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Re: Structure Magazine Questions

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Mark:
If you were consistent in your approach, wouldn't you do the same thing
for shear walls?
I mean how many of us design shear wall overturning forces for an Rw of
1 and then apply a 1.5 safety factor to it?  The principal is the same
is it not?

My "thoughts" are: 
First:
Retaining walls which are properly design for static loading conditions
do not fail in overturning during earthquakes.  Even most retaining
walls that are not designed properly for static conditions do not
overturn in earthquakes (there are some exceptions to this I have seen).
Second:
Shear walls do not fail in overturning during earthquakes.  Unless
liquefaction is an issue, global stability of a wall has never really
been a problem.  I have never seen or read about overturning failures of
shear walls.  There may be some I don't know about, but I do study
actual reports of earthquake failures quite a bit, and I have never seen
this issue mentioned.  For those of you who are getting all excited
about this statement, I am NOT talking about Geotechnical failures that
could lead to overturning issues.

I believe we do not really understand what seismic overturning forces
and resisting forces are present on a wall.  Nor do I believe we really
understand how these forces effect global stability of walls (retaining
or shear) under seismic loading.

I am not suggesting that anyone back off of the 1.5 factor for
overturning, but I would not get too excited if I were evaluating the
performance of an existing walls if the overturning safety factor turned
out to be somewhere between 1.0 and 1.5.  

Do not make the mistake in assuming that the seismic forces we design to
in the Code are anywhere near what the structure will actually see.  Try
to understand where the Code falls short by examining the actual
failures that occur in earthquakes.  And on the other hand, if you are
in the practice of evaluating existing structures for seismic loads, do
not get too excited about certain elements being "over stressed", if
these under designed elements have performed adequately in past
earthquakes.


Lynn 
  



Mark K Gilligan wrote:
> 
> When talking about seismic forces on retaining walls we need to clearly
> understand how these forces were calculated.
> 
>  The forces specified in the building code are less than can be expected.
> The use of the Rw factor is based on the asumption that if you design the
> super structure for the reduced forces there will be some inelastic action
> in the structure but the structure will not collapse.  In reality the
> actual forces in the structure may be considerably higher.  As a result the
> forces that the foundation sees will also be larger than the code forces
> and a factor of safety of 1.5 will often not be adequate.  This is a
> deficiency in the building codes.
> 
> If the earthquake forces on the retaining wall are from the soil behind the
> wall it is my understanding that the procedures typically used produce
> forces that are much closer to the actuall expected loads.  In this
> situation the use of the 1.33 increase in capacity may be reasonable, based
> on the reduced likelyhood that the maximum forces will occur.  A number of
> Geotechnical Egineers typically do not specify lateral forces on retaining
> walls because these forces plus the regular active pressures on the wall
> generally can be resisted by a wall designed for the specified active
> forces.  This can be justified based on the comman practice of using a
> reduced factor of safety when the likelyhood of the loads is rare.  A
> geotechnical engineer should be consulted to verify what I have said in
> this paragraph is totally correct.
> .
> 
> Mark gilligan
> 
> *******************************************
> 
> Can the safety factor for overturning of a retaining wall be reduced below
> 1.5 when
> analyzed subjected to seismic forces (i.e. 1.5/1.33)?
> 
> I would say 'NO', it can not.  The loads from seismic forces are the
> maximum
> probable load for a given return period.  They are not precise, and a
> factor
> of safety is still required.  We design a structure to survive a seismic
> event, for life safety reasons primarily.  Why would we want to reduce the
> life-safety even further by reducing the factor of safety.  It would seem
> to
> me to be a foolish move.   Allowable stresses are increased because for
> short
> term duration a material can be taken to a higher stress before failure.
> But
> the laws of physics for overturning do not change for a short-term loading.
> 
> __________________________________________________
> 
> Richard Lewis, P.E.
> Missionary TECH Team
> rlewis(--nospam--at)techteam.org