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How to Defend Bridges Against Earthquakes
Transcript of How to Defend Bridges Against Earthquakes
Bridge? We choose the cable stayed bridge because our length is 6/10 mile long. Our choices were limited to cable stay and suspension bridge. Examining the structures of both types of bridges, we noticed that the cable stay would fit our
needs the best! The reasons why we chose the cable-stayed bridge over every other type of bridge, particularly the suspension bridge, is because of how how the cable-stayed bridge performs in an earthquake. What is an earthquake? A satisfactory definition of an earthquake is that they occur from the movement of tectonic plates, and occur along the fault lines of the Earth. Let us discuss why they are these movements cause earthquakes, and why they are so dangerous. Viscous Damper -Heat generated by the Earth's core moves the plates.
-The molten rock, heated by the core rises up and cools down to form crust.
-This newly formed crust competes for space with the already existing crust.
-It's because of this that the plates grind against one another, or slip under one another, causing stress to build up and eventually be released.
-The energy released by an earthquake is comparable to 3,000 Hydrogen-bombs. Viscous Damper Viscous Damper One type is where the faults slide past each other in opposite directions, side by side.
The other type is where one fault is sliding under another fault. The one on top pushes against the surface of the fault that is sub ducting. The amount of stress built up by this surpasses grinding of faults several times... Viscous Damper There Are Two Types Plate Tectonics In an earthquake, the ground is shaken violently. This causes:
1. The ground to become liquified.
2. The towers to move along the sand, causing it to dig in and have fall over, eventually
3. The deck to shake with the tower, and possibly snap under the pressure. Solution #4 Solution #1 Solution #2 Solution #3 Why Earthquakes Are A Problem The way to stop the sand from turning into
quicksand is simple: steel rods. By embedding
steel rods into the sand, where the bottom of the towers will rest, the sand will not liquify when the ground shakes. With the rods in place, the towers of our bridge won't sink into the bottom of the lake To stop the ground from digging into the sand, as it moves, gravel can be substituted as the foundation for the base of the tower. Gravel is too large for the tower to be able to dig into. By suspending the deck of the bridge at the top of the tower, the deck can swing freely, as a pendulum independent from the tower, so it won't buckle
and break. Consequence to
Solution #3 Because the deck can swing from side to side, if the earthquake shakes the entire bridge enough, the deck will smash into
the the arms of the tower. If the tower falls so does the deck, under it's own weight. So the way to stop this is through viscous damping. Viscous damping requires the use of a liquid and two moving parts. You slather this liquid (in this case oil) unto two the two parts, and as the piston moves through the cylinder it hits the viscous fluid and produces friction: thus, some of the kinetic energy is turned into heat energy. This allows the deck to swing, but not too far to either side. Another problem that mother nature plagues unto our bridge is wind. When the wind blows past the cables of our bridge, a natural phenomenon happens: vortex shedding. Vortex shedding happens when air passes through a cylindrical object (such as cables). As the wind wraps around the cylinder, it creates other vortices that jostle the cylinder, from side to side and eventually makes the cylinder fall. Mother Nature's Other Problem This Would Be That Type What is Vortex Shedding? Pictures of Helical Strakes The Final Result With all the measures we have taken, our bridge can brave any earthquake, and come out still standing strong. We are very proud of the work we've done and we hope you have all enjoyed this presentation, and learned something in the process Summary To sum up, we have taken several steps in making sure that our bridge won't be destroyed in an earthquake. We have accomplished this by 4 measures:
1. Allowing the bottom of the tower to move safely, with gravel and a series of steel rods.
2. Having the deck swing independently from the rest the tower, by suspending it completely.
3. Using viscous dampers to make sure that the deck doesn't swing too far to either side.
4. Using helical strakes to offset vortex shedding, so that our cables don't fail.