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How the Bridge Vibrates
Suspension bridges vibrate during earthquakes and strong winds. This 1:500 scale model of the Golden Gate Bridge displays vibration patterns (also known as vibration modes) inherent in suspension bridges. Chain is used to model the cables and segmented pieces are used to model the roadway deck, so the vibration patterns are exaggerated and more easily seen.
Swaying in the wind
On the model, push the CENTER SPAN area to one side and release.
This is the fundamental transverse vibration mode. The Golden Gate Bridge takes about 20 seconds to complete one back-and-forth swing. The amount of time to complete one back-and-forth cycle is the period of vibration. Very large and heavy structures like the Golden Gate Bridge move slowly and therefore have long periods of vibration. The model is smaller and lighter than the actual Bridge, so it vibrates faster. When steady westerly winds blow across the Bridge deck, they push the deck to the east, usually bending or bowing a few feet, or about a meter. For very strong winds, the deck is designed to bow horizontally at the middle of the center span 27.7 feet (8.4 meters).
Wind Flutter
Twist and pull down on the CENTER SPAN area.
Notice how the deck on either side twists in the opposite direction. Twist and pull down on the deck section labeled 1/4 SPAN. Notice how the deck on the opposite side twists in the other direction. This pattern of twisting, or torsional vibration mode, led to the collapse of the Tacoma Narrows Bridge in 1940.
Vertical Vibration
Press down on the CENTER SPAN and quickly release it.
Notice the deck oscillates or vibrates up and down about 2 times per second. This is the fundamental vertical vibration mode of this model. The Golden Gate Bridge, 500 times larger, takes about 8 seconds to complete one cycle of this oscillation.
Earthquake
Shake the ANCHORAGE area back and forth.
Notice how waves travel along the deck. Try shaking faster and slower. Which rate makes the bridge model move more? Bridges respond or resonate more intensely to earthquakes when the ground shaking motion matches the period of vibration of the bridge. |