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Re: Structure Magazine Questions

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In a message dated 1/9/99 10:16:12 PM EST, rlewis(--nospam--at) writes:

<< seaint(--nospam--at),Internet writes:
 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

My question would be how many people in seismic zones 3 and 4 when designing
retaining walls even consider the seismic inertia force of the soil wedge
behind a retaining wall.  How many soils reports even include a seismic force
component to design for on a retaining wall.  I have in the pass used an
inertia force of 10-15 pcf for an inverted triangle acting behind the wall as
a rule of thumb, if it is even considered.

I would still require the the 1.5 factor of safety, but would use the 1.33
increase in the allowable soil bearing values.  

Mike Cochran