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Analysis of Large Diameter Ductwork

Analysis of Large Diameter Ductwork

(OP)
We've been asked to analyze the effect of section loss in a 96" diameter exhaust duct with 1/4" walls that is suspended from the roof. I've hit a bit of a roadblock since AISC (chapter F8 in 13th edition) limits the scope of the chapter to round HSS sections that have a D/t of less than 0.45*E/Fy, which this duct is slightly over even before accounting for section loss (384 vs 363). The duct is primarily subject to bending forces since it spans about 35'. I've found a few standards for pipe design that don't account for the potential for the duct to buckle under bending forces, but little else that seems to apply.

Does anyone have any suggestions for standards that might apply to this situation? Otherwise, does anyone have any suggestions for how to apply the AISC standard that would account for the slenderness of the duct walls?

RE: Analysis of Large Diameter Ductwork

There's gotta be some piping design standard out there that would cover it. Anything in CIDECT guides? I've got this: Link. There's gotta be something in there even if it's wildly out of date. Let me know if you get that desperate.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Analysis of Large Diameter Ductwork

AWWA D100 and API-620 tank standards give allowable stress for thin-wall cylinders, and I think ASME B&PV Code Section VIII does as well. The ASME stack standard may include that as well. If it operates hot, go with the ASME standards.

RE: Analysis of Large Diameter Ductwork

(OP)
I just had a thought... Since I'm not that far outside the D/t limit in AISC F8, I could assume the steel is a lower yield strength than it actually is in order to force the limit to be met. I would think this would be a conservative way to analyze the duct if the difference is small... do you agree?

RE: Analysis of Large Diameter Ductwork

I do agree for something that falls within the scope of normal building / AISC stuff in other respects. However, I found this statement rather persuasive:

Quote (jstephen)

If it operates hot, go with the ASME standards.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Analysis of Large Diameter Ductwork

Check out the SMACNA duct construction standards series. You will likely find your answers there.
Dave

Thaidavid

RE: Analysis of Large Diameter Ductwork

(OP)
Good point with the ASME code and a duct running hot. This duct runs around 150 to 170 degrees F. I'll look into the B&PV code and the SMACNA standard.

Thanks, for the responses!

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