Rooftop Units in California

[palves]

Does anybody know where the 2010 CBC (or 2009 IBC for that matter) specifies the minimum weight of a rooftop that needs to be analyzed by an engineer? I seem to remember 400 lbs. being the cutoff that requies an analysis, but just want to find the specific section. Thanks.

 

[jdgengineer]

It's in ASCE 7-05 Chapter 13, Section 13.1.4.

400 lbs is correct.

 

[jdgengineer]

However, that is only for the anchorage of the equipment to the roof. The roof still needs to be checked to ensure that it is capable of supporting the equipment.

 

[palves]

Perfect, thanks. Quick follow-up question. If the unit is 400 lb. and under that exempts us from the seismic design requirements of Chapter 13, but are we also exempt from checking gravity loads? My RTU is a new 333 lb. unit being put on an existing building and is pretty small. Might the city be looking for gravity calcs?

 

[jdgengineer]

In my opinion the 400 lbs requirement is only to exempt from seismic anchorage calculations for the unit. I still think you need to check the roof for gravity / wind loading.

Chapter 34 of the CBC gives an exemption from checking gravity loading of the existing structure if the increased loading does not increase the internal forces of the supporting members by more than 5%. Depending on your roof framing / support of the unit this may or may not be true.

 

[jdgengineer]

Be careful though. The 5% rule is from the ORIGINAL design. Therefore, if there are solar panels, etc on the roof from previous work that was not included in the original calculations than this allowance may have already been used.

 

[msquared48]

If the old RTU is less in weight than the original, the BD should buy that the gravity system does not have to be checked, only seismic anchorage provisions met.

If this is a totally new weight, unless you have the original structural calcs for the roof that specifically allosed for 1 to 2 psf over the entire roof for Mechanical units/piping, etc, and the 2 psf for the concerned area of framing is greater than the weight of the unit, you should check the framing. At the least, anyway, I would check the joists under the unit that span to any purlins.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com