Posted on 11/07/2019 6:02:09 PM PST by Rebelbase
[snip] On November 7, high winds buffeted the area and the bridge swayed considerably. The first failure came at about 11 a.m., when concrete dropped from the road surface. Just minutes later, a 600-foot section of the bridge broke free. By this time, the bridge was being tossed back and forth wildly.
At one time, the elevation of the sidewalk on one side of the bridge was 28 feet above that of the sidewalk on the other side. Even though the bridge towers were made of strong structural carbon steel, the bridge proved no match for the violent movement, and collapsed.
(Excerpt) Read more at history.com ...
https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)
"The bridge's collapse had a lasting effect on and engineering. In many physics textbooks, the event is presented as an example of elementary forced resonance; the bridge collapsed because normal speed winds produced aeroelastic flutter that matched the bridge's natural frequency."
My college prof did an entire class on this in Physics 101 and rewound the film projector many times to watch it over and over again.
A film projector, remember those?
Did the designer go to engineering school
Learned about this in college. The bridge attained its resonant frequency and it was doomed.
My Grandma’s name was Gertrude and she gave that to my Mom as her middle name. So “Galloping Gertie” had special family meaning!
During WW II my dad was a platoon leader in the Marines doing some training at Cornell University. He liked to tell me about marching his platoon across the suspension bridge over the gorges in campus — without breaking cadence! They would hit the natural frequency of the bridge and get it swinging.
All great fun! Dad was majoring in mechanical engineering, too. He was probably demonstrating natural frequency to his men.
Not being an engineering major, what does “natural frequency of the bridge” mean?
Some years ago I took a course in differential equations at North Dakota State University. Excellent math professor. We used “Differential Equations and Their Applications,” by Martin Braun. He added this to the story about the Tacoma Narrows Bridge. (page 123)
“After the collapse of the Tacoma Bridge, the governor of the state of Washington made an emotional speech, in which he declared “We are going to build the exact same bridge exactly as before.” Upon hearing this, the noted engineer Von Karman sent a telegram to the governor stating “If you build the exact same brdge exactly as before, it will fallinto the exact same river exactly as before.”
engineering schools used to always make sure that every engineering student saw this video ... don’t know if they still do ...
Aerodynamic flutter considerations were not taken to account at the time it was designed. All that was required was the bridge had to withstand wind speeds up to a given amount.
P.S. All suspension bridges after that were built to be much more rigid.
Controls guys learn about the Therac 25.
Every physical object has a “resonant frequency” or a “natural frequency.” It’s when a combination of the mass of an object and its springiness combine such that it will start moving when “excited” at that frequency. Each cycle of excitation adds more energy to the object. Think of pushing a child on a swing where you impart just a little more energy with each cycle and pretty soon you’ve added so much energy the child is way up high.
Ever hear an object start buzzing or rattling in your house when a particular note is played on your music system? That’s the natural frequency of that object and it is being “excited” by that musical note.
My dad built spacecraft for a living. They would put the finished satellites in a giant “shake table” and sweep through all the frequencies a rocket engine on the booster would create. He took me in for such a test and, when they hit the natural frequency of the stowed solar panels, they started flapping like bird wings. They had to make sure the satellite would survive launch and the stowed panels would not get destroyed or hit the rocket housing walls. It was an i credible sight!
I heard once that the engineers who designed the bridge were from another part of the country and were unfamiliar with the winds where the bridge was built. The replacement bridge had openings along the sides that allowed the wind to pass through without those sides acting as sails. Not sure if that’s totally accurate but that’s how I remember it anyway!
Why does the bridge sway, sway, sway and then fall down?
I saw a program on that bridge just a day or two ago. It is still an uncanny sight. Surprisingly it kept up the swaying for an hour giving people time to leave the bridge.
No one was injured.
My Father’s WWII Battalion was the 208th Engineer Combat Battalion. I have their entire very long history. One thing which caught my attention was the Commanding Officer, Lt. Col Leslie O. Scott.
Early in the war he got irritated by something and announced there would be a week long session on proper bridge building. From that history it was clear that Col. Scott was a real engineer, not just somebody who had worked in construction.
OK, thanks for the explanation, being a radio guy, I was only familiar with radio frequencies.
It’s been a long time since structures class, but I’ll try. Any structure, from a simple beam to a complex bridge or building, can be ‘excited’ by vibration. At most frequencies the structure will seem ‘stiff’ and won’t bend much. But if the structure has frequencies of vibration where it’s not stiff, those frequencies are called natural frequencies of that structure. If the structure is excited at a natural frequency it will begin to vibrate and the amplitude of those vibrations can grow until the structure actually fails, or breaks. In the case of Tacoma narrows, the bridge had a poor design with a low natural frequency mode, and the high wind was able to excite that mode to the point of very dramatic bridge failure. It might have only taken some additional bracing in the design to make the bridge stiffer at that frequency. Hope this makes sense.
It does, thank you!
The bridge is resonating to a natural frequency forcing a mechanical action.
Gravity took that thing down.
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