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RE: Rooftop Units

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I have seen that done several times also... generally the less that is known
about the load the more of a CYA factor it gets...  Each point load might
even be above the maximum corner load...

You should be able to get corner weights, center of gravity, and other
information from the RTU supplier... some such as Trane, etc. have published
data on the web.

A couple of things to watch for with RTUs and Truss joists.
1) If you can make sure the unit supports are at top chord panel points as
this will help the top chord out quite a bit (no bending)
2) You or the manufacturer (most likely) should be aware of these joint
loads and make sure that the welds/connections are adequate.  Large point
loads can be more critical on joint connections than line loads.
3) Keep the joists stiff (same true for other secondaries)...  if the heavy
RTU sags the joist down too much you are likely to get/cause a ponding
problem around the RTU
4) If possible turn the RTU such that its narrowest dimension is across the
roof (blocking water running down slope... if their is a slope on the roof)
5) (True for any secondary) Make sure the drain line does not drain on metal
components or metal roofs as they will cause early rusting in the metal.
6) Large RTU, Wisconsin... don't forget to check for snow drifts around the
RTU...

HTH,
Greg Effland, P.E.
KC, MO USA

-----Original Message-----
From: Russ, John P [mailto:JPRh(--nospam--at)pge.com]
Sent: Tuesday, September 17, 2002 4:06 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Rooftop Units


Normally, the corner weights account for the distribution of the weights
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 joists
that are affected by the units.  The change in price due to this weight is
generally quite small.

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


I have a large rooftop unit that sits over six joists. It is made of up of
several components including the fan, compressor, coil sections, etc. I am
currently reviewing data from one manufacturer who gives me the overall size
and weight plus the corner weights. I know the compressor section is
probably the heaviest section. I have asked for the weights of the various
components including their lengths so that I can convert those loads to a
plf and then apply the heaviest concentrated load to the joists. In other
words, I need more information on the distribution of weight.

The salesman is claiming that he cannot provide this info. yet I have
received this info. from other manufacturers when I have ask for it. Our
next step is to get try to get the info. right from the manufacturing plant.

I know that there will probably be some load sharing since all the joists
will most likely deflect together. Am I asking for too much or being too
conservative?

Jim Kestner
Green Bay, Wi


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