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Double Off-Grid Method Is Faster
Steve Marusich, senior engineer with San Francisco-based
Forell Elsesser, the company serving as the structural engineer
on the seismic retrofit of the 80-year-old Pasadena City Hall,
calls the job unique because it uses a "double off-grid"
type of isolation system.
He said in standard base-isolation projects, columns are
shored up, followed by the demolition of footings. Then a
new footing is built and the isolator installed, followed
by the existing column being lowered down onto the new isolator,
so that the columns are directly above the isolator.
A drawback to this method is only a few columns in an area
at a time can be worked on, because "you don't want too
much of the building to be supported by temporary means,"
Marusich said.
"But on this project, we have moved the isolators off
of the outside of the columns, so basically they sit on new
concrete transfer beams that then transfer that load from
the column into the isolators.
"The advantage of doing this is that we can install
the new foundation isolators, and a lot of the framing that
supports it, at one time, which helps speed the construction.
And then you can demolish the columns at a later date."
-Greg Aragon
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