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Retaining Wall Design Methodology

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Stan Caldwell wrote:

>>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"!<<

The question that I have with regard to cantilevered retaining walls (and 
footings subjected to overturning) is if, under service loads you have 
uplift, i.e., a triangular soil pressure, why does the uplift frequently 
disappear when you apply load factors in order to design the toe of the 
footing?  (The ACI footing chapter commentary alludes to this, but does not 
address it.)  How can a portion of a structure that is unloaded under service 
loads suddenly become loaded when load factors are applied?  By changing the 
loading from triangular to trapazoidal after applying the load factors, the 
moment arm to the center of pressure decreases.  Is the toe being designed 
with an appropriate factor of safety?

A second point:  A retaining wall will behave as a cantilever wall only 
if it is straight.  If it is curved, it will behave as a shell and will 
require a tension/compression ring at the top.  For a curved wall convex 
outward, closely spaced vertical cracks will develop if the wall is not 
designed as a shell with a tension ring at the top.  (After the cracks 
develop, the wall *will* become a cantilever and will be safe, albeit 
unsightly, *if* adequate reinforcing is in each cracked section.)

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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