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RE: Construction method for Second Story Additions

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Kate,
Years ago I designed quite a few second story additions in Los Angeles with this same concept. The second story was carried by post and beam construction and the shear transfers where made to the first floor "beefed-up" exisitng or new shear elements (walls, frames etc.). 
I believe that the City of Los Angles tried to outlaw (and may have succeeded) the design of post and beam second story structures. I don't remember why unless some may have considered the two systems laterally independent. I don't think that this was valid inasmuch as this is not really a post and beam structure. It still is considered a box system where the use of posts and beams was to reduce the need to undermine and increase the width and depth of the existing foundation system. 
Tim McCormick (who is a frequent contributer) may be prompted to respond to you about this one.
One thing you brought up is the span of the girders. I would not consider trying to span over the entire structure. I would try to place columns where they align with existing interior walls to cut my spans down. There should be sufficient crawlspace to get into to dig and pour a new pad. We have done with many times. We have even kept the homeowner living in the house by separating the second floor from the first with visqueen and leaving as much of the existing roof or ceiling joists as possible. 
As long as you tie the load path from the roof down to the foundation for gravity and lateral, I don't think this would be considered strictly a post and beam type structure. Check with the City of Los Angeles for the most conservative view.
One more thing, if this is in Simi Valley you may be running into some soils problems that could change the way you design - especially if the first floor is a post tensioned slab on grade. Check with Gaddis Farmer - your building official - on this one (if he is still there, it's been a couple of years since we last met).
I'll be looking in to see what responses you get.
Dennis Wish PE


-----Original Message-----
From:	Kathleen  A. O'Brien [SMTP:wildwoman1(--nospam--at)compuserve.com]
Sent:	Wednesday, December 03, 1997 7:44 AM
To:	SEAOC Forum
Subject:	Construction method for Second Story Additions


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
        


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