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Construction method for Second Story Additions

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Another issue:  

Problem: In the interests of saving money, a client has asked me for my
opinion on the following:

When we are adding a second story on top of an existing first story, what
if we were to make an entirely new structure; that is, add beams and
girders and new pad footings, NOT TOUCHING THE EXISTING STURCTURE. We would
therefore have stilts (braced of course) carrying a second story and
bridging over the existing one.

(Something similar was suggested during the Underpinning discussion):

My initial comments are:

        We would have to deal with the soft story problem. This would mean
braced frames or moment frames (maybe not; not stiff enough) or new shear
walls. I don't know that this would save money and be safe at the same
time.
        While digging for the new pad footings, we would have to
shore/brace the existing footings. This also does not sound like a
moneysaver.
        Lateral load distribution would be an interesting problem - most of
the load is concentrated at the top - standard UBC requirements might not
be conservative.
        Girders required to span over an enitre first story would have to
be ENORMOUS. $$$$$.
        As far as safety goes, my thinking is that a wood framed second
story is fairly lightweight. Supporting it by a first story braced frame is
not toally off the wall, as long as we detail properly. Correct me if I'm


wrong but people do still build buildings with atriums, parking garages and
open space in the first floors - they just detiail them better than they
used to, right?

Feedback? Comments?

Kate O'Brien, P.E.
Simi Valley, CA