I’m watching a documentary about the history of bridge construction. At one point, they showed the Tacoma Bridge collapse 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?)
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.
A suspension bridge in France collapsed in 1850 partly due to self-resonance.
Mechanical resonance was known. 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.
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.
Adding to rat avatar’s post:
This is a good article about it:
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”.
Like rat avatar says the cause of the collapse was not “(self) resonance” but self-excitation or aeroelastic flutter.
Resonance, Tacoma Narrows bridge failure, and undergraduate physics textbooks
Here is a NASA 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 from 2015.
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.
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.
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.
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.
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 collapseactually cited. This was probably not an issue for the relatively short spans made of stone or wood in ancient bridges.
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.
There was a very prominant example of this for the Millenium Bridgein London.
Video of Tacoma Narrows collapse. Turn off the sound.
He wasn’t able to get the dog out. 
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.