Exterior duct and support design

[haynewp]

For those of you that provide supports for large exterior ducts on industrial type buildings, does the mechanical engineer provide you with the required duct support spacing or do you determine the spacing and design the duct wall thickness for a given diameter? I’ve never designed a duct before and am having trouble getting someone to provide me with the support spacing they need on a project. These ducts are over 20 ft in the air and are bending under wind and snow loads and need steel supports cantilevered from the ground.

 

[r13]

No, support spacing is usually determined by the structural engineers, as well as the thickness of the duct wall. ME will furnish you with operating characteristics, and weight of miscellaneous equipment inside/outside the duct. Duct design can be quite challenging. For first time in duct design, you shall review a few past designs, and ask guidance from your office.

 

[phamENG]

Haven't designed one, but I do work in a facility with several of comparable size. Looking back at the record drawings, everything's on the structural drawings - size, diameter, support details, etc. This doesn't necessarily answer your question, but I can see where the ME would have said "give me a duct of x diameter from point a to point b" and the structural engineer made it happen. Otherwise, I would expect to see "duct by others" with details for the towers and foundation.

 

[JLNJ]

Our mechanical guys defer to SMACNA for interior duct spans. Once the duct gets outside I get blank stares when I ask them who makes sure the duct is appropriate for wind, snow, span, etc. Exterior duct will probably need structural guidance unless your mechanical guys have some vetted standards the can fall back on.

On the plus side, the duct is usually stronger and stiffer than anticipated, so the allowable spans are usually pretty sizable.

 

[JStephen]

One issue we run into with steel tank piping is that there is a tendency to treat a large steel pipe like a noodle, put supports here and there, and assume they support the pipe. But it's easy to wind up with cases where you're hanging a footing off the pipe rather than the other way around. Pay attention to where flexible joints are (or aren't). Also be aware of possible wind vibration issues.

 

[haynewp]

Thanks for the replies.

Mechanical already has twice the budget we do, apparently I need to start arguing for some of it to design their ducts and connections.

 

[dvd]

I recommend SMACNA Rectangular Industrial Duct Construction Standards. There are also standards for round duct, if that would be the case. Using the industry methodology gives you something to fall back on if there are any issues.