I have a problem with “resonant vibration” as a failure causation in the collapse of this bridge, be it vibration caused by jackhammers or cement mixers or railroad locomotives.
Stop in a traffic jam on nearly any highway bridge and you can easily feel the deck flex as trucks roll on and off the bridge. This is normal, and well within design limits.
When the Tacoma Narrows Bridge failed due to resonant vibration, it had been subject to oscillations up to 30 plus vertical feet in amplitude for several hours before ANY structural failure took place.
Bridges are expected to vibrate and engineers take steps to dampen high amplitude oscillations well before those oscillations can fail the bridge.
If any survivors or eyewitnesses were reporting continuing oscillations, even large enough to be visible or easily felt, then yes, resonant vibration, exceeding bridge design limits, would necessarily require consideration.
In the absence of any such reports, with the sole exception of one reporter claiming construction workers mentioned “wobble”, and refused to repeat these comments to MnDOT interviewers after the collapse, it’s very difficult to justify claims that resonant vibration failed the bridge.
Tesla well understood that the mechanical advantage offered by resonant vibration was its inherent ability to produce high amplitude oscillation is short periods of time, and with little applied energy added with each successive wavefront. It’s a mechanism which sums many low amplitude deflections into large amplitude oscillations. It’s not an effect that hides inside a structure’s molecular structure. If the vibration is so small as to be imperceptible, then it is probably well within normal design limits.
Loss of structural strength due to long exposure to small amplitude deflections falls under the heading of metal fatigue, not resonant vibration.
If a jackhammer’s impact causes a weld to crack or steel itself to crack, again, this isn’t resonant vibration as much as it is simple energy transfer through impact. The key element of resonant vibration is wavefronts which reflect and combine to create high amplitude oscillation.
It could be argued that ultrasonic waves can shatter brittle materials such as glass, or possibly even cast iron, but I don’t see this happening in comparatively ductile structural steel, and in any event, these types of oscillations are unlikely to be generated by trains, cement mixers, or jackhammers.
I won’t rule out resonant vibration in this failure, but it’s a long way away from the first place I’d look.
I’m saying more as a cause for initiation of the terminal failure event, not as a cause for the overall failure.
It’s pretty clear from the pics in that report, and a quick skim of the text, that this bridge has been in the process of failing for many years. I was just speculating on a not-unlikely cause for the trigger for the final catastrophic failure.
The flexing caused by trucks is likely to be at a very different frequency from a jackhammer. Even though the energy from the latter might be much lower than from trucks, if it, say, hit a resonant frequency in a structural member with a pre-existing crack, the jackhammering might be the final proximate reason for a catastrophic crack propagation and bridge failure.
Even if this speculation is correct, it would only be the metaphorical “blasting cap” added to the “large pile of dynamite” represented by years of maintenance neglect on the structure.