This rustic bridge is popular with fishermen during the Salmon season from Summer through Fall. The Duwamish supports chinook, coho, chum and steelhead, as well as the more rare sockeye, sea-run cutthroat trout and bull trout. Pink salmon run in the millions every odd-numbered years in recent history.
I recently rode my bike across the bridge and stopped in the middle. I then jumped up and down on the bridge and recorded the resulting response using my Android smartphone accelerometer. The time history response in the the vertical axis is shown in the figure below. The sharp spikes occurred as my feet struck the bridge. The curve-fit for the free-decay reveals a natural frequency of 1.9 Hz with 2.7% damping.
Accelerometer App link
– Tom Irvine