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SMACNA GUIDELINES

Volume 12, Issue Number 1 October 29, 2008

The Importance Of Estimating Design Loads For Riser Supports

Selecting riser supports is critical because failure at one level can lead to a catastrophicchain of failure at other levels unless sufficient conservatism has been exercised inestimating design loads. The question is one of balance among the various considerations:conservatism (to prevent such a course of events), reasonable cost, and simplicity.SMACNA believes that the load recommendations for riser supports in SMACNAsRectangular Industrial Duct Construction Standards, second edition, published in 2004,are just that, a balanced approach to riser support loads applicable to any riser system inround or rectangular duct.

If two supports are used, the lower support shall have the capacity of supporting the entirevertical run, while the upper support shall be capable of supporting at least one half of thetotal vertical load, plus the entire load attributable to any portion of duct extending upwardbeyond the top support, if not already included. See accompanying graphic where W1 andW2 represent the weight of duct sections of length L1 and L2.

For duct supported at more than two levels, the lower support shall have the capacity tocarry the full load attributable to the lower two spans of the duct. The other supports shalleach be capable of carrying at least 1.5 times the load contributed by the average weight of the two adjacent spans, on either side of the support. And finally, the top support shall becapable of carrying one-half the load of the top span, plus the entire load attributable to anyportion of duct extending upward beyond the top support.

Vertical duct runs supported off building walls, or off the building structure, should besupported at each story level. Additional intermediate support may be necessary to limit themaximum distance between supports to 20 feet.

Rectangular Industrial Duct Construction Standards, second edition, expands the scope of the 1980 version, updating original text to incorporate a revised theory of design, materials,and more user-friendly tables. Tables for stainless steels and aluminum, expanded chapters

on materials, welding practices, and a guide specification are included. The manual coversthe simple, low or moderate temperature and pressure (or vacuum) indoor systems as well asthe more complex outdoor systems that operate at moderate to high temperature and pressure(or vacuum), and are subject to higher and more complex external loading.

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