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RE: Footing sliding resistance - Friction

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Chris,

Look at 97 UBC Table 18-I-A. In the table , the last two columns are
"Lateral Sliding" with footnote #4 and #6.

Footnote 4 says lateral resistance and bearing can be combined.... no
big deal

Footnote 6 says "Lateral sliding resistance value to be multiplied by
the contact area. In no case shall the lateral sliding resistance exceed
one half the dead load".

Only type 5 "clays" have a value in the last column of 130 psf. The
other soils do not list a value. Type 5 soil does not have a sliding
coefficient given. So it appears to me that if you have type 5 soil, you
calculate the sliding resistance by multiplying your contact area by 130
rather than using a friction coefficient. You can increase the footing
size to achieve an equivalent coefficient of friction of 0.50. It seems
that it is advisable to have footings on clay resist sliding by passive
pressure.

SO if you do have type 5 soil, then the footing size matters to resist
sliding. If you don't or have a soils report with these parameters
defined, the contact area doesn't matter except to increase the dead
weight and passive pressure resistance.

Just for information purposes really. I doubt you have this soil
classification on a hillside.

Hth,
-gm

-----Original Message-----
From: Christopher Haffner [mailto:chaffner(--nospam--at)gorge.net]
Sent: Monday, June 20, 2005 1:17 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Footing sliding resistance - Friction

Thanks all.  You have confirmed my thinking on the footing/soil
interface
values.

Does anyone count on the fill (granular or compacted) below a slab (a 1'
to
6' wedge in this case) as adding any sliding resistance if the original
grade is benched?  There has to be some added benefit.  I don't think I
need
it in this case but it would be good to know for future reference.

Chris
 
Haffner Consulting Engineering
www.haffnerconsulting.com
Office & Fax:  541-478-3052
 

-----Original Message-----
From: Daryl Richardson [mailto:h.d.richardson(--nospam--at)shaw.ca]
Sent: Monday, June 20, 2005 12:24 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Footing sliding resistance - Friction

Christopher,

        The coefficient of friction of 0.45 simply means that 45% of the

vertical load can be relied upon to resist lateral loading.  This is not

related to the width of footing, HOWEVER, the weight of footing can also
be
included in the vertical load as can the weight of backfill on top of
the
footing.  As a result, increasing the footing width will also increase
the
weight resisting the loading.

Regards,

H. Daryl Richardson

----- Original Message -----
From: "Christopher Haffner" <chaffner(--nospam--at)gorge.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, June 20, 2005 12:50 PM
Subject: Footing sliding resistance - Friction


I'm having a Monday morning brain freeze and thought this was a
no-brainer
but can't find a definitive answer.

I am being told by the Geologist on a sloping (2H:1V) residential lot
that
the coefficient of friction is .45 (no units).  He is also indicating
that a
wider footing will add some needed resistance to sliding.

I don't think this is correct in that the coefficient should only depend
on
the amount of gravity (vertical) load and not on the area of the bearing
area.  Therefore, making my footings wider won't really help my sliding
problem other than to add a little more weight.  Am I thinking clearly
here?


Another question.......I will have some granular fill (varies from 1' to
6'
thick) below a basement floor slab.  I think this helps my sliding
resistance by adding weight to the system as it interfaces with the soil
below.  Since I am having the slope benched I would like to use this
resistance (weight x coef of frict).  Opinions????????

Thanks for your input!!!!!

Chris

Haffner Consulting Engineering
www.haffnerconsulting.com
Office & Fax:  541-478-3052



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