the devil?s advocate, chapter 1806A.1 of the CBC states in the second
paragraph, ?Retaining walls higher than 12 feet, as measured from the top of
the foundation, shall be designed to resist the additional earth pressure
caused by seismic ground shaking?. Since 1806A was adopted by DSA and
OSHPD, which are generally more restrictive, would it be reasonable to argue
that in general walls less than 12 feet really do not need to be designed for
personally feel that this provision makes good sense, design for a seismic
surcharge only if the wall is higher than 12 feet. Of course the project
geotechnical engineer would have to be convinced.
do you think?
Cruz, CA 95060-3836
831.426.3186 x 118
consider the environment before printing this e-mail
Sent: Monday, November 10, 2008 2:18
To: dfisher(--nospam--at)fpse.com; seaint(--nospam--at)seaint.org
Soldier Beam Retaining Wall Design Practice
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:
typically use method #1.
David L. Fisher SE PE
Phil Doody [mailto:phil(--nospam--at)m-me.com]
Sent: Monday, November 10, 2008
Subject: Soldier Beam
Retaining Wall Design Practice
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
Basically, there appears to be at least three
ways of designing wood lagging that I have encountered:
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
#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
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
Thanks for your input, Phil Doody
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