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Retaining Wall Design Methodology

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Cantilevered retaining walls are probably one of the most easily understood
types of structures, and are routinely analyzed and designed by engineers
all over the world without any great difficulty.  Nevertheless, there is at
least one aspect of retaining wall design that is still open for debate.  My
colleagues and I have identified three different methodologies for the
design of the heal footing, and we are currently debating which alternative
to adopt as our future standard.  As described below, the issue is how to
treat (upward) bearing forces when designing the heal footing for shear and
moment.  For short walls, this does not make much difference.  However, for
tall walls, the difference can be quite significant.  For a modest 15 ft.
high wall, the resulting heal footing design can vary by as much as 40% in
concrete thickness, and 25% in reinforcement area.  If you are designing
hundreds of feet of retaining walls, that soon adds up to "real money"!

Here are the three alternative methods, all of which appear to be code
compliant, within the realm of good engineering practice, and in common use:

1)  For determining soil bearing pressure, the full width of the footing is
used and results in a trapezoidal (sometimes triangular) soil pressure
distribution.  In designing the footing, however, the soil pressure is
applied only under the toe.  No soil pressure is assumed acting under the
heal.  This leaves the heal being conservatively designed as a heavily
loaded cantilever.  This method is described in many popular texts,
including the 5th Edition (1992) of Reinforced Concrete Design by Wang &
Salmon.  It is the only method available in Daystar software, and is an
optional method in Enercalc (and RetainPro?) software.

2)  For determining soil bearing pressure AND for designing the heal and toe
footing, the full width of the footing is used and results in a trapezoidal
(sometimes triangular) soil pressure distribution.  Some believe that this
method is not always conservative.  This method is described in several
popular texts and manuals, including the 4th Edition (1988) of Foundation
Analysis and Design by Bowles, and the 1996 CRSI Design Handbook.  It is an
optional method in Enercalc (and RetainPro?) software.

3)  For determining soil bearing pressure AND for designing the heal and toe
footing, a uniform soil pressure distribution is applied over a portion of
the footing, excluding a width of "2e" at the end of the heal, where "e" is
the eccentricity between the center of the footing and the resultant
vertical force.  This method is the newest, and appears to be strongly
supported by the research of G. G. Meyerhof, and others.  This method is
described in the 5th Edition (1996) of Foundation Analysis and Design by
Bowles.  Although this method might well be the most logical of the three,
it does not appear that it is currently supported by any commercial
software.    
          
My question to all of you (and especially to the software authors) is very
simple:

What method do you use, and why do you use it?

I look forward to any and all responses.

Regards, 

Stan R. Caldwell, P.E.
Vice President
*************************************
HALFF ASSOCIATES, INC.
Engineers+Architects+Planners
8616 Northwest Plaza Drive
Dallas, Texas    75225
Phone:  (214)346-6280
Mobile:  (214)236-9696
Fax:      (214)739-0095
Email:   scaldwell(--nospam--at)halff.com 
Website:  http://www.halff.com
*************************************
 
 


     

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