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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: Mon, 23 Aug 2004 19:17:02 +0000
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 -0400I am struggling with understanding the intent of references and codes forseismic analysis of retaining walls. Several references suggest the Seed andWhitman 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 walland P = additional lateral soil force due to earthquake applied at 0.6Habove 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? Andis the lateral inertia due to the weight of soil above the heel and toe alsoadded? The US Army Corps of Engineers manual EM 1110-2-2502 recommends the Mononobe-Okabe method to determine the lateral force due to earthquake andalso states to add "the inertia force of the wall, including that portion ofthe 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 isnot a rigid mass. Including these soil masses can significantly increase thelateral 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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