VISCMA

Q23

How do I run restraints for ductwork in fire rated hallways? ANSWER: This is a great example of the need for good phasing on a project. It may be that the restraints have to be installed crossed over the top of the ductwork and before the walls are put in place. It may mean that …

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Q22

Can I use wall penetrations as restraint points for ductwork? ANSWER: Yes, to some degree. It is best determined by a qualified seismic design engineer. Common practice is to use wall penetrations as lateral restraints if the wall is considered strong enough and the design forces small enough. Fire dampers, since they are made to …

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Q21

What is the size exemption for round or flat oval duct? ANSWER: The cross-sectional size exemption is for any shape of duct. SMACNA used to require all round ductwork 28″ larger in diameter to be restrained. This requirement is not used in IBC 2012/ASCE 7-10

Q20

Can I use the “12 inch rule” for ductwork? ANSWER: The “12 inch rule” exemption has been misapplied in many cases. The only way the 12″ exemption rule can apply is if the supports are designed to prohibit significant bending for all hangers on a run of duct. One method is to select hangers that …

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Q19

How far apart do the braces need to be? ANSWER: Industry standard practice uses a maximum of 30′ between transverse restraints and 60′ between longitudinal restraints for ductwork. Actual restraint locations will be determined by the seismic design engineer based on duct construction and design force levels.

Q18

What duct needs to be restrained? ANSWER: All ductwork that is 6 square feet and over in cross-sectional area or over 17 lb./ft. and does not fall into another exemption category shall be restrained to meet the applicable building code. Other exemptions that may apply include ductwork that has not been assigned an importance factor …

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Q17

I have a structure with long support spans and in order to keep the vibration generated by the roof mounted equipment from causing problems, a spring with more than 2 inches of deflection is needed. However, when I put equipment on these springs , it rocks excessively in a breeze or when I push against …

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Q16

When should I use inertia blocks with the vibration isolators? ANSWER: There are three major reasons for including an inertia mass with the isolation. One is that it lowers the center of gravity of the system. For a tall narrow unit, this could make the installation more stable in a seismic event. Secondly, some pieces …

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Q15

If the vibrating equipment is located outside on the ground, do I still need to use vibration isolators? ANSWER: Even if the equipment is on grade on a separate slab from the building, vibration transmitting to the building structure is still a concern. Also, the addition of isolators will reduce enough vibration that it is …

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