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Re: [Fwd: eccentric footings]

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Yes, "calculated" bearing pressures on typical single story residential
footings can be pretty high.  The problem is more significant in older
residential buildings where typical exterior footing is trapezoidal with 8"
stem at the top 12" wide at base, 12" ~ 14" thick with top of footing at grade
or marginally above grade.  Exterior face is typically flush with outside face
of stud wall.  I have reviewed many older residential structures in the East
Bay area.  Many show signs of settlement and tilt causing cracks in the
footing, exterior stucco but reaching a stable state.  In most cases, you will
find that the eccentricity is beyond the middle third and the pressures on
exterior corner quite high.  As Bruno Cote mentioned, the resultant does not
necessarily have to be in middle third.  However, in soft soils, you will
experience settlement, tilt and resulting distress to the structure.  The
problem is less significant in conventional T footings.

Note that the "calculated" pressures are very sensitive in small footings.  A
lot would depend on the assumptions, loads, soil conditions, slight
differential in interior and exterior grade, etc.  You could get radically
different answers by small changes in your assumptions.

I have repaired many old trapezoidal footings by reinforcing from outside or
by adding small footings at about 4' to 6' o.c. below existing footing.  If
you have a footing on property line, be sure to tie the floor slab (if on non
expansive soil) or provide grade beams at regular interval and designed to
take the eccentricity.

Anand Nene, S. E.