The Tuned Mass Damper: How Science Could Earthquake-Proof the Skyscrapers of Tomorrow

Unlike many of nature's deadly forces, earthquakes almost always strike without warning. These destructive and devastating forces can topple cities in seconds, leaving behind rubble and tragedy in their wakes. Fortunately, engineering has answered the call with the tuned mass damper.

Although most earthquakes are just small tremors, it only takes one to cause millions of dollars in property damage and thousands of deaths. For this reason, scientists continue to pursue new technologies to limit the destruction that earthquakes can dish out.

How Tuned Mass Dampers Work

Given the significant threat that earthquakes pose, especially to towering skyscrapers and long-span bridges, a lot of energy and effort has gone into the development of solutions that can dissipate the violent energy released during seismic events. One such innovative solution is the tuned mass damper (TMD).

A TMD is a device that uses a tuned mass to counteract the oscillations of a structure. By doing so, it absorbs and dissipates the energy that could otherwise cause damage or destruction.

Imagine a child on a swing. If you push the swing, it starts to move back and forth. Now, if you try to push the swing again but at a time when it's coming back towards you, the swing's motion is disrupted. This is the basic principle behind TMDs. The essential damping devices are designed to "push" against the structure's movement during an earthquake, thereby reducing the oscillations.

A reliable tuned mass damper system is essential, but there's more magic to the engineering of these devices, including the innovative "smart fluid" known to the pros as MR.

The Role of Magentorheological Fluid

Much of the effectiveness of mass dampers is a unique substance called magnetorheological fluid (MR fluid). This fluid is used inside large dampers to stabilized buildings during earthquakes. MR fluid is a liquid that changes to a near-solid when exposed to a magnetic force, then back to liquid once the magnetic force is removed.

During an earthquake, MR fluid inside the dampers will change from solid to liquid and back as tremors activate a magnetic force inside the damper. Using these dampers in buildings and on bridges will create smart structures that automatically react to seismic activity.

This will limit the amount of damage caused by earthquakes. In this edition of How Stuff WILL Work, you will learn more about MR fluid and its ability to change states. We will also look at how buildings, new and old, can be turned into smart structures.

What is MR Fluid

Looking at it in a beaker, MR fluid doesn't seem like such a revolutionary substance. It's a gray, oily liquid that's about three times denser than water. It's not too exciting at first glance, but MR fluid is actually quite amazing to watch in action.

A simple demonstration by David Carlson, a physicist at the North Carolina lab, shows the liquid's ability to transform to solid in milliseconds. He pours the liquid into the cup and stirs it around with a pencil to show it's liquid. He then places a magnet to the bottom of the cup, and the liquid instantly turns to a near-solid. To further demonstrate that it's turned to a solid, he holds the cup upside down, and none of the MR fluid drops out.

Typical MR fluid consists of these three parts:

Carbonyl Iron Particles -- 20 to 40 percent of the fluid is made of these soft iron particles that are just 3 to 5 micrometers in diameter. A package of dry carbonyl iron particles looks like black flour because the particles are so fine.
A Carrier Liquid -- The iron particles are suspended in a liquid, usually hydrocarbon oil. Water is often used in demonstrating the fluid.
Proprietary Additives -- The third component of MR fluid is a secret, but Lord says these additives are put in to inhibit gravitational settling of the iron particles, promote particle suspension, enhance lubricity, modify viscosity and inhibit wear.

How MR Fluid Works

So, what is it that gives MR fluid its unique ability to transform from liquid to solid and from solid to liquid quicker than you can blink an eye? The carbonyl iron particles. When a magnet is applied to the liquid, these tiny particles line up to make the fluid stiffen into a solid. This is caused by the dc magnetic field, making the particles lock into a uniform polarity. How hard the substance becomes depends on the strength of the magnetic field. Take away the magnet, and the particles unlock immediately.

While scientists have just recently discovered many new applications for MR fluid, it has actually been around for more than 50 years. Jacob Rabinow is credited with discovering MR fluid in the 1940s while working at the U.S. National Bureau of Standards (now the National Institute of Standards and Technology).

Until about 1990, there were few applications for MR fluid because there was no way to properly control it. Today, there are digital signal processors and fast, cheap computers that can control the magnetic field applied to the fluid. Applications for this technology include Nautilus exercise equipment, clothes washing machine dampers, shock absorbers for cars and advanced leg prosthetics.

In the next section, we will look at the seismic applications of this MR technology, which may have the biggest impact on saving lives and preventing the collapse of buildings.

Buildings and Bridges

Tall buildings, long overpasses, and pedestrian 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, strong steel structures like 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 mechanical vibrations.

Damping System Types

A damping system in a building is much larger and is also designed to provide vibration control and 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.

**A full-scale MR fluid damper that is 1-meter long and weighs 250 kilograms. This one damper can exert 20 tons (200,000 N) of force on a building.**
Photo courtesy Lord Corp.

How An MR Fluid Damper Works

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 main chamber of the seismic damper. 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.

**Buildings equipped with MR fluid dampers will mitigate vibrations during an earthquake.**

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.