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RE: Basement Walls

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-----Original Message-----
From:	Horn [SMTP:scott.horn(--nospam--at)usa.net]
Sent:	Wednesday, August 06, 1997 10:37 PM
To:	seaoc(--nospam--at)seaoc.org
Subject:	Basement Walls

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
[Dennis S. Wish PE]  Although I have not design many basement 
walls in my area of the country, I did grow up in the suburbs of 
Chicago (Skokie) and our home like the majority, had a basement.
A basement wall is, as you indicated, pinned at the top by 
virtue of the floor framing. As such, the moment in the stem 
will occur at the center of the stem. Tension will be on the 
outside (opposite soil) face of the wall.
Until the diaphragm is in place, the wall will act like a 
typical retaining wall with the maximum moment at the base of 
the stem. Footing width would not be critical for a basement 
wall since the diaphragm prevents most of the rotation of the 
footing and the slab at the bottom prevents sliding.
However, what will prevent rotation in the stem after the wall 
is loaded and before the diaphragm is in place. If you can 
prevent the wall from being loaded (backfilled) before 
completing the diaphragm, I would guess you could design it as a 
basement wall only. However, if you live in an area with heavy 
rains you might experience loading of the wall by accumulation 
of water, or by collapse of the backfill.
In either case, I have designed conservatively;
1. Model the wall and design stem steel and footing width for a 
retaining wall,
2. Model and provide stem steel for tension in the face of the 
wall as required for a pinned connection.

Finally, I'm not sure you need to consider lateral in your area 
of the country, but I would consider (if you are joining a slab 
to the top of wall) the friction of the slab against soil to 
help prevent rotation at the top of the wall.

Also, If you are in the Chicago area, you will not have to 
consider lateral due to seismic and if this is a face brick 
structure as is common in the area, the weight of the structure 
should exceed the lateral component for wind.

Hope this helps,
Dennis S. Wish PE