California 2016 CBC - Seismic bracing of ductwork?

[JPohling]

I have a plan reviewer that is requiring seismic bracing on small cross section ductwork. They are citing ASCE 7-10, 13.6.7 Ductwork. I believe exception #2 is exactly what we have. They are saying that somehow we need to satisfy both exception #1b and #2. Our ductwork is small diameter with the largest being 24".

We are under the impression that seismic bracing is required for ductwork greater than 6 SF in area and on hangers greater than 12" in length.

What are your interpretations? Are there any commentaries on this for clarification?

 

[mtlogcabin]

There is also a requirement to weigh 17 lbs/ft or less. Are you meeting that?
If you are I do not see a problem if the hangers do not exceed 12 inches.

 

[JPohling]

Definitely meet the 17lb requirement. The issue is our hangers are longer than 12"

 

[mtlogcabin]

The issue is our hangers are longer than 12"

You do not have to meet both exceptions only one of them
You can't meet 1a because it is not a trapeze assembly.
You can't meet 1b because the hangers are over 12 inches from the support point to the structure.

That leaves exception 2 for you to use. You can meet that because the duct-work is less than 17 lbs/ft and cross sectional of less than 6 sq ft.
the plans examiner is wrong to ask you to meet both requirements.

 

[Phil]

The plan reviewer must be new. Typical ducts smaller than 6 sq ft have been exempt for decades (the actual requirements have been tweaked a little bit over the years). You might show the plan reviewer Mason Industries OSHPD preapproved details general notes which is more explicit: " . . . are not required for HVAC Ductwork and Factory Built Vent Duct where any of the following are satisfied: . . ." The pertinent page is attached. The entire document can be downloaded at https://www.oshpd.ca.gov/FDD/Pre-Approval/OPM-0043-13.pdf. This has passed the scrutiny from OSHPD. Note: CBC 1616A.1.26 revised ASCE 7's 17 plf to 10 plf.