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Re: Construction method for Second Story Additions

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>One of the problems we have encountered adding these second stories is that
>often one has to strengthen the existing story. Some jurisdictioins do not
>require this, but even when they don't I do not think it wise to put a
>newer, stiffer story over an older one, especially if it has no existing
>ply sheathing or other LFRS. A lot depends upon the age of the existing
>structure of course; sometimes this is not a problem.

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


A building department that would allow a vertical addition without a lateral
and vertical analysis of the lower levels is apparently deferring to the
engineers judgement and not enforcing the code. Section 3403 of the 94 UBC
is pretty clear. In San Francisco, *any* structurally connected vertical
addition, short of a 150 S.F. roof penthouse, requires a seismic upgrade of
the entire building, not just the areas tributary to the vertical addition.
So, in SF if you are making a vertical addition you should consider maxing
out the addition floor area. The UBC is a bit more flexible with the
interpretation. In general, additions or alterations should not increase
seismic demands or decrease seismic capacity.