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RE: Duct Saddle Support

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   We have used variations of the saddle shown on 6-4 in the SMACNA 'Round
Industrial Duct Construction Standards' for large diameter ducts. We've
supported ducts up to 10' diameter with this detail.  Generally, with these
bigger ducts, you should be getting into some thicker gages anyway.  The
detail should work fine, as long as you make the saddle wide and tall enough
to prevent ovalling or flattening of the duct.  SMACNA does not provide
formulas or criteria for the design of the saddle;  Roark's 'Formulas for
Stress and Strain' would be a good starting point.
 
    For the design of the duct itself, you may want to look over your
mechanical engineers' shoulders a bit.  Sometimes they ignore such things as
wind and snow loads on exterior ductwork.  Also be careful of flex
connection locations in relation to supports.  For the saddle design, you
may need some 'roller' type connections to accommodate expansion in hot
ducts.  Also, be careful of water accumulation due to sprinklers within the
duct.
   

Rich Kipke, PE
Structural Engineer
Haden inc.
1399 Pacific Drive
Auburn Hills MI 48326
direct:(248)-475-5144 Fax: (248)-475-5227
RKipke(--nospam--at)Haden.com