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RE: Retaining Wall Design Practice (Seismic)

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Somewhere in the body of the '07CBC Title 24 portiion, (DSA and OSHPD requirements), is the exclusion of seismic design for retaining walls less than 12' in height, (left over from the '98 CBC). I'm not at my office now so I can't site the section- but it is there. I have found that jurisdictions in Ventura County, (of South. Calif.), accept this as well as the local soils engineers. Consequently, I have yet to design retaining walls for seimic forces if less than 12' in retained earth height.
Larry Hauer S.E.

Subject: Retaining Wall Design Practice (Seismic)
Date: Mon, 10 Nov 2008 14:31:05 -0800
From: PFeather(--nospam--at)
To: seaint(--nospam--at)

The inclusion of seismic is actually driving the design on our subterranean projects now.  My question, "where is the justification for this"; or as Frank Lew used to say, where are the bodies?
Is anyone aware of a bunch of basement wall failures or retaining wall failures due to seismic?  How about any failures?  I would really like to be able to point to something to explain the added cost to the client besides, "it's the code now".
Paul Feather PE, SE

From: Rhkratzse(--nospam--at) [mailto:Rhkratzse(--nospam--at)]
Sent: Monday, November 10, 2008 2:18 PM
To: dfisher(--nospam--at); seaint(--nospam--at)
Subject: Re: Soldier Beam Retaining Wall Design Practice

Well, I'm the last person to be considered an authority on anything related to soil design values, but I would like to point out that now that we're required to consider earthquake forces in our design of retaining walls (at least in my area of California), I think in terms of 1/3 increase in earth pressure is more or less offset by 1/3 increase in allowable timber stresses.  Where does that leave us if we've already taken a 40% reduction in loads??

Ralph Hueston Kratz, S.E.
Richmond CA USA

In a message dated 11/10/08 2:12:01 PM, dfisher(--nospam--at) writes:
I typically use method #1.
David L. Fisher SE PE


From: Phil Doody [mailto:phil(--nospam--at)]
Sent: Monday, November 10, 2008 4:01 PM
To: seaint(--nospam--at)
Subject: Soldier Beam Retaining Wall Design Practice

Is there a consensus on the design of wood lagging for soldier beam retaining walls? Several design methods appear to be in use and each gives very different answers.
Basically, there appears to be at least three ways of designing wood lagging that I have encountered:
#1 Determine the uniformly applied load  on the lagging due to the soil pressure at a given depth. Select the lagging based on the required section modulus, S = (Moment)/(Allowable bending stress).
#2 Same process as above but instead apply only 60% of the theoretical uniform load.  The justification for the 0 .6 reduction factor is that soil movement causes the lagging to flex outward, and induces a redistribution of soil pressure away from the center of the lagging thus reducing the bending moment. This method is described in the Caltrans Trenching and Shoring Manual. 
#3 The third method of sizing lagging is based on FHWA recommendations contained in Federal highway Administration Report Number FHWA-RD-75-130.  A table in this report provides the minimum thickness lagging for various soil conditions, soldier pile spacings and excavation depths.  This method cannot be used if there are surcharges behind the wall.
The most conservative method is the first method and the one which I am most accustomed.  However, I would like to know if others use methods #1 and #2 and if anyone has observed lagging failures using these methods?
Thanks for your input, Phil Doody

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