Oscillation and resonance

Systems tend to oscillate with different amplitudes at different frequency. This variation in Osciallation is called resonance. Resonance can occur in various types of systems such as Mechanical, acoustic, electromagnetic, and nuclear systems. Resonance in relation to bridges is referred to as mechanical resonance. Resonance occurs when a system easily transfers energy into different forms, such as kinetic and potential. In mechanical resonance, a system tends to have a certain frequency at with it will store more energy, meaning that it will oscillate more violently. This is called the systems natural frequency.

 

This image above demonstrates oscillation at different amplitutdes at different frequencies. The peak that goes off the chart is the natural resonance. If a system can achieve this, it will break down.

On November 7th, 1940 the Tacoma Narrows Bridge collapsed. This collapsed was a result of violent oscillations in the bridge. When it was build, the engineers made it solid, and didn't provide for any air to get around the bridge. Instead the wind was diverted above and below the bridge.

 

Shortly after the completion of the bridge, it was discovered that the bridge would osciallte when mild winds occured. The engineers at the time figured that the mass of the bridge would keep the bridge in place. The problem is that oscillations in a system don't occur linearly. When the resonance of a system is close to the systems natural frequency, then the amplitude of the osciallations become much greater. The real trouble in the system came when it entered a "torsional vibration mode". This means that the left side of the bridge would go up while the right went down. In other words, the bridge would twist. When this occured, the center of the bridge was unaffected, and two men even walked accrossed the center of the brdige unaffected.