Rooftop Ductwork Structural Support

[trence76]

I'm a young structural engineer and I get alot of oddball projects. Latest project is providing support for a rather large, 6'-0" tall x 10'-0" wide cross section, ductwork running along the roof, with the bottom of the duct being approximately 3'-0" from the finished roof surface. The project started as a review, with the MEP engineer providing simple roof curbs parallel to and directly above the roof structure, specifically light steel beams at 7'-0" o.c.

My concerns include the lateral wind pressure against the duct and the uplift and suction pressures that may want to pull the ductwork off of the roof. While it is 18gage ductwork, it is not heavy enough to overcome or make me feel comfortable with the suction forces I am developing in my calculations. As a result, I feel positive attachment to the roof structure is going to be required. The problem is that I do not want to overdesign structural supports and I am looking for anyone's input, and or experience for these types of applications.

Thanks for taking the time to provide your input.

 

[msquared48]

You are very correct in your thinking here. The only adjustment you need to make, in my mind, is being concerned about over designing the structures. I would be far, far more concerned with under designing them. That's where the liability comes in here.

On more than one occasion, I had to use a section of far greater capacity than needed for reasons of section availablility, relative cost, connections, whatever.

Just CYA here. You'll do fine.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

Do you have snow accumulation in your area?... can be a problem if the duct is big enough... once had to reinforce a 100' high x 600' long building for a new duct... 34' dia...

Dik

 

[MiketheEngineer]

Yes - snow could easily collapse a large 18 ga duct. Typical wind load would be in the 20 psf range sideways and 10-12 psf up or down. Those too could collapse the duct - again at 18 ga and depending on the size.

This is just a guess as I have NO idea where you are.

 

[JedClampett]

Don't be afraid to add stiffeners to the duct to allow it to span from side to side and up to down. It's not unusual to add angles to ducts, especially large ones, to increase bending capacities.
Also, don't forget about internal pressures in the ducts. One inch of w.c. doesn't seem like much, until you try spanning light gage ductwork 10 ft. with it.
Like Mike and Mike said, follow the numbers down to bedrock, if necessary, and believe them.