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RE: Sliding

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Scott Haan Noted:

>As a plan reviewer I saw  alot of people say they were using the slab on
grade to transmit lateral force into
> the soil or to other gravity footings, but they forgot to calc it out and
detail it.  There is more to the design 
>than just checking concrete bearing at the end of a wall or pedestal which
is a dubious mechanism at best.  
 
>Most S.O.G. have isolation joints at structural elements and  get cut up
for plumbing repairs etc....  Also what 
>kind of a friction coefficient is there between concrete-visquene-soil or
a sand bed, visquene and soil.  Using 
>a S.O.G. is not a magic cure to take out shear wall and braced frame
footing reactions and requires some design.

In high seismic regions especially near a fault, frictional resistance
alone often cannot prevent a building from sliding.

First consider that most geotechnical reports give friction coefficients of
0.4 while the actual ground accelerations are often in excess of this.  The
code base shear coefficient may be less but when you consider the
overstrength in the system you will find that the effective R factor is
less than assumed by the code and as a result the real base shears are
more.

No body includes the mass of the slab on grade when calculating the
inertial forces that must be transmitted to the soil yet this is not
supported by logic.  First the slab on grade is constrained to move with
the columns and shear walls and while it will not see the same
accelerations as the super structure it will be subject to an acceleration
of Ca.  Ca is the expected acceleration of the ground and can be greater
than the coefficient of friction provided by the geotechnical engineer. 
Secondly the slab on grade has a membrane under it so the  coefficient of
friction is even lower than for the rest of the building.  As a result the
slab on grade contributes additional inertial forces that must be resisted
by the friction at the footings.

Even when we ignore the mass of the slab on grade and only use the code
calculated base shear it is often necessary to engage the frictional
resistance of all of the footings.  This often requires that the slab on
grade be designed as a diaphragm to tie the shear walls and footings
together.

Since we do not see a lot of sliding failures there must be other
mechanisms mitigating the problem.  Some of the likely reasons include.

    - The coefficients of friction are actually higher than assumed.
    - Passive resistance and cohesion play a bigger role than assumed.
    - Even in major earthquakes there is a large variation in ground
accelerations and many sites see significantly 
       less than the maximum accelerations.
     -Buildings may slide more than assumed.

While I do not claim to have all of the answers I would like to suggest 1)
that engineers often ignor the transfer of the seismic loads between the
ground and the building and 2) that we lack a good understanding of how
lateral forces are transfered between the ground and the building.

Options include:

  -Ignore the problem.  This I find troubling.
  -Do little and accept the damage.  This assumes you can quantify the
likely damage and that the client will 
     accept the damage.
  -Formally design for the "real"  forces with keyways etc.  This is
probably excessive.
  -Tie the foundation together and do not worry about sliding.  This is my
preference but we need better 
    methodologies to determine the magnitude of the forces to be designed
for.


Mark Gilligan

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