N.B. When you were a child, and were on a swing, you always would pump your legs at a certain part of the cycle, and you would swing higher if you pushed at the right point. That was an application of resonant frequency. Most things (swings, bridges, cars) have at least one, though many engineered projects nowadays are designed to have several so that they cancel each other out. Part of the reason for this was the first Tacoma Narrows Bridge (also known as Galloping Gertie.) The lightweight suspension bridge, spanning a wind-filled gorge, was noted for its bounciness. Some drivers got seasick from the crossing. Alas, the novelty didn't last long; within a few months of its opening, the windspeed was just right and the bridge tore itself to pieces. The film of its destruction is a common sight in frosh-level engineering courses.

The footbridge at the Bowl and Pitcher in Spokane has a resonant frequency quite near the timing of a human stride. And now you know what started this whole mess.