Why does the bridge shake when a car passes over it?
2 Answers
Because resonance occurs when vehicles pass over the bridge. Below are the relevant explanations: 1. Resonance solution: Since vehicles passing over the bridge generate resonance, which in severe cases can lead to bridge collapse, modern bridge designs use flexible structures instead of the previous rigid structures. 2. Resonance: Resonance, also known as "harmonic oscillation" in mechanics and electronics, refers to the phenomenon where an object's intensity is amplified due to resonance. For example, when two identical springs are connected, if one undergoes simple harmonic motion, the other will also perform simple harmonic motion. One of the key conditions for resonance is elasticity, and when an object is subjected to an external frequency, its frequency must match or be very close to the latter's frequency.
Every time I drive over a bridge, I feel the deck swaying, which I believe stems from the bridge's physical characteristics. The bridge deck has elasticity, much like a spring. When a car presses down on it, the weight causes the bridge to bend downward, and when released, it springs back up. This repeated action creates vibrations. The faster the speed or the heavier the vehicle, the greater the force applied, resulting in more noticeable vibrations. Another key point is resonance: if the bridge's natural frequency aligns closely with the vehicle's movement frequency, the vibrations can be amplified several times. For example, when heavy trucks travel at a specific speed, they can cause the bridge to shake violently. The bridge's material also plays a role; steel bridges are more prone to movement than concrete ones, and older bridges may vibrate more due to structural looseness. Modern bridge designs take these factors into account, incorporating dampers to absorb energy and reduce fluctuations, but some slight swaying is normal. This experience is common when crossing suspension bridges on highways.
