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RE: Rooftop Seismic Anchorage

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I would agree that the 2.0 increase should not be applied to the entire assembly, but just to the anchorage using the wedge anchors.  Aren’t you still limited to a max of 4.0 Ca Ip Wp at strength level?  Your ASD force is over 4.0 Wp?  Just checking…


David A. Topete, SE


From: Gerard Madden, SE [mailto:gmse4603(--nospam--at)]
Sent: Tuesday, June 12, 2007 10:37 AM
To: seaint(--nospam--at)
Subject: Rooftop Seismic Anchorage


Got a small rooftop fan that is required to be seismically anchored to the roof deck. Simple enough.

The unit is supported on Vibration Isolators, so that kicks my Ap=2.5 and Rp=1.5

The Vibration Isolators are fastened to wood sleepers and the wood sleepers are bolted to the concrete fill O/ metal deck roof with Hilti Kwik Bolt 3's (expansion anchors).

Using expansion anchors requires the force being doubled per the footnotes in CBC Table 16-0

All said and done, this leads to an ASD force level of 4.19wp  (or 4.19 g )

I'm just wondering if I have to apply such a high level of lateral load to the entire assembly or just once it leaves the vibration isolator. I'm just hoping to reduce the Overturning moment by not applying the 2.0 increase except at the sleeper to roof deck connection where the expansion anchors are being used..