People stepping up and down should not be a problem, and even the 1-Hertz sideways back-and-forth movement of humans walking should not have been a problem, as everyone is likely to be stepping at different times. For anyone pushing with their right foot, another person would be pushing with their left, and all the forces would pretty much cancel each other out. This sideways resonance would only be a problem if enough people walked perfectly in step. This is the synchronous in synchronous lateral excitation from pedestrians. On the Millennium Bridge, people did start to walk in step, because the movement of the bridge affected the rhythm at which they were walking. This formed a feedback loop: people stepping in sync caused the bridge to move more, and the bridge moving caused more people to step in sync. Humble Pi: A Comedy of Maths Errors