Understanding the Tacoma Bridge Collapse: Causes and Differences in Resonance

[Ukitake Jyuushirou]

i remember in high school my physics teacher used the example of tacoma bridge to teach us resonance. what exactly is it that caused tacoma to collapse?

how is forced resonanace different from normal resonance?

 

[FredGarvin]

https://www.physicsforums.com/showthread.php?t=62264

 

[Ukitake Jyuushirou]

thank u very much

another question, in an earthquake does resonance cause buildings to collapse?

 

[Cliff_J]

Well, the real problem is movement and its effects on the loading. If a structure like the Tacoma Narrow Bridge are constructed with a resonance frequency that is similar throughout the structure, then it will not damp motion well and instead allow it to amplify motion with the external excitement. If you search on modern building techniques used in Japan (and elsewhere) you'll find a few examples of active or passive damping systems to control movement of a building when an outside force is applied, either from wind or an earthquake.

There are a lot of other reasons for structural collapse in an earthquake and the type of earthquake too. It could be liquifaction of the soil (ground acts like liquid) or the geography of the area like Mexico City that amplifies seismic waves like a bowl of jello. Or even more common is likely just plain poor building techniques and materials not designed to handle the loading from the movement.

 

[Ukitake Jyuushirou]

i am interested in this resonance phenomenon which i do not understand much about.

i understand that resonance is the tendency of a system to absorb more energy when the frequency of the oscillations matches the system's natural frequency of vibration.

does this mean that all humans have a resonant frequency?

how far reaching is the effects of resonance?

 

[Cliff_J]

Think of simple harmonic motion (SHM) of a small weight and a spring. If the spring is held vertically supporting the weight below it and you pull the weight down a little, it will spring back. However, it doesn't stop at the starting point, it goes right past, then back down to close to where you pulled it, and back up past the starting point...and so on. Add more weight and it will happen more slowly, add more spring and it will happen faster. Place the weight/spring sideways on a table where there is friction, and now it will damp more quickly, add some oil and it will continue longer. Et cetera.

If you knock your knuckles on your desk it makes a sound, if you knock your knuckles on a door it will make a different sound, and if you knock your knuckles on the a musical drum it will also sound different. So your knuckles aren't a very good sound producer, but those surfaces are good at making sound to certain degrees. When you knock on them, you deliver a sharp pulse to them, and this pulse in effect excites all frequencies. Yet you only hear some of them - why? Because of the resonance of each surface, and the damping is how long they 'ring' after you strike them.

Musical instruments are based on resonance, think of a guitar string. By placing your fingers at different spots on the fretboard, you are changing both the spring and mass ratio of the string (like SHM), and hence its fundamental vibration frequency.

Human bodies don't really work well for resonance, the body overall is too well damped. However, vocal chords are a part of the body in tension to add more spring, and along with low mass and they do resonate very well to make audible sounds. Also, if you were to fart, it has its own sound, again caused by a resonance in that area.