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Re: Underpinning

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Like I said in my pevious post, I design the second floor framing such that
if the new second floor will cause overstress on the soil, I'll add a post
and beam system in this area only and add a new pad footing under the
existing continuous footing. If an exterior bearing wall carrying the new 2nd
floor joist will cause overstress in the existing continuous footing, I
change the direction of the floor joist, add floor beams and provide new
posts and pad footings at the ends of the beams. It takes a little bit more
time trying a variety of framing layout to come up with a working system. It
does not always work all the time especially if flush floor beams are
required for no drops in the ceiling below or if there are not enough
interior walls in the first floor to support flush floor beams. But having
done more than a hundred second story additions here in Southern California,
I managed to get it to work without underpinning, so far. I even have to use
steel beams to create a flush floor beam condition.

Most owner's objection to underpinning is that they think it is expensive.
They also don't want to get a new soils report  because again of the cost and
they want to build right away and they think that having a soils report will
take some time. Besides, what if after convincing the owner to get a soils
report, it turns out that the soil can only carry 1000 PSF and he needs to
underpin anyway, I'll bet he'll backcharge you for the cost of the soils
report. You or the Soils Engineer cannot guarantee the favorable results of a
soils report so he won't want one. 

I think it is reasonable to convince a non-engineer homeowner that if you are
going to add a second floor to an existing one story house, you need new
support or foundation. What they might object to is the continuous
underpinning of the existing footing which they might think is excessive even
if you convince them through calculations and cost estimates that this is the
best way to go. I have better luck convincing them to spend extra money on
more floor beams, posts and a few additional pad footings plus a few extra
$$$ for for a more complicated structural design to make it work without
underpinning.

Ernie Natividad