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Re: More problems with the 1997 UBC

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Gary:

Your understanding of the 1997 UBC provisions for your equipment hung from a
concrete slab and supported by vibration isolation springs is essentially
correct.  This is a dynamic problem involving flexibly-supported equipment at
the roof of a flexiblwe building.  A few thoughts that might help your
situation.

1) If you want one design that will work on all floors, you have to use the roof
value and must expect the design force to be higher.  If you provide two or
three designs for different elevations within your building, you can use Formula
32-2 to provide lower design forces lower in the building.

2) Section 1632 allows you to rationally evaluate the value of a-sub-p.  If you
know the fundamental frequency of the building , and the fundamental frequency
and damping ratio for your vibration-isolated equipment, you may be able to
reduce a-sub-p from 2.5.

3) Expansion anchors have a history of "failure" during earthquakes and deserve
the design force penalty.  You don't seem to be considering undercut anchors in
your evaluation.  They are very good for tension applications.

4) Even in your worst case evaluation, your seismic design force is only 3.02
Kips.  This doesn't seem like a very high number to design to.  If you are using
structural steel supports, your minimum AISC ASD connection load is 6 kips.  As
for the anchorage to the concrete, a couple of undercut anchors will easily take
the load.  And you may need more than two anchors anyway because of the geometry
of your details.

There will be no revisions or supplements to the 1997 UBC.  The next code we
will be dealing with is the 2000 IBC, already to the printers.  This code will
have supplements.  Since you are obviously interested in improving the code, I
suggest you get a copy of the 2000 IBC, and take a stab at suggested revised
wording.  There is a published procedure for proposing revisions to the IBC.
The SEAOC State Seismology committee, besides initiating proposals to the IBC,
is also in a position to facilitate proposals of individual SEAOC members.

Regards,

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA