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RE: SEISMIC: Roof Supported Equipment

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Title: SEISMIC: Roof Supported Equipment

If you are confident that the weight of the additional equipment is just a drop in the bucket as far as the overall building structure is concerned, then you really only need to design the attachments of the new equipment to the existing (refer to ASCE7-02, section 


If you meet the criteria, just use ASCE equation to get the amplified lateral force that acts on the equipment at the connection elevation.


Eli Grassley

PSM Engineers

Seattle, WA


-----Original Message-----
From: Polhemus, Bill [mailto:bill.polhemus(--nospam--at)]
Sent: Tuesday, May 17, 2005 4:14 PM
To: seaint(--nospam--at)
Subject: SEISMIC: Roof Supported Equipment


Because you evil Left-Coasters have foisted this upon us, I now have to foray into the "dark side" of structural engineering, calculating seismic forces.

My untutored brain does register grudging respect that the ICC (in reality, many of you SEAOCers) have done a creditable job digesting all this NEHRP stuff for me, but I'm still agog at the sheerness of the learning curve.

Anyway, I've got a situation that would be a no-brainer if it were just a building. But instead, I've got some rather massive equpment sitting on a steel platform which in turn is place on the ROOF of an existing building, and I want to determine what the effect of this will be.


Again, it will probably turn out to be nothing, but I'm undecided enough about it that I know I have to make the effort.

So instead of just a building with a site factor involved, I've got a building with a site factor, and then the equipment on top of that. Surely the building will act as a sort of giant energy absorber for the equipment such that the equipment seismic effect will be much diminished.

The building is a single (high-bay) story, all precast concrete.

Do I do this as a Two DOF system?

What would YOU do?

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