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RE: footings a beam or a slab?

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Mike,

It is the shear that is maximum at  distance d from the face of a wall or
column. Flexure is theoretically maximum at the face of the support however
in this case it is so small that  the min. requirements for temp. steel will
govern.  The  footing is more like a one-way slab and that placing minimum
temp. bars  for transverse reinforcement  is a good practice.


Alex C. Nacionales, C.E.

-----Original Message-----
From:	Mike Brown [mailto:mike.brown(--nospam--at)cshqa.com]
Sent:	Thursday, October 01, 1998 6:57 AM
To:	seaint(--nospam--at)seaint.org
Subject:	Re: footings a beam or a slab?

Marlou:

Several things can be argued about this situation.  To begin with the
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 made
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.
>
>Thanks in Advance
>
>Marlou B. Rodriguez, EIT
>Robert Englekirk Inc.
>Honolulu, HI
>
>
>