# Until the Tacoma Bridge disaster, self resonance in bridges was unknown? **URL:** \ **Category:** Factual Questions\ **Created:** [September 30, 2019, 6:53pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058 "2019-09-30T18:53:03Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![EinsteinsHund](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/einsteinshund/32/8400_2.png) [@EinsteinsHund](https://boards.straightdope.com/u/EinsteinsHund)\ **Post date:** [September 30, 2019, 6:53pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/1 "2019-09-30T18:53:03Z")
I’m watching a documentary about the history of bridge construction. At one point, they showed the [Tacoma Bridge collapse](https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)) and noted “Until this disaster, the phenomenon of self resonance in bridges was unknown”. WTF? This was in 1940, 25 years after freaking general relativity, and mechanical self resonance was unknown? Or it was known, but not for bridges? The mind boggles, so I console myself that a TV program once again told bullshit. Please set me straight. (any architects around?) ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [September 30, 2019, 6:59pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/2 "2019-09-30T18:59:33Z")
> [@EinsteinsHund](#): > > WTF? This was in 1940, 25 years after freaking general relativity, and mechanical self resonance was unknown? Or it was known, but not for bridges? Hard to say without seeing the documentary, but “unknown” might mean “never been observed in a bridge” rather than “never conceived of in principle”. Perhaps the era of the Tacoma Narrows Bridge was about the time when bridges started getting light and long enough for this kind of thing to happen. ---
**Author:** ![Dewey\_Finn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dewey_finn/32/4222_2.png) [@Dewey\_Finn](https://boards.straightdope.com/u/Dewey_Finn)\ **Post date:** [September 30, 2019, 7:04pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/3 "2019-09-30T19:04:40Z")
A [suspension bridge in France](https://en.wikipedia.org/wiki/Angers_Bridge) collapsed in 1850 partly due to self-resonance. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 30, 2019, 7:07pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/4 "2019-09-30T19:07:04Z")
[Mechanical resonance was known](https://en.wikipedia.org/wiki/Broughton_Suspension_Bridge). This particular effect based on wind and vibration probably wasn’t seen before. Long span bridges that lightweight may not have been that common before then. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [September 30, 2019, 7:07pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/5 "2019-09-30T19:07:15Z")
The “resonance” explanation is the _‘pop-science avoid the math’_ explanation and it is more complex than that. It wasn’t mechanical self resonance that really brought down the bridge it was _flutter_ and the effects that are studied under a field called bridge aerodynamics-aeroelastics. Aeroelastic fluttering also caused crashes of the L-188 Electra and other issues. You are talking about non-linear dynamical systems involving fluid dynamics which is not a “solved” portion of physics. That said they had found a fix and approved it two days before the Tacoma Narrow bridge collapsed. Even today CFD is in the realm of [supercomputers for numerical approximations.](https://www.nasa.gov/ames/image-feature/nasa-highlights-simulations-at-supercomputing-conference-like-aircraft-landing-gear) ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [September 30, 2019, 7:25pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/6 "2019-09-30T19:25:29Z")
Adding to **rat avatar’s** post: This is a good article about it: > **[Science Busts The Biggest Myth Ever About Why Bridges Collapse](https://www.forbes.com/sites/startswithabang/2017/05/24/science-busts-the-biggest-myth-ever-about-why-bridges-collapse/#18401b541f4c)** > > If you've ever thought the Tacoma Narrows Bridge collapsed due to resonance, you've fallen for the same myth that scientists did. ---
**Author:** ![EinsteinsHund](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/einsteinshund/32/8400_2.png) [@EinsteinsHund](https://boards.straightdope.com/u/EinsteinsHund)\ **Post date:** [September 30, 2019, 7:26pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/7 "2019-09-30T19:26:40Z")
Ok, thanks to all for the quick responses. I thought I was losing my mind. So it’s the typical case of a TV show dumbing it down and fucking up their statements of “fact”. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [September 30, 2019, 7:28pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/8 "2019-09-30T19:28:33Z")
Like **rat avatar** says the cause of the collapse was not “(self) resonance” but self- **excitation** or [aeroelastic flutter](https://en.wikipedia.org/wiki/Aeroelasticity#Flutter). [Resonance, Tacoma Narrows bridge failure, and undergraduate physics textbooks](https://aapt.scitation.org/doi/10.1119/1.16590) Here is a [NASA PDF](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960004074.pdf) about the history of flutter in airplanes. The U.S. first started to study that in any systematic manner in 1941 (after the bridge disaster). So yes, understanding of flutter is relatively recent, started less than 100 years ago, and is still not fully solved. Flutter testing is still a scary part of new aircraft testing today and still leads to headaches and design changes like this [Boeing 747-8 airworthiness directive](https://airwaysmag.com/industry/faa-to-issue-airworthiness-directive-for-boeing-747-8-wing-flutter/) from 2015. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [September 30, 2019, 7:34pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/9 "2019-09-30T19:34:29Z")
> [@leahcim](#): > > Perhaps the era of the Tacoma Narrows Bridge was about the time when bridges started getting light and long enough for this kind of thing to happen. This, sort of and in part. Everything about its design made it unusually elastic, considerably more so than previous suspension bridges. It was only two lanes wide, and the structure under the roadway was only 8 feet deep. Contrast this with (e.g.) the Golden Gate Bridge, which was six lanes wide and featured an road deck structure that was ~25 feet deep, much more rigid. There was also the use of plate girders to form the structure of the road deck, rather than open trusses, leading to high aerodynamic loads in crosswinds. Again, contrast with the GG bridge, which used an open-truss design. Finally, the particular problem that these factors contributed to was actually aeroelastic flutter. This is a bit different from simple resonant forcing, in that the distortion of the bridge deck from its neutral position resulted in increased aerodynamic load. This meant that the wind hit the right speed, the amplitude of the oscillations built up very rapidly to destructive levels. AIUI, it wasn’t simple resonance that was unknown in bridges at the time - it was aeroelastic flutter, which hadn’t been anything for bridge designers to previously consider. It’s harder to predict mathematically, and where bridges are concerned, it had never been an issue before since nobody had previously build a bridge that was so long and flexible and with large aerodynamic surfaces. Ninja’d by **Frankenstein Monster** , but posting anyway. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [September 30, 2019, 7:45pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/10 "2019-09-30T19:45:10Z")
I was going to ask how the disaster impacted the engineer’s reputation and career, “unknown” effects or not, but he died of a heart attack not that long afterwards and never had the chance to build another bridge. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [September 30, 2019, 8:00pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/11 "2019-09-30T20:00:50Z")
Regarding the OP, I seem to recall that the Roman Army had commands to ‘break stride’ when marching over bridges, way back in the First Century BC. So presumably they already knew that this phenomena could occur. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [September 30, 2019, 8:19pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/12 "2019-09-30T20:19:05Z")
Roman engineers knew some stuff, but I doubt anybody had made a study of complicated aerodynamics. Though if they did I would love a reference. ---
**Author:** ![Rick\_Kitchen](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rick_kitchen/32/522_2.png) [@Rick\_Kitchen](https://boards.straightdope.com/u/Rick_Kitchen)\ **Post date:** [September 30, 2019, 8:21pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/13 "2019-09-30T20:21:20Z")
Didn’t ancient armies have to be told not to march in step when walking over bridges? ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 30, 2019, 8:32pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/14 "2019-09-30T20:32:14Z")
> [@Tim\_T-Bonham.net](#): > > Regarding the OP, I seem to recall that the Roman Army had commands to ‘break stride’ when marching over bridges, way back in the First Century BC. So presumably they already knew that this phenomena could occur. > [@Rick\_Kitchen](#): > > Didn’t ancient armies have to be told not to march in step when walking over bridges? I have heard of ancient references to this but only ever seen the [Broughton Suspension Bridge collapse](https://en.wikipedia.org/wiki/Broughton_Suspension_Bridge)actually cited. This was probably not an issue for the relatively short spans made of stone or wood in ancient bridges. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [September 30, 2019, 9:12pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/15 "2019-09-30T21:12:41Z")
It would make sense for an army to propagate the semi-myth of a footbridge collapsing due to resonance to make sure soldiers avoid stepping in sync, for the simple fact that it feels terrible when you’re doing it. I’ve never stepped across a bridge in formation myself, but at the swinging bridge at Tom Sawyer’s Island at Disney World, if you step more than a few steps in sync with yourself it feels like you’re jumping in one of those bouncy castles. ---
**Author:** ![Novelty\_Bobble](https://avatars.discourse-cdn.com/v4/letter/n/a3d4f5/32.png) [@Novelty\_Bobble](https://boards.straightdope.com/u/Novelty_Bobble)\ **Post date:** [September 30, 2019, 10:25pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/16 "2019-09-30T22:25:49Z")
There was a very prominant example of this for the [Millenium Bridge](https://en.wikipedia.org/wiki/Millennium_Bridge,_London)in London. > [@](#): > > The bridge’s movements were caused by a ‘positive feedback’ phenomenon, known as synchronous lateral excitation. The natural sway motion of people walking caused small sideways oscillations in the bridge, which in turn caused people on the bridge to sway in step, increasing the amplitude of the bridge oscillations and continually reinforcing the effect.[6][7] On the day of opening, the bridge was crossed by 90,000 people, with up to 2,000 on the bridge at any one time. ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [September 30, 2019, 10:35pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/17 "2019-09-30T22:35:29Z")
[Video of Tacoma Narrows collapse](https://www.youtube.com/watch?v=j-zczJXSxnw). Turn off the sound. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [September 30, 2019, 11:35pm UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/18 "2019-09-30T23:35:50Z")
> [@Chefguy](#): > > [Video of Tacoma Narrows collapse](https://www.youtube.com/watch?v=j-zczJXSxnw). Turn off the sound. He wasn’t able to get the dog out. ☹ ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [October 1, 2019, 10:43am UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/19 "2019-10-01T10:43:13Z")
> [@Novelty\_Bobble](#): > > There was a very prominant example of this for the [Millenium Bridge](https://en.wikipedia.org/wiki/Millennium_Bridge,_London)in London. [Video of bridge oscillations here.](https://www.youtube.com/watch?v=eAXVa__XWZ8) ---
**Author:** ![DorkVader](https://avatars.discourse-cdn.com/v4/letter/d/73ab20/32.png) [@DorkVader](https://boards.straightdope.com/u/DorkVader)\ **Post date:** [October 1, 2019, 10:59am UTC](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058/20 "2019-10-01T10:59:03Z")
> [@Chefguy](#): > > [Video of Tacoma Narrows collapse](https://www.youtube.com/watch?v=j-zczJXSxnw). Turn off the sound. Thats always a fascinating thing to watch, simply because at a gut level it is so counterintuitive for that much steel and concrete to have that much flexibility or any really. [Next page](https://boards.straightdope.com/t/until-the-tacoma-bridge-disaster-self-resonance-in-bridges-was-unknown/841058.md?page=2)