Selecting riser supports is critical because failure at one level can lead to a catastrophic chain of failure at other levels unless sufficient conservatism has been exercised in estimating 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 SMACNA’s “Rectangular Industrial Duct Construction Standards,” second edition, published in 2004, are just that, a balanced approach to riser support loads applicable to any riser system in round or rectangular duct.
If two supports are used, the lower support shall have the capacity of supporting the entire vertical run, while the upper support shall be capable of supporting at least one half of the total vertical load, plus the entire load attributable to any portion of duct extending upward beyond the top support, if not already included. See accompanying graphic where W1 and W2 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 to carry the full load attributable to the lower two spans of the duct. The other supports shall each 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 be capable of carrying one-half the load of the top span, plus the entire load attributable to any portion of duct extending upward beyond the top support.
Vertical duct runs supported off building walls, or off the building structure, should be supported at each story level. Additional intermediate support may be necessary to limit the maximum 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 covers the simple, low or moderate temperature and pressure (or vacuum) indoor systems as well as the 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.
“Rectangular Industrial Duct Construction Standards” is available to SMACNA members at the member price of $77, to architects and engineers at the discounted price of $212, and to others at the list price of $295. It is also available on CD-ROM and in PDF format. For more information or to purchase “Rectangular Industrial Duct Construction Standards,” visit the Publications Store section of the SMACNA Web site.