Rooftop HVAC Unit - Seismic Isolator "Seismic Force Ratio"

[dreber]

Hi all, I've got a tough one for you.

In CA, A reputable manufacturer, Mason Industries, has produced anchorage demand load calculations for a rooftop unit which uses their spring isolators.

The city building department has asked that I reproduce the Mason Industries demand load calculations on my letterhead.

In doing so, I came upon a term "Seismic Force Ratio = 3.75"

I've been doing this stuff a long while and never come across "seismic force ratios".

Any idea what this is, where it comes from, or how it is used?

Also included in the calculations was this sheet, which didn't solve my immediate problem, but you may find useful.

 

[phamENG]

Looks like it's the inverse of the Seismic Response Coefficient.

Wp=10670#.
N= 6 isolators
Vy=Max Shear per Mount = 6669#

6 mounts * 6669#/mount = 40014#

40014#/3.75 = 10670 = Wp.

 

[RPMG]

It's everything except for the weight, including the overstrength factor.

 

[dreber]

Thanks guys. I'm feeling silly now. Leaving it up for the shame.

 

[phamENG]

No worries. I feel that way at least once a day...