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# Re: Roof Equipment

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Roof Equipment
• Date: Sat, 9 Nov 2002 09:22:00 -0800

```Timothy,

I saved your post from months ago because I wanted to comment on it, only to
realize now that I had forgotten.  Sorry.  None the less, I still want to
comment because I respectfully disagree.  I think most roof top air handlers
need to be designed as flexible.  If not, first I think the burden of proof
is on you that the period is less than 0.06 seconds.  Good luck with that
one.  Second, I believe these air handlers typically have internal
isolators.  If they didn't, they would vibrate the heck out of the roof.
Those same isolators that prevent vibration serve to amplify the seismic
mass.  Sorry, but I think that until the wording of the code changes, a sub
p = 2.5 is the correct way to go unless you can prove T < 0.06 seconds.

West Covina, CA

----- Original Message -----
From: "Timothy P. Spengler" <timothyps(--nospam--at)cleanpak.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, July 15, 2002 7:17 AM
Subject: RE: Roof Equipment

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-----
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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