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RE: Seismic loads on Retaining walls

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Thanks Harold, that is a good reference. Unfortunately, it does not
specifically address the factor of safety to be applied for the given
seismic loads. But it does address one question: it recommends using Kh =
Sds/2.5, which is the design ground acceleration per NEHRP. However, this
raises the same question as is being discussed related to R-values and
overturning of buildings - should an R-value also be applied? 

I discovered in the USACE document TI 809-04, Seismic Design for Buildings,
a recommendation to use Kh = 0.5*Sds/2.5 (page 9-16). Thus, they appear to
use an equivalent R-value of 2. Since retaining walls typically slide before
they overturn and if a small amount of lateral movement can be tolerated
during an earthquake, it seems that the seismic force might be permitted to
be reduced below ground acceleration. 

William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com
 

> -----Original Message-----
> From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com] 
> Sent: Monday, August 23, 2004 1:17 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Seismic loads on Retaining walls
> 
> Bill,
> 
> Take a look at the NEHRP Commentary 7.5.1 "Earth Retaining 
> Structures" 
> http://www.bssconline.org/NEHRP2003/comments/.
> 
> The commentary was authored by Chet Soydemer of Haley and 
> Aldrich and Maurey Power of Geomatrix.  It is the best 
> description of seismic effect on retaining walls to date.  It 
> originally appeared in teh 2000 NEHRP Commentary and was not 
> altered too much in the 2003 NEHRP.  It has great references 
> to both Monobe-Okabe and Seed and Whitman.  And discusses 
> yielding and non-yielding walls.
> 
> Keep in mind that seismic engineering assumes a nonlinear 
> response to an unfactored seismic induced load.  Another 
> reference you may want to consider is NCEL Technical Report 
> R-939, by Robert Ebeling and Ernest Morrison.  It is from 
> NAVFAC in Port Hueneme, CA.
> 
> Regards,
> Harold Sprague
> 
> >From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
> >Reply-To: <seaint(--nospam--at)seaint.org>
> >To: "SeaInt Listserver (seaint(--nospam--at)seaint.org)" <seaint(--nospam--at)seaint.org>
> >Subject: Seismic loads on Retaining walls Date: Sun, 22 Aug 2004 
> >13:54:39 -0400
> >
> >I am struggling with understanding the intent of references 
> and codes for seismic analysis of retaining walls. Several references 
> suggest the Seed and Whitman approach to determine additional lateral soil
force 
> >due to earthquake:
> >
> >P = (3/4*Kh)*1/2*Ws*H^2 = 3/8*Kh*Ws*H^2
> >
> >where: Kh = peak ground acceleration
> >	 Ws = soil unit weight
> >	 H  = soil height behind wall
> >    and P  = additional lateral soil force due to earthquake 
> 	applied at 0.6H above the base
> >
> >Using IBC, the peak ground acceleration would be 0.40*Sds.
> >
> >But the references I have do not show examples of the application of 
> >this lateral force in combination with other forces for evaluating 
> >sliding stability or wall strength. For example, is the 
> lateral inertia of the wall mass calculated separately and included in the
lateral 
> >force analysis? And is the lateral inertia due to the weight of soil 
> >above the heel and toe also added?
> >
> >The US Army Corps of Engineers manual EM 1110-2-2502 recommends the 
> >Mononobe-Okabe method to determine the lateral force due to 
> earthquake and also states to add "the inertia force of the wall, 
> including that portion of the backfill above the heel or toe of the wall".

> The inertia is determined by multiplying the mass directly by the peak
ground 
> >acceleration.
> >
> >It makes sense to me to add the inertia due to the wall 
> mass, but also adding the inertia due to the soil weight above the heel
and toe can 
> >add considerable lateral force and seems overly 
> conservative, since the soil is not a rigid mass. Including these soil
masses can 
> significantly increase the lateral seismic forces for design. So how do 
> others handle the combination of lateral forces?
> >
> >The code is also unclear about the application of the R-value for 
> >retaining wall design. Table 1622.2.5(1) in the 2000 IBC does not 
> >explicitly refer to retaining walls, although item 14 for "other 
> >distributed mass cantilever structures" would appear to 
> apply. But is the R-value applied for sliding and overturning analyses as 
> well as for concrete design? It makes sense to me to apply R for 
> concrete design to account for the ductility of the material. But R should
not 
> affect the sliding and overturning, since stability is directly impacted
by the 
> >ground acceleration and since inclusion of R would reduce the effective
factor of safety.
> >
> >Furthermore, Section 1806.1 of the 2003 IBC states that "Retaining 
> >walls shall be designed for a safety factor of 1.5 against lateral 
> >sliding and overturning." It does not state whether this 
> safety factor applies to seismic loading conditions nor whether this
applies using 
> >service loads or factored loads. What is the intent of the code? I 
> >usually reduce the required factor of safety to 1.1 for seismic 
> >conditions vs 1.5 for normal loading conditions, based on EM 
> 1110-2-2502, Table 4-1.
> >
> >In general, it is rather unclear as to what the intent is of the 
> >building codes for such designs. I am interested in how 
> other engineers have interpreted the seismic provisions for retaining
walls.
> >
> >William C. Sherman, PE
> >(Bill Sherman)
> >CDM, Denver, CO
> >Phone: 303-298-1311
> >Fax: 303-293-8236
> >email: shermanwc(--nospam--at)cdm.com

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