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Structure sabotage

Measures the natural frequency of a structure to replicate that frequency with an actuator
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A device could be installed on a metal beam that would measure the natural frequency of it by striking it with a solenoid and measuring the output using strain gauges. Following the initial measurement, the device could keep striking the structure at the measured frequency until it collapsed.

Ideally, the response would be continuously measured to insure that the striking frequency is just.

snus, Feb 15 2008

Tesla's Earthquake Machine http://www.excluded....com/earthquake.htm
[jutta, Feb 15 2008]

Stump Shaker Stump_20Shaker
This was my version. I see Tesla got there first. If only I had a Manhattan address and a moustache. [bungston, Feb 15 2008]

Vibrations of different sorts and structures http://www.sciencen.../20071124/bob10.asp
Things fall down all the time. [bungston, Feb 15 2008]

Annotated Mythbusters: Tesla's machine http://kwc.org/myth...hquake_machine.html
DIdn't work. But I don't think they properly measured the resonant frequencies of the thing they were attached to - they just tried different frequencies. [jutta, Feb 15 2008]

Wikipedia: Mechanical resonance http://en.wikipedia...echanical_resonance
The phenomenon at the root of this effect. [jutta, Feb 15 2008]

xkcd webcomic http://xkcd.com/228/
another application [Gamma48, Jun 06 2009]

[link]






       I have a bad feeling that i'm going to be inciting some arguing here, but here goes:
This won't work. Also, if it were to work, this implementation would not be effective (solenoid/strain gauges). I know it sounds good, and all the stories and urban legends say it will, but it won't. For several reasons, a few of which are:
1)Most structures are more fluid than they appear, the idea of a single "Resonance frequency" is science fiction at best. Parts of the building would vibrate. not much would happen. You might be able to do something to a backyard shed, maybe make it bounce around, but not a cement and steel building.
2) if you managed to find an effective way to induce a standing wave in the building, it would take a tremendous amount of force to make it destructive. I hesitate to even mention this because i really can't think of a way to do this in a properly designed** building.
3) implementation alone: strain gauges are wildly sensitive to all motion. people walking around in your building, wind loading, trucks driving by... would all show up on it and cloud your results.

I would be remiss if I did not go into the finer details of mechanical resonance (what you're attempting to use in your evil plan). The idea involves putting a small amount of energy into a building at a time interval such that every time the energy is imparted, it is cumulative instead of subtractive. This requires a few things: 1) that the energy is not dissipated at a rate faster than it is added (it will be) 2) that the timing is perfectly in synch to accumulate the power 3) that the increase in energy does not change the properties of the structure in such a way that it causes a change in the way the structure reacts to the energy (it will).
Sorry, dude, you're going to have to stick to explosives.

**Yes, Yes, Tacoma Narrows bridge. A) A tremendous amount of wind energy was necessary to take this down, not a bouncing solenoid. B) Clearly, it was not properly designed.
ericscottf, Feb 15 2008
  

       /1) that the energy is not dissipated at a rate faster than it is added/   

       That, right there, is why this will fail. The ability of a large structure to absorb energy through structural damping is far beyond what you can impart with anything less than a wrecking ball. [-]
david_scothern, Feb 15 2008
  

       It is not rare for vibrations to wreck structures. Bridges are at a special disadvantage. Science News had a recent review of the phenomenon, linked. One would think that there would have been failures of brick warehourses in the early part of the 1900s related to machinery. I know considerable trouble is taken to dampen vibrations produced by these machines - before anything falls, anyone nearby can see that the vibrations are causing trouble. An example: our new upstaires front loader on high spin - it shook the entire house alarmingly. Rubber dampers under the feet made that stop.   

       Solidly built structures fall all the time from earthquakes. More loosely built structures dampen the vibrations and survive.
bungston, Feb 15 2008
  

       All buildings will have a resonant frequency based on their mass. It will be very low (<1Hz) and require vast input energy. Harmonics may be used to trigger this, but to less effect.
wagster, Feb 15 2008
  

       this was done on Mythbusters
simonj, Jun 06 2009
  

       I'm with [wagster] the device would need to impart enough energy with every oscillation to overcome the natural damping forces present in the system. If you can't overcome friction all you are going to produce is diffuse localized heating. I imagine that for solid structures this would require quite a lot of energy for quite a lot of time.
WcW, Jun 06 2009
  

       I agree...a building is a fairly heavily damped oscillator, so I'm thinking this is bad science.
sninctown, Jun 06 2009
  
      
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