RTU on 'isolation curb'

[Triangled]

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

 

[sandman21]

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.

 

[dik]

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

 

[Triangled]

upload reattempt>>>


Yes, much better...

 

[dik]

Thanks very much...

Dik

 

[Triangled]

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
F_p =

0.4 * a_p * S_DS * W_p * ((1+2*(z/h)
( R_p / I_p )

I normally use R_p = 6.0 for standard RTUs.

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

 

[wannabeSE]

If the unit is on vibration isolators, I use the a_p and R_p 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.

 

[Triangled]

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

 

[dik]

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

Dik

 

[wannabeSE]

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

 

[Triangled]

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.