And they don’t have to be as wide either. It is probably well worth it to pay for that square-to-round coming right off the unit. For large ducts you may need something heavier than a standard U-channel strut or more than two “feet” in your support.ĭon’t forget, your duct support system will be much simpler if you use round spiral duct instead of rectangular. That weight and the support width determine your beam deflection. That weight will dictate the size of “feet” you need. If the one you are looking at is 8’-0” from the next support one way and 4’-0” from the next support the other way, you can consider that it is holding up 6’-0” of duct (the sum of half the distances). There are a lot of sources for weight charts for ducts. Recommended practice is, once you come up with a duct support layout, number your supports, then figure out exactly what you have sitting on each one. If the beam is 60” wide, it can deflect no more than ¼”. The allowable deflection for a horizontal support beam is L/240. Many duct supports use structural sections and hardware such as U-channel strut, angle brackets, nuts and bolts. Most support manufacturers have more than one size, and you can choose based upon your duct weight and the allowable PSI. You should know the allowable PSI for mechanical equipment based upon roof construction. Size can matter, because those feet turn “pounds into PSI”. Non-penetrating rooftop support systems use plastic or rubber feet to distribute duct weight. We see more ducts being put on single-ply flat roofs, and in those cases you want to minimize roof surface penetrations. You will still need to determine height and any hardware to restrain the duct. If you use curbs or equipment rails, these guidelines give you quantities and widths. Now that you know where you need to put your duct supports and how many you need, you can start figuring out the parts and pieces you need to make the supports. Remember, the real reason behind these span recommendations is to have no more than two transverse duct connections between a pair of supports, and that is unlikely to be the case there. If you are supporting 5’-0” lengths of rectangular duct and you have a span that is 10’-6”, you really don’t need to use two supports. Please use a little common sense with these guidelines.For round duct maintain a spacing of not more than 12’-0”. For 5’-0” sections of rectangular duct, maintain a spacing of not more than 10’-0”. For 4’-0” sections of rectangular duct, maintain a spacing of not more that 8’-0”. Based on the shape of the duct and the length of your duct joints, you can now start locating the rest of your supports. Now that you’ve made it this far, you should be left with nothing but straight segments of duct.That applies to the inlet, straight-through outlet and branch outlet sides. Locate supports within 4’-0” of any branches.If you have vertical or horizontal offset ducts, locate supports within 2’-0” of the ends. Use an intermediate support on elbows where the center line length plus 4’-0” is greater than the support spacing for the duct type.Locate a support within 2’-0” of the ends of any elbows (including the elbows that may be turning vertically to penetrate the roof).This prevents the duct weight from affecting the mechanical connection of duct to unit. Locate your first support about 2’-0” from the discharge. And as you go through the exercise you can imagine how this duct support system would be different if you used a 60-inch round duct instead of the 96 inch x 32 inch duct shown. Since gravity is the first force you must design for in a roof support system, you can see where a lighter duct will be desirable.Ībove is an example rooftop duct layout. Round spiral ducts are generally 30 percent less weight than equivalent rectangular ducts. And round spiral duct is typically provided in 10 foot lengths and greater. That explains the 8 to 10 feet apart recommendation when you are installing 4 and 5 foot joints of rectangular duct. But the underlying “common sense” behind those recommendations is pretty simple: have no more than two duct joints between supports. There is no comprehensive research that sets support spacing for every combination of duct size, shape, gauge and joining method. The joining methods may range from a few screws up to welded and bolted flanges. The weakest aspect of that “box section beam” are the joints between the segments. A duct section has been described as “a box section beam of considerable strength.” Round tubes make a stronger beam than square or rectangular profiles of the same dimension and thickness. You’ve probably seen recommendations that tell you to space duct supports 8 to 10 feet apart and 12 feet or more apart for round ducts.
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