Interesting article about the Brooklyn Bridge swaying under the thousands of people crossing it during the blackout. Suspension bridges are designed to sway, but people were actually feeling seasick and weaving as they walked. There\’s also this interesting bit of trivia:

Contrary to logic, pedestrian traffic is actually heavier than vehicular traffic. Pack people into the same square footage of a car or SUV, and the humans weigh more than the vehicle.

When people cross a bridge they vibrate it in a chaotic manner that engineers can\’t design for.