Reply #13: it all depends

[melm00se]

a structure may be built to support 5000 people under normal circumstances but if they all start to move in synch and hit the harmonics of the structure you have a recipe for a disaster.

the harmonic issue is of sufficient concern that many buildings have harmonic dampers to prevent this issue.

See: http://en.wikipedia.org/wiki/Tuned_mass_damper#Dampers_in_buildings_and_related_structures, specifically:

Mechanical human sources

Masses of people walking up and down stairs at once, or great numbers of people stomping in unison, can cause serious problems in large structures like stadiums if those structures lack damping measures. Vibration caused by heavy industrial machinery, generators and diesel engines can also pose problems to structural integrity, especially if mounted on a steel structure or floor. Large ocean going vessels may employ tuned mass dampers to isolate the vessel from its engine vibration.

but even dampers can be ineffective if the mass is large enough.