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RE: Roof Equipment

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If the rooftop equipment is considered flexible, as defined by the code, then there's no question about it, by current code that's what you need to use.  I don't know the specifics behind your situation, but my experience has been that most roof top mounted equipment can be defined as rigid, resulting in a a.p of 1.0.  The blue book provides some additional guidance on this matter, but my take on it is that the specific question as to whether the equipment is truly flexible or rigid is a rather difficult one to answer.  It seems unfortunate that going conservative on this interpretation results in such a substantial design hit.

As a side note, there has been considerable discusion that the multiplier of 3.0, shown in formula 32-2 of the UBC, should actually be 2.0 as shown in the IBC's equivalent formula.  While to the best of my knowledge an errata has never been issued from the UBC that would officially correct this, some building officials have told me they would be willing to accept designs based on this reduced formula. 

Tim Spengler SE
Portland, Oregon





-----Original Message-----
From: lrhauer(--nospam--at)earthlink.net [mailto:lrhauer(--nospam--at)earthlink.net]
Sent: Saturday, July 13, 2002 10:51 AM
To: SEAOC
Subject: Roof Equipment


I'd like to get some opinions on what seismic forces others are using for
anchoring roof top mounted mechanical equipment, particularly with a Ca of
0.57 for Southern California.



Table 16-O is pretty explicite in Item 3,C that an Ap of 2.5 and Rp of 3.0 be
used, therefore Formula 32-2 seems to kick in, since these values are not in
the "non-building" formulas. With Hx=Hr, Formula 32-2 results in a strength Fp
of 1.9Wp and ASD force of 1.36Wp, which seem awfully high. 



Any opinions?



Larry Hauer, S.E.

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