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RE: depth of footing above bottom reinforcement
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: depth of footing above bottom reinforcement
- From: "Scott Maxwell" <smaxwell(--nospam--at)umich.edu>
- Date: Sun, 2 Nov 2008 23:38:56 -0500
won't bother with whether or not one should consider a slab as a "structural
foundation", but instead avoid that mess and just stick to ACI 318. ACI
318 tends to make the issue of slabs on grade easy...it is does NOT cover slabs
on grade...per section 1.1.6. Now, it does get a little bit "murky" when
you consider that it specifically states: "This code does not govern design and
construction of soil-supported slabs, unless the slab transmits vertical loads
or lateral forces from other portions of the structure to the soil." That
last part kind of mucks things up a bit. Would this mean that a load
bearing light gage wall on a slab "force" that slab to be under ACI 318 since it
would be transfering load from "other portions of the structure" (i.e.
transfering load from a roof or floor that the load bearing wall
supports)? But, then a slab that only supports a storage rack NOT be per
ACI 318 (after all, it would be taking the "load" directly, unless you consider
the storage rack to be "part of the structure")?
in relation to the original provision, it would NOT apply to "slabs" anyway as
section 15.1.1 specifically limits chapter 15 of ACI 318 to isolated footings,
combined foots and mats.
the original question, I would say it is pretty clear that the dimension in
question is the minimum depth from the bottom most layer of reinforcement, which
will be "d", unless there are multiple layers of bars. After all the
provisions specifically states: "Depth of footing ABOVE BOTTOM REINFORCMENT
shall not be less than 6 in for footings on soil, nor less than 12 in for
footings on piles." (emphasis is mine). The point is the provision
specifically goes out of its way to define where the depth of footing is
relative to...the bottom reinforcement. Now, it is not clear if that means
location of centroid of bottom reinforcement or absolute bottom of
reinforcement. For one layer of bottom steel, the difference will be
neglibile...for multiple layers of bottom steel, it would still be small, but
not as small as one layer.
really want to know the intent behind the provision, I could see what I can find
out. I likely would not be able to get an answer for a little bit...the
Fall ACI Convention is taking place right now in St. Loius, so I don't know how
quick of a response I will get from my friends/contacts.
not sure what the original intent of this requirement is. May be Scott
can find out from one of his committee friends. Most building slabs are
not considered structural foundations so it may or not apply to some of the
conditions you site. If I was to guess, I would say this requirement
just comes from experience that there should be some type of minimum thickness
requirement for footings and pile caps just like a lot of other minimum
concrete requirements such as minimum temperature steel. May be it was
just a bunch of old farts from the 1940's smoking cigars around the table and
thought it was a good idea?!?
Thomas Hunt, S.E.
10/31/2008 08:10 AM
Please respond to seaint
|RE: depth of footing above bottom
is this specification intended to be a cover requirement for the steel? What
is its purpose? Does it matter if there is no earth above the footing as in
monolithic construction? Could this be interpreted to prevent the use of
structural slabs on grade used to support industrial racks; or monolithic
footing & slab mat construction where the slab is reinforced? What of
interior monolithic wall footings for light gauge steel partions as in slab
construction for self storage facilities?
Thanks again. These are all hypothetical
questions as this point. I'm just researching the history and application of
I would disagree and say it is a just a
dimension/distance value of 6 inches or 12 inches and has nothing to due with
"d" (i.e. distance from extreme fiber.....). So if you had a pile cap
with #8 bar each way for bottom reinforcement and 4 inches of cover then your
overall depth (h) of the pile cap would need to be a minimum of 18 inches (4"
clr + 2 -#8 bar + 12").
Thomas Hunt, S.E.
PO Box 10129, Brooksville, FL,34603
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