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

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Many of us usually assume that there is only one or two roof top units.  But is some cases, there might be 100 units and these also have to be considered in your seismic analysis.  For example:  One project at Verbatum a number of years ago:  Only saw four or five units the first time I was on the clients roof.  About five years later called back and found about 100 units on the same roof.  Huge problem as some of the units were over the production line.   The tilt-up building had to be retrofitted both for the vertical loads and for the seismic.

Another issue would be a visual screen that many jurisdictions require to hide the mechanical units.

In Nevada, client's M.E. specified a certain heating unit to hang from steel trusses.  Unbeknownst to both the M.E. and myself as the E.O.R., the client made a deal with the mechanical contractor to substitute a much heavier unit.  After finding out about this, the "new, cheaper" unit had to be placed on a mezzanine instead of under the trusses.

Little games client's play.

Neil Moore, S.E.

At 12:39 PM 1/4/2005, Mlcse(--nospam--at) wrote:
I would consider them a dead load. 
The RTU may be moved in the future, and the support framing would have to be checked for the new location.  You know the weight of the unit (or you know the intended weight + another 100 to 200 pounds additional weight, just in case the contractor needs to change it since that particular model is not available during construction), why use the Live Load factors instead of the Dead Load factors when using LRFD.
The higher Live load factors in LRFD or strength design is because we have less certainty about what the actual loads will be, since live loads are considered to be transient loads.  It would be conservative to consider the RTU's as a live load if using LRFD.  It would be unconservative to include as part of the live load if you are reducing the live load based upon tributary area to the framing member (bending and shear).  As a sanity check, I would want to check the framing member for all the know dead loads, including the RTU, and what would be a reasonable number of 150 to 200 pound people (live load) standing in the tributary area to this member.
If using ASD design, you need to consider them as a dead load, there is no short term increase allowed for the supporting wood members (assuming the roof to be wood framing and not steel framing) and you probably should be checking the dead load only case.  Wood values have changed over the years, and for Douglas Fir there have been some significant reductions in allowable bending stress (1994 NDS). 
Michael Cochran
In a message dated 1/4/2005 6:58:22 AM Pacific Standard Time, dboltz(--nospam--at) writes:
I've had different answers from various Senior Engineers.  Some say that since the RTU weight does not vary in magnitude and location, it is a DEAD LOAD.  Others say that if the RTU is changed or moved in the future, it's weight and location would be considered a LIVE LOAD. 

Any help is appreciated.