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Rooftop Units

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As John Russ said, if you have the corner weights you have everything you need for gravity loading.  These things sit on curbs which are pretty stiff, enough so to distribute loads on joists (assuming the long axis is crossing the joists), so the corner weights are sufficient to get the varying load under the curb.  If the unit is parallel to the joists, that's different, and you'll probably be threading some shallow heavy wideflanges under it.
It sounds like you're crossing joists, so...
You didn't say if these are new joists or an existing roof.  If they're existing (and assuming they're capable of carrying the extra load), you need to either make sure the curbs fall on the panel points (which is nearly impossible) or stiffen the top chord (L 4x6 or similar welded to the chord and diagonals, LLV).  The fallback, as always, is to thread a couple of wideflanges in.  This is always difficult, because of those mechanical and electrical guys.
If they're new joists, do yourself a favor and make them the wideflange kind.  The most beautiful Special Joist Load diagram in the world isn't worth beans once the mechanical contractor starts substituting units.  We always try to put beams in under mechanical units and we've never regretted it.  Neither has any of our clients.  If you really want to do it right, camber the beams to match the joists.  I would not include deflection under the mechanical in the camber, as it won't be there when the deck installers are installing.
And yes, remember the drift load.  No matter what the UBC loophole says.
Mike Hemstad
St. Paul, Minnesota
-----Original Message-----
From: Russ, John P [mailto:JPRh(--nospam--at)]
Sent: Tuesday, September 17, 2002 4:06 PM
To: 'seaint(--nospam--at)'
Subject: RE: Rooftop Units

Normally, the corner weights account for the distribution of the
based on rigid body distribution.  You could merely scale the weights
between the corners and aply them to the joists.  If this is new
construction, I have just used the maximum corner load at all the
that are affected by the units.  The change in price due to this weight
generally quite small.

-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)]
Sent: Tuesday, September 17, 2002 2:04 PM
Subject: Rooftop Units