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RE: Roof Equipment

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The multiplier of the 3.0 in the 1997 UBC component equation is correct for
the accompanying R-sub-p values and is in no need of errata.  This is the
correct value, based on the 1994 NEHRP Provisions.

The multiplier of 2.0 in the 2000 IBC is also correct for the accompanying
R-sub-p values and is in no need of errata.  this is the correct value,
based on the 1997 NEHRP Provisions.  The 1997 NEHRP Provisions changed both
the multiplier and the R-sub-values such that you would get essentially the
same answer with both codes.

The 2000 NEHRP Provisions remains unchanged from the 1997 NEHRP Provisions
on this matter and will form the basis for the 2003 IBC and 2002 NFPA 5000.

Needless to say, you can't mix and match multipliers and R-sub-p values.

Regards,

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA

*********



                                                                                                                   
                    "Timothy P.                                                                                    
                    Spengler"            To:     <seaint(--nospam--at)seaint.org>                                               
                    <timothyps@clea                                                                                
                    npak.com>            cc:                                                                       
                    07/15/02 08:17                                                                                 
                    AM                   Subject:     RE: Roof Equipment                                           
                    Please respond                                                                                 
                    to seaint                                                                                ..... 
                                                                                                                   
                                                                                                                   




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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