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Heck, you can lift a house 3 feet in a minute, if you
can muster the horsepower. The 1/8 inch per day is an old rule
of thumb I first heard in college 20 years ago, and it applies
to houses with drywall or plaster that is meant to be saved.
You are right about using
cribbing for a whole-house lift. My article doesn't show
cribbing because when lifting the center of a house, the
middle of a floor, or a corner of a house there is no risk of
the structure falling over sideways. But when you raise a
structure completely off it's foundation, there is no longer
any friction to prevent it from sliding sideways. In this case
a very stable base is needed. Think pyramidal. A pyramid is an
inherently stable structure. (Have you ever seen a pyramid
fall over? No. I thought so!) If you build your cribbing with
an extremely wide base (perhaps 6 feet) with a less-wide top
(perhaps 2 feet or a bit less) then there should be little or
no risk of the structure shifting sideways. But... here's the
clincher... technically you only need one or two of these
highly-stable supporting structures (unless the house splits
apart during the lift) and the others could be less-stable
supports, such as narrow cribbing (entirely 2' square,
non-pyramidal) or even simple lally columns. Personally, I
wouldn't use lally columns for anything but
"interstitial" supports, that is, supports in
between highly-stable cribbing supports. You could spend a
fortune on wood for cribbing.
Another thing: One article I
read (This Old House magazine, I think) mentioned using 6x6
blocks of oak for cribbing. This would be an investment
akin to a king's ransom, unless you had some oak trees in your
backyard. Oak is extremely hard and resistant to crushing,
which is the reason they use it. But Southern Yellow Pine is
about half as hard as oak, yet twice as hard as typical
spruce/pine/fir lumber.
About that 4x10
"needle" beam. (I've never heard that term, but I've
wondered if there was a word for such a beam parallel to the
sill.) I would consider something heavier, unless you plan on
placing supports every 5 to 6 feet under this beam. I would
try using triple 2x12's nailed or bolted together, then the
supporting structures could be placed farther apart, maybe 8
or 10 feet on center.
Which brings me to the
important issue: Beam sizing and support spacing. This is a
highly technical topic that requires some understanding of
engineering. I recently wrote an
article about replacing a beam in a porch roof, and I went
into a discussion of determining the size of a beam. Beam
sizing and span (which dictates the spacing of the supporting
posts or cribbing) depends on many factors, such as
- The width of the building,
- The number of stories,
- The presence of a
center-bearing wall (most old houses used this technique)
and
- The live load on the roof.
This last factor is not
important because you can choose to do this work when there is
no chance of snow... although there is always a chance of
wind, which can cause some loading problems. Anyway, you can
use the best-case roof live load of 20 pounds per square foot,
which allows you to use longer spans for your needle-beams and
center beam.
One handy reference book I
looked at is "The U.S. Span Book For Major Lumber
Species" published by the Canadian Wood Council.
Apparently they have a web site
with an online span calculator. I haven't looked at this site,
but the little handbook is pretty easy to follow.
You seem to be on the right
track.
Bruce W. Maki, Editor.
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