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Part II: Use of Side Friction to Resist Overturning Moment

• To: SEAINT(--nospam--at)seaint.org
• Subject: Part II: Use of Side Friction to Resist Overturning Moment
• From: "Robert Rogers" <robert.rogers(--nospam--at)woolpert.com>
• Date: Wed, 16 Dec 98 07:11:34 EST

``` I appreciate the opinions with regard to this subject matter.  I apologize
for the length of this follow-up, but perhaps I may be enlightened by a
helpful respondent.  Some things which make me suspicious of these
calculations are:

calculation assumes a side friction value of 0.50 ksf.  The report only
states to design the spread footings for an allowable soil bearing
pressure of 2,500 psf (typical for the medium to stiff clay soil found at
the site in question) and that it may be cast against undisturbed earth
or engineered fill.

(2) The overturning resistance by friction was calculated as follows:

+++++  <----- 60 Kips
Footing is              +   +
7.5 ft. wide         3' +   +
+   +          F
/|\ ++++++   +++++++    |
|  + 2'           +    |
|  ++++++++++++++++   \|/
<---- 7.5'----->
F

Friction Resisting Force = F = (2 ft.)*(7.5 ft.)*(0.5 KSF)= 7.5 Kips
Resisting Moment Due to Friction = (7.5 Kips)*(3.75 ft.)*2 = 56.25'K

And then for the sides of the footings a methodology similar to an
eccentric load on a weld pattern is utilized:

friction = (Resisting Moment Due to Side Friction)*(c)
------------------------------------
(Ip)

where (Ip) is the polar moment of inertia and (c) is the distance to
the extreme fiber:

0.500 ksf =            (M) * (7.5' / 2)
---------------------------------------
{(1/12)* 7.5' * 2' (7.5^2 + 2^2)}

Thus the Resisting Moment to friction is: 10 'K

Thus the total resisting moment due to friction is:

(56.25'K)+(2)(10'K)  = 76.2'K

The traditional treatment of friction is a normal force multiplied by a
friction coefficient to calculate a force resisting movement.  If I
understand this correctly (which I may not), to achieve a 7.5 Kips
resisting force on the side of the footing the normal force which must be
applied would be:

7.5 Kips
--------------------------   =  Normal force
u = Coefficient of Friction

Assuming u=0.5 the normal force would have to be 15.0 Kips (applied
uniformly to the side of the footing to ensure 0.5 ksf).  Now once the
footing rotates, allowable soil pressure is exceeded, and the footing
starts to pull up on one side and push down on the other, then we get into
the discussion about the shear strength of the soil, cohesion, angle of
internal friction, failure planes, etc. .

Frankly I don't understand how you obtain this much lateral force on the
sides of the footing (which is only 5 ft. deep to the bottom of the
footing) to generate this type of resisting moment.  Am I on line or can
someone show me the light ?

```