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Re: Structure Magazine Questions

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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
probable load for a given return period.  They are not precise, and a
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
me to be a foolish move.   Allowable stresses are increased because for
term duration a material can be taken to a higher stress before failure. 
the laws of physics for overturning do not change for a short-term loading.


Richard Lewis, P.E.
Missionary TECH Team