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Re: roto-hammers, masonry & city of LA

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In a message dated 98-01-18 16:48:33 EST, Tom VanDorpe, P.E. wrote:

<< f I am not mistaken, Anaheim Stadium had some very serious problems with
this
 song in the early '70s due to concert fans bouncing to the music.  The
 solution at that time was to attach cables to  the cantilevered portion of
the
 club level  in order to pull the cantilevered end down a bit and hold it in
 tension.   >>


[Bill Cain]  In the late '70's, when I was working for the Office of the State
Architect, I was called on to evaluate the balcony at the San Jose Center for
the Performing Arts for vibrations occurring during dance performances.  The
problem was encountered when a Greek dance troup was performing and the
audience started moving in time with the music producing uncomfortable
vibrations.

The balcony is a bridge type structure with two large plate girders spanning
about 125 feet. The balcony capacity is about 750 people. Its period of
vibration was somewhere in the 1/2 second range.  I remember standing in the
middle of the balcony and jumping at approximately the structure period.
After about 6 to 8 jumps, the resonance was sufficient to make me queasy and I
don't get motion sickness easily.

We did some testing to verify the period and damping.  The damping as I
recall, was something less than 1/2% when we pulled a tension at the center
and released it rapidly.  Our analysis showed that it was unlikely that
dangerous levels of vibration could be produced by an active audience but the
vibrations became very uncomfortable quickly.  

The City of San Jose did further investigations on ways to solve the problem.
I don't know what, if anything, they came up with.  I know they quickly
rejected the idea of adding columns under the plate girders at midspan   :<).

Bill Cain, SE
Albany, CA