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RE: Pedestrian Bridges

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Someone else mentioned this before, but pay close attention to vibration. A
local pedestrian bridge, already designed and being fabricated, was later
discovered to have major problems with pedestrian vibration, that is, the
case where pedestrians tend to "march" in unison creating a natural
resonance. Of course, the span of this bridge is much longer as it goes over
an interstate, and may even be cable supported. But I know besides a lot of
Maalox, that poor engineer had a nice claim to help pay for the extra
stiffeners. However, this is FL and I believe it was at first to be a
completely open bridge, but then they changed it to have an overhead type
structure to provide the necessary stiffness. Since you bridge sounds
completely enclosed you will have the advantage of a box-like structure that
should make it more then stiff enough to handle any vibrations. BUT, my
understanding of vibrations and natural resonance is quite limited, so it is
likely much more complicated then that!

Another quick example, besides Ole Galoping Gurdy (more of a wind vibration
problem), is the Millenium Bridge (I think that is it) in London. This was a
multi-million dollar state of the art pedestrian bridge over the Thames, and
was designed by one of the big boys. Well, it opened up to great fanfare,
but was soon closed down because the lightweight structure began vibrating
with the unified footsteps of the pedestrians. I believe the bridge was not
just made out of steel, I think it was a composite? This is all from memory,
I am sure someone else knows more about it or you can check the web. Anyway,
I think they closed it down, studied the problem, and installed a tuned
damper (not cheap).

Andrew D. Kester, EI
Structural Engineer
Bentley Architects & Engineers
665 W. Warren Ave.
Longwood, FL 32750

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