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

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Daniel Bolt wrote:

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.
--Dan

Dan,
I've never considered a roof top A/C or Swamp Cooler to be a dead load. however I can see where this is considered a professional opinion. If you are really concerned, then design for the worst load condition. Swamp Coolers weigh next to nothing and generally less than the live load of the area used by the cooler. An A/C unit can exceed the live load and if this is the case remember that you can also increase the stresses to the joists or beams by an allowable 25% (in our area) as an appropriate load duration factor (LDF). You need to check what your location will allow for LDF increases. Permanent or Dead Loads are the roofing material, the underlayment, sheathing, insulation, framing, ceiling (if applicable) and any miscellaneous load you need to add. As I said before, I don't consider the weight of serviceable mechanical equipment as part of the live load.

Dennis

PS. I have not taken the time to check the code references to this and you might look in the commentaries or contact companies like Trus-Joist to see how they handle mechanical loads in the design of their proprietary products. This doesn't create a standard for compliance, but they generally consider the conservative approach.

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