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RE: Minimum reinforcing in mat slab foundation

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Dmitri and David,
Yes I have been aware of the .0018 Ag criteria.  I was asking (probably not
very clearly) if there was any special provisions for the mat slabs.  But
thanks for the advice on skin reinforcement.  I will check into that.
Thank you for the replies,
Joe

Joseph R. Grill, PE
Verde Valley Engineering, PLLC
email: VVEng(--nospam--at)cableone.net

-----Original Message-----
From: Dmitri Wright [mailto:dmitri(--nospam--at)cascade-structural.com] 
Sent: Saturday, July 31, 2010 1:08 PM
To: Seaint
Subject: Re: Minimum reinforcing in mat slab foundation

Joe,

I agree with David, and use the same design philosophy.

A couple of other comments:  I usually use a unit strip method for mats, 
taken at the column or other load points.  I have not run a parametric 
study, but I'm pretty sure this is a bit more conservative than a finite 
element model.  Depending on the mesh and the boundary conditions, FEM 
models tend to be a bit underconservative in many instances.  Fortunately, 
mat designs are  typically not that sensitive to the subgrade modulus. 
Values of 50 pci to 300 pci are common, and should not have a dramatic 
effect on the design.  One other design check that I do is for skin 
reinforcement.  I don't have my ACI with me, but I think it is in chapter 
10, or check the index under "skin reinforcement".  It is a requirement for 
deep beams, but the concept applies to any concrete that you want to design 
to minimize surface cracks.  The equation calculates a bar spacing, which is

independent of bar size.  So I check for the spacing first, then chose a bar

size to meet the bending or reinforcing ratio requirements.

As far as Rich's comment, I think this is a requirement for beam design, 
where it is crucial to avoid brittle failure.  Brittle failure of a mat is 
not likely to be a concern, since it is continuously supported by the soil.

Dmitri Wright, PE
Cascade Engineering, Inc.
245 SE 4th Ave, Suite B
Hillsboro, OR  97123-4033
dmitri(--nospam--at)cascade-structural.com
503-846-1131




Subject: Re: Minimum reinforcing in mat slab foundation
From: David Topete <d.topete73(--nospam--at)gmail.com>
To: seaint(--nospam--at)seaint.org

--e0cb4e8878273ba722048c9fceea
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: quoted-printable

I have checked for min. temp and shrinkage steel, which typically governs
over bending and shear requirements in (thicker) mats.  So, I'll check for
0.0018 Ag.  For an 18" thick mat, for example, the minimum steel needed
would be 0.0018*18"*12" =3D 0.39 in^2 per foot of mat width.  In that case,=
 #4
@ 12"o.c., top & bottom.  Depending how thick, I've added intermediate
steel.

No one has called me on it yet.

On Fri, Jul 30, 2010 at 10:52 AM, Joseph R. Grill <vveng(--nospam--at)cableone.net>wrote=
:

>  I am designing a buried structure to be used as a shelter.  The depth of
> the structure and the soil conditions have required the design of a mat
> foundation (my first).  Not having software that directly designs the mat=
, I
> have set up a model on Risa3D (I don=92t have Risa Foundation yet).  I ha=
ve
> checked punching shear and sized reinforcing for flexure.  What is the
> minimum temperature shrinkage requirements for the mat.  ACI 350 suggests
> some parameters based on joint spacing which I would like to do without.
> But the ACI 350 suggestions require a lot of minimum steel and probably
> warranted.  The slab is 26 ft. x 54 ft. and will probably have no issues
> with groundwater.
>
>
>
> Thanks, Joe
>
>
>
> Joseph R. Grill, PE
>
> Verde Valley Engineering, PLLC
>
> email: VVEng(--nospam--at)cableone.net
>


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