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RE: Basement Walls

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Scott,

You have raised some interesting issues.

Be careful about designing a basement wall as a retaining wall.  As a
retaining wall the wall must be free to translate at the top.  Tension
will occur on the soil side.  As a simple span wall tension occurs on
the inside of the basement wall.  When the wall has 2 layers of rebar
the design is easily accommodated.  With 1 layer (most common for
residential) you have to decide where to place the rebar to be most
effective.

I have designed them as both a retaining wall and a simple span for
commercial buildings where the contractor wanted to back fill prior to
the floor being placed.  In residential structures it is most common to
design them as a simple span wall with a "generous" continuous spread
footing to accommodate some degree of back fill.  Residential back fill
is generally not compacted so that the applied back fill loads are not
as great as commercial projects.  It is best to add a note to preclude
back filling until the floor is in place.  Also be mindful of the
details to transfer the forces into the diaphragm.

Regarding seismic soil loads on basement walls is a hole other topic.
In the BSSC, I tried my best to see if we could get some consistency in
the geotechnical community on how to even calculate seismic induced soil
loads and was met at the gate by an angry mob of geotechnical engineers
carrying torches.  Some in the field say there is no seismic induced
lateral soil load.  Others say that there is, and they give values.  As
a structural engineer I would love to have a consensus among the
geotechnical engineers on methodology, but if my experience in the BSSC
is any indication that time is not soon.

Good luck,
Harold Sprague
Black & Veatch
BSSC TS 13 Chairman
 ----------
From: Horn
To: seaoc(--nospam--at)seaoc.org
Subject: Basement Walls
Date: Thursday, August 07, 1997 12:36AM

My brother is an Architect and we have been involved in the design of
several custom homes.  We have an on-going argument about the design of
basement walls.  He insists that homes the Salt Lake City area

with basements require only 24" wide footings for basements walls.
Basically, the walls is pinned-pinned!  I've been able to be involved in
homes with owners with enough money to not complain too much about the
cost, and an Architect who responds well to some coaching.



I live in Illinois now and I've been looking at homes around here
(most have basements) and they do the same thing.  There's no way these
walls are evaluated as retaining walls.  I've checked the loads on this
type
of configuration and the shear that has to be dumped into the
diaphragms is considerable.  The idea of preloading a wood diaphragm
with large
amounts of deadload and then needing additional reserve capacity for
seismic loads of the structure and additional soil lateral loads is not
very
appealing to me.  I have submitted to considering the retaining wall as
fixed at the base and pinned at the top in some instances.



Has anyone else been pressured to not design a basement wall as a
retaining wall?  If there is anyone who has / does do this, please let
me
know what the justification is.



Scott Horn