Duct trapeze Support

[Maturin]

I'm analyzing some trapeze hanger supports for a 72" diameter duct and I'm not sure about the load distribution from the ducting to the hangers.

The hangers are semi-circular straps, unreinforced, attached to 3/8" hanger rods while the ducting is 12 ga. thick.

Would the duct self-weight be uniformly distributed to the strap, (i.e. combination of cases 12 and 20 from Table 9.2 of Roark, or would the distribution be localized to a few locations on the strap. Is anyone aware of a design guide for these types of supports?

 

[jdgengineer]

I've always based design of these on tributary are to the strap. Essentially just the length of duct tributary to the support times the weight per foot of the duct.

Do you need lateral loads as well?

 

[Maturin]

Thanks for your response jdgengineer.My question is more about the localized load effects at the support. If it is determined that the load at the support is 1500# then how is that load transferred to the support. Is there a uniform radial compression load between the strap and duct over the strap contact surface or is the distribution different? Is there high circumferential bending in the duct at the support?

 

[rb1957]

uniform radial pressure is one ...
cosine dist'n is another ... peak at the bttm of the arc, zero at the diameter) ... possibly a little more real.

yes there would be bending in the duct ... a continuous beam at a support ... you could simplify to two 1/2 bays, cantilevered at the ends with a support.

 

[topsisu]

If this work is in the US, you may also want to review the design requirements from SMACNA for round industrial duct construction standards. Also available in a rectuangular duct if needed. These are ANSI standard documents.

 

[Maturin]

Thanks RB1957.

When I mentioned circumferential bending I was referring to the transverse direction (i.e. bending around the duct circumference). I'm not really concerned about longitudinal bending.

 

[msquared48]

I have to agree with RB, and that would be an ideal distribution assuming a very rigid duct in relation to the strap.

However, the duct will deflect laterally under the load from the strap, so there will bed a point of equilibrium of forces and deflections that determine the actual force pattern.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com