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RE: Seismic loads on Retaining walls[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: RE: Seismic loads on Retaining walls
- From: "Harold Sprague" <spraguehope(--nospam--at)hotmail.com>
- Date: Thu, 26 Aug 2004 19:48:55 +0000
The geotechs do things a bit differently. They will take a dynamic load and convert it to an equivilent static load. No R required. The document that will clarify the best recommendations is the NCEL Technical Report R-939, by Robert Ebeling and Ernest Morrison, NAVFAC in Port Hueneme, CA.
The resistance and strength are dependent on whether or not the soils are yielding or non yielding and whether the wall is a yielding wall or an anchored wall. The best thing about the R-939 is that it contains worked examples on regular retaining walls, sheet piles, tied walls in all types of soils.
Regards, Harold Sprague
From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com> Reply-To: <seaint(--nospam--at)seaint.org> To: seaint(--nospam--at)seaint.org Subject: RE: Seismic loads on Retaining walls Date: Thu, 26 Aug 2004 15:05:42 -0400 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 touse an equivalent R-value of 2. Since retaining walls typically slide beforethey 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 soilforce > >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 thelateral > >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 shouldnot > 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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