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RE: Retaining Wall Design Practice (Seismic)

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As mentioned it is section 1806A page 165. While one would expect the DSA OSHPD provisions to be more restrictive, that language is not in the primary section of the code. I have been told that some California building departments have administrative bulletins that exclude the seismic requirement under certain conditions. San Francisco has an admin bulletin that provides exceptions to when a soils report is required, but nothing official for the seismic requirements that I am aware of. I have also been told that there is a california committee that is working on an addendum to the seismic requirements for retaining walls, but this is just hearsay.
 
For conventional retaining walls it is very difficult to resist the sliding forces that include the seismic increment, the footing keys become so deep that piers may be a better solution. This seems to be a bigger deal than the requirements for the overturning and wall strength. I just passed on a residential project that had 10' landscape retaining walls. The building department did not have an official policy and I know my design would not be well recieved.
 

From: Larry Hauer [mailto:lhauer(--nospam--at)live.com]
Sent: Tuesday, November 11, 2008 6:24 PM
To: Struct. Eng. Assoc.
Subject: RE: Retaining Wall Design Practice (Seismic)

Paul,
 
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)se-solutions.net
To: seaint(--nospam--at)seaint.org


Ralph,
 
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)aol.com [mailto:Rhkratzse(--nospam--at)aol.com]
Sent: Monday, November 10, 2008 2:18 PM
To: dfisher(--nospam--at)fpse.com; seaint(--nospam--at)seaint.org
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)fpse.com writes:
Phil:
 
I typically use method #1.
 
 
David L. Fisher SE PE
 
 

 



From: Phil Doody [mailto:phil(--nospam--at)m-me.com]
Sent: Monday, November 10, 2008 4:01 PM
To: seaint(--nospam--at)seaint.org
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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