Skyscrapers and long bridges are susceptible to resonance created by high winds and seismic activity. In order to mitigate the resonance effect, it is important to build large dampers into their design to interrupt the resonant waves. If these devices are not in place, buildings and bridges can be shaken to the ground, as is witnessed anytime an earthquake happens.
Dampers are used in machines that you likely use every day, including car suspension systems and clothes washing machines. If you take a look the How Stuff Works article on washing machines, you'll learn that damping systems use friction to absorb some of the force from vibrations. A damping system in a building is much larger and is also designed to absorb the violent shocks of an earthquake. The size of the dampers depend on the size of the building. There are three classifications for dampening systems:
- Passive -- This is an uncontrolled damper, which requires no input power to operate. They are simple and generally low in cost but unable to adapt to changing needs.
- Active -- Active dampers are force generators that actively push on the structure to counteract a disturbance. They are fully controllable and require a great deal of power.
- Semi-Active -- Combines features of passive and active damping. Rather than push on the structure they counteract motion with a controlled resistive force to reduce motion. They are fully controllable yet require little input power. Unlike active devices they do not have the potential to go out of control and destabilize the structure. MR fluid dampers are semi-active devices that change their damping level by varying the amount of current supplied to an internal electromagnet that controls the flow of MR fluid.
Inside the MR fluid damper, an electromagnetic coil is wrapped around three sections of the piston. Approximately 5 liters of MR fluid is used to fill the damper's main chamber. During an earthquake, sensors attached to the building will signal the computer to supply the dampers with an electrical charge. This electrical charge then magnetizes the coil, turning the MR fluid from a liquid to a near-solid. Now, the electromagnet will likely pulse as the vibrations ripple through the building. This vibration will cause the MR fluid to change from liquid to solid thousands of times per second, and may cause the temperature of the fluid to rise. A thermal expansion accumulator is fixed to the top of the damper housing to allow for the expansion of the fluid as it heats up. This accumulator prevents a dangerous rise in pressure as the fluid expands.
Depending on the size of the building, there could be an array of possibly hundreds of dampers. Each damper would sit on the floor and be attached to the chevron braces that are welded into a steel cross beam. As the building begins to shake, the dampers would move back and forth to compensate for the vibration of the shock. When it's magnetized, the MR fluid increases the amount of force that the dampers can exert.