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Re: [Fwd: eccentric footings]

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Mark & Jessica Pemberton wrote:
> 
>     ---------------------------------------------------------------
> 
> Subject: eccentric footings
> Date: Tue, 17 Mar 1998 12:24:51 -0800
> From: Mark & Jessica Pemberton <jmpember(--nospam--at)softcom.net>
> To: seaoc(--nospam--at)seaoc.com
> 
> Has anyone else been asked to analyze a standard
> single story residential footing as eccentrically loaded?
> I doesn't calc out (resultant force is far from middle third),
> but its the way they've always been detailed.  And how about
> rigid frame pad footings?  I rarely see these loads concentric
> yet the calculations (by others) always assume this.
> 
> Mark Pemberton, P.E. --(Does anyone put C.E.?
>                                        or is that chemical engineer?)


When you mention standard residential spread footing, I am assuming that
you are referring to the strip footing underneath the foundation walls.
The eccentricity would be caused by the soil pressure acting on the
outside of the wall.

The requirement of the resultant being in the middle third is
conservative. Being outside the middle third does not mean that the
footing will be unstable. It means that the soil pressure will be
increased at the toe of the footing. Soil pressure near the heel of the
footing will be zero. It is good practice to design foundations with the
resultant between the middle third because this prevents settlement
problems that could occur if the toe is overstressed.

The stability of the foundation wall would become a problem if the
overturning moments exceed the stabilizing moments. On the projects I
worked on, the stabilizing moments exceeded the overturning moments by a
factor of two when I sized the footing based on middle third criteria.

This might explain why the middle third calculation does not work, but
walls are still standing.

As far as rigid frame footings, anchorage of the base plate to the
footing is often designed to behave as a pin connection. No significant
moment would be transmitted to the footing in this case. I have
considered the eccentric loading in the footing whenver the anchorage
allowed the transfer of moment to the footing.

Bruno Côté
BOCTE(--nospam--at)ibm.net