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Re[2]: Basement Walls

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The general order of construction is:
1.  Cast grade beam footing
2.  Place walls
3.  Place floor slab directly over the grade beam against the wall

The wall is designed as a simple span.  Backfilling is supposed to take
place after the diaphragm is in place, but in practice the contractor
will often place some of the back fill after the wall forms are stripped
and the damp proofing is applied.  The contractor does this to make life
easier for the framing crew.  Finish grading will be done by the
landscaping crew long after the diaphragm is in place.

Generally the contractor gets by with this because the wall can't slide
due to the basement slab, and the back fill is not compacted.  On rare
occasions the back filling can push the wall over.  That is why you put
the note on the drawings that tells him not to back fill until the floor
diaphragm is in place.

Once an unreinforced concrete or CMU wall cracks, you are depending on
the soil to support itself.  I reinforce all basement walls.  Clays will
expand at various rates with the addition of water.  Play it safe and
reinforce.

Kansas City had so many wall failures due to unreinforced CMU that it is
difficult to sell a home and impossible to gain acceptance for new homes
to use CMU for basement walls.

Regards,
Harold Sprague
Black & Veatch
 ----------
From: JOHN HUBERT
To: seaoc(--nospam--at)seaoc.org; SpragueHO(--nospam--at)bv.com
Subject: Re[2]: Basement Walls
Date: Thursday, August 07, 1997 9:53AM



     Doesn't the basement floor provide a significant amount of lateral
     resistance, therefore placing less importance on the footing?

     However, if backfilling is performed prior to the diaphragm being
     installed, then the wall would experience the deflection associated
     with a cantilevered retaining wall versus a simple span.  I have
never
     designed a residential basement wall, but it is hard to believe
that a
     wall with just a typical spread footing can develop enough
resistance
     against overturning without utilizing the weight of soil by
extending
     the footing farther into the soil.  Is the diaphragm generally
     required to be in place prior to backfilling so that there is at
least
     spring resistance at the top of the wall.

     Incidentally, residential basement walls in this area (western New
     York) are 6" or 8" thick unreinforced concrete or 8" unreinforced,
     ungrouted CMU. The backfill for the walls is generally clay.
Although
     cracking is a predominant problem, it does not seem like there are
as
     many actual failures of these walls as there should be.  This has
been
     puzzling to me for quite some time.  Can anybody explain this?

     John Hubert