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Re: Structure Magazine Questions

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Lynn:

I was trying to make the point that the earthquake forces on retaining
walls from the soil are calculated differently than those for buildings. 
In the former case the forces are an approximation of the actual forces the
structure will see, while in the later case the calculated seismic forces,
as specified by the code, are often less than the forces the structure has
to resist. My point was also that in the first case a factor of safety less
that 1.5 for seismic loads may be appropriate while in the second case
designing the retaining wall for the code forces would overestimate the
factor of safety of the wall.

I was not suggesting that we design the shear wall or the retaining wall
for an Rw=1.0, but rather that we design the retaining wall for the forces
that can be transmitted to it from the rest of the structure.  

The use of an Rw>1.0 for the super structure is justified because of the
ductility that we have designed into specific elements.  Unless the
retaining wall is able to exhibit some ductility and still remain stable,
then it needs to be designed for the forces that can be transmitted to it. 
This statement is consistent with the rest of the code but because it is
not explicitly stated in the code, many people do not follow it, and
instead just design for the code forces.

We have to be careful with statements such as; "Retaining walls which are
properly design for static loading conditions do not fail in overturning
during earthquakes".  This sort of thinking has lead us to be complacent
about some aspects of seismic design only to be rudely reminded of the laws
of physics when an earthquake occurs.   Some of the problems with making
decisions solely based on previous earthquake experience arise from changes
in design and construction practices, and because the past earthquakes may
hot have been all that large.

The reason why most retaining walls do not fail in earthquakes was
addressed in the second part of my posting.  If the only earthquake forces
on the retaining wall are from the soil mass behind the wall, there is
rarely a failure.   The problem is that the original question did not make
it clear how the earthquake forces were computed.  

On the other hand, I have seen designs that rely on a cantilever retaining
wall to resist building seismic forces acting normal to the face of the
wall.  Admittedly, this is not a normal situation so it is not surprising
that few such failures have been reported, but that is not to say that this
failure mode cannot occur.


Mark Gilligan