(--nospam--at)saudioger.com>
- Date: Sun, 16 Nov 2003 11:45:54 +0300
original post on the subject was: "expansion joints requirements for a Slab-on-Grade footing
for a multiple houses.
do you think a REQUIREMENT should encompass?
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?
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
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
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.
is no need, in my opinion, to discuss the CONSTRUCTION joint which is
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 am able to make my points thru. Best regards,
Syed Faiz Ahmad; MEngg, M.ASCE
Senior Structural Engineer
Saudi Oger Ltd
Riyadh, Saudi Arabia
remember, the original post asked about expansion joint requirements for
a slab on ground footing, not recommended practice for joint
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
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
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.