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RTU on 'isolation curb'

RTU on 'isolation curb'

RTU on 'isolation curb'

(OP)
The attached is from ASCE 7-10.

For determining connections for an RTU to an isolation curb and the isolation curb to the roof, would one use R=2.0 (or 2.5) and then multiply by factor of 2 per footnote (b)?

...attachment is from CHAPTER 13 SEISMIC DESIGN REQUIREMENTS FOR NONSTRUCTURAL COMPONENTS, ASCE 7-10
Table 13.6-1 Seismic Coefficients for Mechanical and Electrical Components

RE: RTU on 'isolation curb'

It depends on the type of isolation curb mech. is specifying. Ask for the specifications on the curb from the mech., this should give you the information you need to determine the Rp and if it has seismic restraints so you can avoid doubling the loads.

RE: RTU on 'isolation curb'

OP:
Do you have a different link to that table?

RE: RTU on 'isolation curb'

Thanks very much...

Dik

RE: RTU on 'isolation curb'

(OP)
What I am trying to grasp is the ramifications of utilizing an 'isolation curb' under a typical RTU on a typical one story wood or metal deck roof. I am digging into some detail per sandman1, and haven't gotten there as yet.

EQN 13.3-1
Fp =

0.4 * ap * SDS * Wp * ((1+2*(z/h)
( Rp / Ip )

I normally use Rp = 6.0 for standard RTUs.

If correct so far, and then utilizing Rp from middle section of Table 13.6-1, say 2.0, I've effectively tripled Fp. And then does the footnote criteria double that again?



RE: RTU on 'isolation curb'

If the unit is on vibration isolators, I use the ap and Rp from the "Vibration Isolated Components and Systems" section. To avoid doubling this force per the footnote, I typically specify snubbers or restraints to restrict movement to 1/4" or less. Often times, the restraints are built into the isolators.

RE: RTU on 'isolation curb'

(OP)
wannabeSE, thanks for confirmation on both the section and the footnote.
For RTUs on a standard manufacturer's curb, do you use Rp 6.0?

RE: RTU on 'isolation curb'

I'd use the 2.5 value for vibration isolated components, and would make sure the equipment was on vibration isolators...

Dik

RE: RTU on 'isolation curb'

Triangled,
Large air handling units often times have internal isolators on the fans. When this is the case, I use the ap=2.5 and Rp=2.0. But a good argument could be made for scaling the forces based on the weight of the fans with ap=2.5 and Rp=2.0 and weight of the rest of the unit with ap = 2.5 and Rp = 6.0.

About 75% of my work is in hospitals where special seismic certification (shake table testing) is required. In that case, I use the ap and Rp used for certification.

RE: RTU on 'isolation curb'

(OP)
wannabeSE, Dik and sandman21,
Thank you very much for your insights and recommendations. I feel like I have a better handle on the loads now.

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