One of the teaching units I enjoyed most in my physics classroom was on wave motions. Almost everything is capable of some sort of waving motion, or oscillation. The motions come in a wide range of frequencies and amplitudes dependent upon the object. Smaller objects tend to have high frequencies and small amplitudes. Large objects tend to have low frequencies and large amplitudes of motion. It is a fascinating field of study.
Some objects respond to an input of energy of some specific frequency and begin oscillating with the same frequency as the source. Their motion can grow in amplitude as the source of energy continues. A simple example is a pendulum with a child on a swing. Pushing the child at the right time inputs energy to drive the amplitude larger. The energy of drawing a violin bow across the strings of a violin sets some of the strings into vibrations that are large and produce sound. This response to the driving action goes by the name of resonance. A previous post of mine on resonance can be found here.
The resonance response can be so strong that large vibrating objects reach amplitudes that can be easily seen or felt. Between July and November 1940, the new suspension bridge across Tacoma Narrows responded to various strength winds by doing some gentle waving motions as motorists crossed it. Some called the bridge Galloping Gertie.
On 7 November 1940, a half mile long section of the suspension bridge was set into a waving motion none had seen before by a steady 42 mph wind from the side. It collapsed and fell into the narrows. The video of that event is a classic one used by physics teachers every year to dramatically demonstrate the effect of resonance.
Every year I showed that video I thought it would be fascinating to actually see the bridge location and the replacement constructed in 1950. I finally got to do that. While vacationing in Tacoma, we were staying only a few miles from the bridge. I had to go see it and walk out on it if possible. We found a small War Memorial park next to a parking lot a short distance from the highway leading to the bridge.
After crossing the highway, we were treated to this view. Today, there are two bridges. The one on the right is the 1950 replacement of the one which fell into the narrows. It carries one-way traffic across to the other side. The left bridge was opened in 2007 and carries one-way traffic the other direction. The walkway was only available on the new bridge.
The total shore-to-shore bridge length is 1 mile (1.6 km). It is 0.5 miles from the near to the far support towers. We walked to the halfway point of the left bridge. At the lowest point on the walk is where the steel suspension cables are attached.
The scale of the bridges is huge. The towers rise nearly 500 feet above the piers. Due to the high speed traffic flow, the noise was quite intense. It would have been good to have ear plugs. There was a lot of road dirt blown into our eyes, too.
We neared the center of the span and saw two workers walking on the cables doing some inspections.
Finally, we reached the middle. This intrepid explorer posed for the shot. I finally had my up close and personal experience with the Tacoma Narrows bridges. It was the final lesson for me about that fascinating physics event. I was happy.
Gertie doesn’t gallop any more. Engineers modified the structure to make it and future bridges safer. I didn’t get to be on the actual replacement bridge either. However, the view was excellent. This is toward the northwest from the center of the span.