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Re: Seismic loads on retaining walls

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My point was that structural and geotechnical engineers operate in their
own world and do not appreciate the assumptions made by the other
dicipline.  It is not that they do not understand their own dicipline but
rather that they do not understand enough of the others to effectively
comunicate.

A related point is the 33% increase in allowable soil pressure.   The
allowable soil pressures for dead and live loads are often based on the
need to limit long term settlements.  Earthquake loading is a short term
load and thus the allowable bearing values should typically be based on the
ultimate bearing capacity of the soil.  Thus we would expect that, in many
instances, allowable bearing stresses under earthquake loading would be
significantly greater than the 33% increasess yet this almost never
happens.  Is this because the 33% increase is blindly used?  

Part of the problem may be due to the geotechnical engineers desire to
limit deflection under seismic loading.  Under earthquake loading greater
foundation deflections are often acceptable.  If we are expecting
considerable inelastic distortion in the superstructure why should we not
allow more damage in the foundation?  Is this a decision for the
geotechnical engineer to make on his own without consultation?  Here again
I do not believe that there is good communication between the geotechical
egineer and the structural engineer.

The fact that Caltrans always considers seismic active pressure does not
mean that it is always the right solution.  It is  very possible that
Caltrans has adopted this approach without considering all aspects of the
problem.  At present I am not convinced that these pressures should always
be explicitly considered.  I am very clear in my mind that when you start
talking about the interaction of seismic loads from the building with the
ground that there are a lot of inconsistencies and a lot of fuzy thinking. 


The statement by Mr. Kiss that "as competent geotechnical firms should
always do" is a subjective opinion.  In my experience the more established
and sophisticated geotechnical firms do not always provide active seismic
pressures.  It is my opinion that similar broad statements result in
unnecessary work.  We need to think what is appropriate for the project and
not get boxed in by generalities.

Mark Gilligan


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