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RE: Use of Side Friction to Resist Overturning - Spread Footing

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I've also seen this before, and the checking engineer who involved me in the
discussion rejected it.  In this case the soil was cohesive and the argument
was that cohesion was available on the vertical wall.  We consulted with a
geotech who pointed out that the shrinkage associated with cohesive soils
may actually mean the material isn't wholly in contact with the footing.

However, a shear failure of the overburden soil must occur to mobilise the
restoring mass.  I think it is fair to use an inclined angle to define the
soil mass on the tension side equivalent to the angle of internal friction,
on the basis that during uplift the material on the footing does provide
shear resistance with the adjacent soil and failure would probably develop
along this angular line rather than vertically.

I don't see much engineering justification for a blanket 300 psf applied to
the base wall.  For the dimensions given I wouldn't have thought the footing
design would be too much larger if this supposed friction was not considered
(the normal design approach).

Dave Meney
Structural Engineer

Yenem Engineering Services
WESTERN AUSTRALIA 6076

>-----Original Message-----
>From: Robert Rogers [mailto:robert.rogers(--nospam--at)woolpert.com]
>Sent: Tuesday, 15 December 1998 9:58 pm
>To: SEAINT(--nospam--at)seaint.org
>Subject: Use of Side Friction to Resist Overturning - Spread Footing
>
>
> I have recently seen a set of calculations for the design of reinforced
> concrete spread footings (for a new building) which support building
> columns which are subjected to large lateral loading.  The spread
>footings
> are approximately 5 ft. deep (to bottom of footing), 2 ft. thick,
>and have
> a 3' tall square pier which receives the square column.  The calculations
> assume a value of 500 psf for side friction on the 2' thick footing.  The
> forces generated by this side friction are supposed to help resist the
> overturning moment imposed to the footing.  I have never seen
>this type of
> analysis used for spread footing design.  I think the methodology is way
> out in left field !  Any other opinions ?
>
>
>
>