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# Fw: footings a beam or a slab?

• To: "seaoc list" <seaint(--nospam--at)seaint.org>
• Subject: Fw: footings a beam or a slab?
• From: "T. Eric Gillham" <gk2(--nospam--at)kuentos.guam.net>
• Date: Thu, 1 Oct 1998 10:12:19 +1000

Mike:

The only thing I would take exception to is the statement that "there is no
tension zone in the transverse direction", for the following reason:

Draw a section of the footing with dimensions.  I agree that because of the
depth of the footing relative to its width in the transverse direction, and
the 8" wall assumption, the reaction from the soil WILL transfer directly
up to the wall.

But it you draw a diagonal compression strut up from the edge of the
footing to the wall, you will see that it is inclined (it has to be).  This
means that there is a horizontal component which must be added to the
vertical component.  Looking at the footing like a truss, you will need a
tension "chord" across the bottom face of the footing to account for that
horizontal component being added to the vertical reaction to make it
inclined (hope that makes sense).  So IMO, there IS a horizontal tension
force, which is resisted by the one manifested on the other side of the
footing (since it is symmetric, or assumed to be).  But something,
preferrably reinforcement, must transfer that tension across the bottom
face of the footing from one side to the other.

Another way of looking at it would be to check the ACI-318 requirements for
a corbel.  There is a tension component that must be reinforced for
(disregarding the fraction of the vertical load that is required to be
applied horizontally) at the top of the corbel.  Just invert a symmetric
double corbel and you have a situation similar to the footing being
discussed here (although the load is distributed, not concentrated, ).

Having said that, it is likely, IMO, that minimum reinforcement would be
sufficient, but it would depend on the particular footing I guess.  Also, I
am ASSUMING an infinitely stiff footing (standard assumption for regular
stuff like this), which if you really looked at it from an FEA standpoint,
wouldn't be the case but how much of a difference it would make I can't
say.

Does this make sense?

T. Eric Gillham PE

----------
> From: Mike Brown <mike.brown(--nospam--at)cshqa.com>
> To: seaint(--nospam--at)seaint.org
> Subject: Re: footings a beam or a slab?
> Date: Thursday, October 01, 1998 8:56 AM
>
> Marlou:
>
> footing will act like a beam across any weak soils in the longitudinal
> direction (I think this issued is resolved for you).
>
> As for the reinforcing in the transverse direction, an argument can be
> stating that there is no flexure in this direction (assuming the wall is
> centered on the footing).  To back this argument up, the maximum flexural
> stress occurs at a distance 'd' from the face of the wall ('d' is the
depth
> of the footing to the tensile reinforcing).  With a 16 inch deep footing,
> d=16-3-0.25(#4bar)=12.75 inches.  Assuming an 8 inch wall thickness and
the
> wall is centered on the footing, you have 8 inches from the face of the
wall
> to the edge of footing which is less than the 12.75 inches, therefore no
> flexure.  The vertical forces from the wall are transferred directly to
the
> soil without putting the footing in flexure in the transverse direction.
In
> other words, there is no tension zone in the transverse direction.
>
> However, we do use transverse reinforcing in our footings.
>
> If anyone does not agree with the above, please respond.  I would like to
> hear other opinions, especially if I'm not seeing the whole picture.
>
> Mike Brown, P.E.
>
> -----Original Message-----
> From: MRodrig273(--nospam--at)aol.com <MRodrig273(--nospam--at)aol.com>
> To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
> Date: Wednesday, September 30, 1998 4:14 PM
> Subject: footings a beam or a slab?
>
>
> >I have a case where I have a footing that is 24 inches wide and 16
inches
> >deep.  I found that the footing only requred minimum steel
reinforcement.
> In
> >the longitudinal direction I used 3 #5 bars and in the transverse
direction
> I
> >used #4 @ 6".  I was told that my #4 bars were not needed and that I can
> just
> >space them at 18 inches.  This was argued that the footing acts like a
> beam,
> >which I understood, and in a beam you don't need to satisfy the 0.0018bt
> >critiria.  But as I was reading the ACi.  In the flexural reinforcement
> >section 10.5.4 it states that "For Structural slabs and footings of
uniform
> >thickness the minimum area of tensile reinforcement in the direction of
the
> >span shall be the same as that requred by 7.12.  Maximum spacing of this
> >reinforcment shall not exceed the lesser of three times the thickness
and
> 18".
> >If my footing acts like a beam, does it span the length of the footing
or
> does
> >it only span the 24 inches in the transverse direction.
> >
> >What does everyone else use as the minimum reinforcement for the
transverse
> >direction for a 24 inche wide beam.
> >
> >
> >Marlou B. Rodriguez, EIT
> >Robert Englekirk Inc.
> >Honolulu, HI
> >
> >
> >
>
>
>