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I thought it was to avoid the possibility of resonant freuency of the bridge being triggered. A good modern example was the millenium bridge on the Thames, the oscillations were increased as people started to sync with the already present oscillations due to poor design. It was a strange experience when I was on it as it was hard not to move in sympathy with it or even stand. Half an hour later the bridge was closed.
The most famous example has to be the Tahoma Bridge incident in the US around 1940. The oscillation was due to wind but the same effect can be achieved if many people are walking in step.
Am I right in asuming this? I noticed on Chelsea bridge there is a sign telling troops to break step when crossing.
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