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RE: Structure Magazine Questions - Retaining walls

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The whole issue of retaining wall design is currently under debate.  The
geotechnical engineers are not in agreement as to the applied seismic load
or even if a seismic load exists.  

That issue is being debated in the Building Seismic Safety Council (BSSC)
Geotech subcommittee.  Once they decide on a load, the BSSC Nonbuilding
structures subcommittee will try to develop the rules on safety factors,
overturning, etc.

To keep it simple, I am inclined to retain the 1.5 factor, even though it
might only be realistically applicable to long period events.

Stay tuned.

Harold Sprague
The Neenan Company

-----Original Message-----
From: Bill Sherman [mailto:SHERMANWC(--nospam--at)]
Sent: Monday, January 11, 1999 8:09 AM
To: seaint(--nospam--at)
Subject: Re: Structure Magazine Questions - Retaining walls

> 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
As some others have pointed out, code prescribed seismic forces are
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
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
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
a general reduction in safety factors, such as from 1.50 to 1.50/1.33, 
although it is not explicitly stated.