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RE: Are Roof Top Units considered Dead or Live Loads?

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I consider them to be dead loads; the only reason to consider as live would
be the higher LRFD factor.  To account for this, design for the weight given
to me +25% to account for the concentration of piping and ducts,
installation and repair workers, and a "fluff" factor.

For bar joists, I include an outline of the unit (with dimensions) and list
the fluffed-up weight.  If the weight changes, all I have to do is check the
beams and change the weight on the plans.  The joist designer may either
design custom joists for the RTU or use standard joists that will work.  I
also include these notes:

In the "Design" section of the general notes:
 - RTU loads are preliminary as provided by xxxxxxx Engineering.  The
Contractor is responsible for coordinating the roof joist design with the
weights of the final units selected for installation.

In the "Steel" section of the general notes:
 - RTU locations and loads are approximate and should be verified before
designing or fabricating associated roof joists.  For final design, assume
the final RTU location may shift 4'-0" in either direction along the joist
length.

This usually takes care of any minor changes associated with different
manufacturers.  Going from 40kip to 160kip, IMHO, should be a design change
order if the design has already been completed.


> -----Original Message-----
> From: Rich Lewis [mailto:sea(--nospam--at)lewisengineering.com]
> Sent: Tuesday, January 04, 2005 12:13 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Are Roof Top Units considered Dead or Live Loads?
> 
> I don't know about the rest of you, but I have been burned too many
> times designing a roof structure to support mechanical equipment.  The
> mechanical engineer gives me one weight value and two weeks later it's
> different.  Sometimes the construction submittal comes in with different
> weights and the steel is already fabricated.  I have learned to add a
> "fudge factor" to all mechanical equipment loads on the roof so that I
> design the structure once, not 2-3 times.  I have never been able to
> recoup the redesign fee of the roof structure from a mechanical
> engineer.
> 
> I have also had equipment move after being given locations by the
> mechanical engineer.
> 
> That being said, and considering LRFD load factors, I am inclined to
> consider them as live loads.
> 
> Rich Lewis
> Lewis Engineering

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