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Re: Structure Magazine Questions - Retaining walls

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> Q:    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)? 
 
Based on a literal reading of the wording in the 1997 UBC, I would say "no".
 
Section 1611.6 requires a safety factor of at least 1.5 and does not exempt 
seismic load cases.  Allowable stresses should then be based on load 
combinations in Section 1612.3.1.  When permitted by this section, the 
one-third increase in allowable stresses for materials should be allowed but 
there is no provision to reduce the overturning and sliding factors of
safety. 
  
 
As some others have pointed out, code prescribed seismic forces are
generally 
less than the actual expected forces and rely somewhat on ductility of the 
structure.  However, I also think that it is true that overturning of 
structures is not a common failure mode due to seismic forces.  This is
likely 
due to the fact that seismic motions reverse before a structure reaches full 
overturning in one direction.  It is interesting to note that UBC Section 
1621.1 requires that overturning due to wind not exceed 2/3 of the dead-load 
resisting moment, but I don't believe that there is a similar requirement
for 
lateral seismic forces.  Personally, I've always felt that a lower factor of 
safety against overturning should be allowed for retaining walls for seismic 
load cases, and an argument could be made that the codes' "intent" is to
allow 
a general reduction in safety factors, such as from 1.50 to 1.50/1.33, 
although it is not explicitly stated.