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Re: Seismic on restrained retaining walls

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I have used this wording in plan check responses under the old code when this came up and it has been found acceptable/reasonable by the plan checkers.


1.      Seismic design for residential retaining walls is normally not performed. No seismic design criteria for the walls were presented in the soils report. The 2001 CBC code section 1611A.6 states the following:


"retaining walls higher than 12 feet, as measured from the top of the foundation shall be designed to resist the additional earth pressure caused by seismic ground shaking".


This height threshold is not defined in the regular building code as this applies to school and hospital projects. Since the school and hospital (DSA & OSHPD) code provisions, by law, cannot have less stringent criteria than the regular building code, it is logical to state that seismic design need not be considered for these site walls, which do not exceed 6 feet in height. If this were a school or hospital project, the walls would still be exempt from seismic design because they are not taller than 12 feet


On Fri, Apr 25, 2008 at 1:11 PM, Tarek Mokhtar <Tarekmokhtar(--nospam--at)> wrote:

The 12' retaining wall exception appears in chapter 18A, DSA requirements, but disappears
in chapter 18. Further more section 15.6.1 appears to imply that all retaining walls shall
be designed with additional seismic loads. Does that mean that six or eight foot
retaining walls in backyards get additional seismic loads?

Tarek Mokhtar, SE
Laguna Beach, Ca

And the rule of thumb shall be to keep all of our retaining walls 11'-11" or less in height.

On Thu, Mar 20, 2008 at 9:12 AM, Hugh Brooks <hbap(--nospam--at)> wrote:
The Mononobe-Okabe equations do not apply to restrained walls where activation movement is restricted.
For seismic lateral on restrained walls Retain Pro uses  P = (soil density) x (k<sub h>) x (retained height)^2.  Application is approx. 0.60 x (retained height).  This is a slightly trapezoidal loading but with only slight error a uniform loading could be assumed.   K<sub h> is the acceleration factor appropriate to the site.
See Kramer, Geotechnical Earthquake Engineering, Section 11.6.2.
Hugh Brooks, SE
Retain Pro Software



Tarek Mokhtar, SE
TMM Structural Engineers, Inc
31645 S. Coast Hwy
Laguna Beach, CA., 92651
949-499-2777 Fax