Tacoma Narrows Bridge & Aeroelastic Fluttering By: Matthew Miller & Jonathan Haefner

What is Aeroelasticity? Aeroelasticity is the interaction among inertial, elastic, and aerodynamic forces. Aeroelasticity can create a potentially catastrophic phenomenon called flutter. A classic example of this phenomenon is the case of the Tacoma Narrows Bridge. Flutter occurs in every airplane flight. Engineers are always striving to design models of airplanes that better resist this phenomenon.

History of the Bridge The old Tacoma Narrows Bridge was located in Washington State on Route 16. The longest span of the bridge was 2,000 ft (853 m). It was opened on July 1, 1940. Just four months later it was destroyed on the date of November 7, 1940. The bridge was later rebuilt at the same place. Being more than a mile in total length, it was the third largest bridge in the world when it was built. The architect who designed the first bridge never designed another bridge.

What is Flutter? Flutter is a self-starting vibration that occurs when a lifting surface bends under an aerodynamic load. A vibration refers to any repetitive back and forth motion of particles. This bending creates violent compressions and rarefactions above and below the object forcing it to bounce up and down.

What Happened at the Tacoma Narrows Bridge? Flutter destroyed the Tacoma Narrows Bridge, however, people at the time believed that this was the work of resonance. The collapse of this bridge is a classic example of a longitudinal wave. It is true that no people died in this accident. The bridge was later rebuilt in 1950. It was engineered to easier resist this flutter.

What is Resonance? Resonance is the tendency of an object to vibrate with high amplitude when excited by energy at a certain frequency. This frequency is known as the object's natural frequency of vibration or resonant frequency. The characteristics of flutter and resonance are very similar, making many people believe it was resonance. The frequency of the flutter could have been measured in hertz, however, it would have been too dangerous to measure at the time.

Creative Ways to Stop the Crisis The destruction of the Tacoma Narrows Bridge could have been prevented if there was enough destructive interference. However, one would have to make sure there was not any further constructive interference. The destructive interference would cancel out the other wave, and the bridge would be saved. (Cancel meaning to make the wave flat line.)

Mechanical Waves The wave that destroyed the Tacoma Narrows Bridge was a mechanical wave. A mechanical wave is a wave that requires a medium to travel through. This kind of wave always travels in longitudinal form. The determining factors of how we perceive these waves are pitch, amplitude, and frequency. The power of the wave is determined by the wave speed, amplitude and phase. (Wave Speed = Frequency x Wave Length)

Interesting Facts As the Tacoma Narrows Bridge swayed back and forth, it increased its amplitude every sway until destruction. The winds at the time were forty miles per hour, however, the center of the bridge barely shook at all. The reason the center of the bridge wasn’t shaking was because that it was the node or resting point of the bridge.

It’s Quiz Time! 1.Q: What is aeroelasticity? 2.Q: What is flutter? 3.Q: Give two examples of flutter. 4.Q: Mechanical waves travel in what type of wave? 5.Q: What must there be for a longitudinal wave to travel? 6.Q: What is the tendency of an object to vibrate with high amplitude when excited by energy at a certain frequency? 7.Q: What could have stopped the crisis?

Answers!!!!!!!!!! 1. A: Aeroelasticity is the interaction among inertial, elastic, and aerodynamic forces. 2. A: Flutter is a self-starting vibration that occurs when a lifting surface bends under an aerodynamic load. 3. A: Airplanes, Tacoma Narrows Bridge. 4. A: Longitudinal. 5. A: A medium. 6. A: Resonance. 7. A: Destructive Interference.

Glossary Compression: A region of higher density or pressure in a wave. Constructive Interference: Interference that results in a wave that has a greater amplitude. Destructive Interference: Interference that results in a wave that has a smaller amplitude. Frequency: The number of waves over time (waves/time). Hertz: The unit used to express frequency; one hertz is one cycle per second. Longitudinal Wave: A wave in which the particles of the medium vibrate along the path of travel. Medium: A substance through which a wave can travel. Pitch: How high or low a sound is perceived to be. Rarefaction: A region of lower density or pressure in a wave. Wave Speed: The speed at which a wave travels.

Bibliography http://en.wikipedia.org/wiki/Mechanical_wave, “Mechanical Wave.” Wikipedia. Date Accessed 10/13/06. http://en.wikipedia.org/wiki/Resonance, “Resonance.” Wikipedia. Date Accessed 10/14/06. http://en.wikipedia.org/wiki/Vibration, “Vibration.” Wikipedia. Date Accessed 10/15/06. http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge, “Tacoma Narrows Bridge.” Wikipedia. Date Accessed 10/15/06. Holt, Rinehart, and Winston. Sound and Light. Austin: Cable News Network, 2000. 6-11.