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In the years to come, we could see structures equipped with hundreds of massive dampers, each containing MR fluid, designed to absorb seismic vibrations and ensure the stability of the building. The diagram below illustrates the operation of these dampers during an earthquake.
Photo courtesy Lord Corp.Unlike many of nature's unpredictable forces, earthquakes typically strike without warning. These powerful and catastrophic events can level cities in mere moments, leaving devastation and sorrow in their aftermath. Thankfully, engineering has come to the rescue with the tuned mass damper.
While most earthquakes are minor tremors, a single significant one can lead to billions of dollars in damages and countless lives lost. This ongoing threat has driven scientists to develop innovative technologies aimed at minimizing the destruction caused by seismic activity.
Understanding the Functionality of Tuned Mass Dampers
Considering the immense risk that earthquakes present, particularly to massive skyscrapers and long-span bridges, considerable resources have been dedicated to creating solutions capable of absorbing the violent energy released during seismic events. One of the most inventive approaches is the tuned mass damper (TMD).
A TMD is a device that employs a precisely tuned mass to counteract the vibrations of a structure. In doing so, it absorbs and disperses the energy that could otherwise lead to damage or collapse.
Imagine a child on a swing. When you push the swing, it begins to move back and forth. If you push the swing again when it's coming toward you, the motion is interrupted. This is the core concept behind TMDs. These essential damping devices are engineered to "push" against the movement of a structure during an earthquake, thus reducing excessive oscillation.
While a reliable tuned mass damper system is crucial, there is more to the engineering magic behind these devices, including the cutting-edge "smart fluid" that experts refer to as MR.
The Role of Magnetorheological Fluid
A key factor in the efficiency of mass dampers is a remarkable substance known as magnetorheological fluid (MR fluid). This fluid is utilized within large dampers to help stabilize buildings during earthquakes. MR fluid is unique in that it transforms into a near-solid state when exposed to a magnetic field, and reverts back to a liquid once the magnetic field is removed.
When an earthquake occurs, the MR fluid within the dampers transitions between solid and liquid states as tremors activate a magnetic force within the damper. Implementing these dampers in buildings and bridges will result in intelligent structures that autonomously respond to seismic events.
This will minimize the damage caused by earthquakes. In this edition of How Stuff WILL Work, we will explore MR fluid and its ability to alter its state. We will also examine how both new and existing buildings can be transformed into smart structures.
What is MR Fluid
Above, MR fluid before magnetization. Below, the fluid solidifies after being magnetized. Observe the glossy surface of the liquid in the top photo and the matte surface in the lower one.At first glance, MR fluid may not appear to be anything extraordinary. It looks like a gray, oily liquid, denser than water by about three times. While it doesn't capture much attention at first, MR fluid is surprisingly fascinating when observed in action.
In a straightforward demonstration by David Carlson, a physicist from the North Carolina lab, the liquid's remarkable transformation into a solid within milliseconds is clearly displayed. First, he pours the liquid into a cup and stirs it with a pencil to show its liquid state. Then, by placing a magnet at the bottom, the liquid instantly solidifies. To demonstrate its solidity, he tilts the cup upside down, and the MR fluid remains intact, not a drop spilling out.
MR fluid is made up of three essential components:
- Carbonyl Iron Particles -- Comprising 20 to 40 percent of the fluid, these small iron particles, measuring just 3 to 5 micrometers, have a consistency similar to black flour due to their fine size.
- A Carrier Liquid -- The iron particles are suspended in a liquid, typically hydrocarbon oil, though water is often used in demonstrations of the fluid's properties.
- Proprietary Additives -- The third ingredient is a closely guarded secret. These additives are designed to prevent the iron particles from settling under gravity, maintain particle suspension, improve lubrication, alter viscosity, and reduce wear.
How MR Fluid Works
What is it that grants MR fluid its remarkable ability to rapidly shift between liquid and solid states? The answer lies in the carbonyl iron particles. When exposed to a magnetic field, these microscopic particles align, causing the liquid to harden into a solid. This process occurs as the dc magnetic field forces the particles to adopt a uniform polarity. The rigidity of the substance depends on the strength of the magnetic field. Remove the magnet, and the particles immediately return to their original, unlocked state.
Although MR fluid has only recently found new applications, it has actually existed for over 50 years. Jacob Rabinow is recognized for discovering MR fluid in the 1940s while working at the U.S. National Bureau of Standards, now known as the National Institute of Standards and Technology.
Up until around 1990, MR fluid had limited applications due to the lack of precise control mechanisms. Today, advancements like digital signal processors and affordable, fast computers make it possible to regulate the magnetic field applied to the fluid. This technology is now used in various applications, including Nautilus exercise machines, washing machine dampers, car shock absorbers, and sophisticated leg prosthetics.
In the upcoming section, we will explore the seismic applications of MR technology, which could have the greatest potential in saving lives and preventing the collapse of buildings.
Buildings and Bridges
Tall skyscrapers, extensive overpasses, and pedestrian bridges are vulnerable to the resonance effects caused by high winds and seismic movements. To minimize these effects, it's crucial to integrate large dampers into the structure's design, which will interrupt these resonant waves. Without such devices, strong steel structures like buildings and bridges could be severely shaken or even collapse during an earthquake.
Dampers are commonly found in everyday devices such as car suspension systems and washing machines. For instance, according to the How Stuff Works article on washing machines, damping systems utilize friction to absorb some of the mechanical vibrations and reduce their impact.
Damping System Types
In buildings, damping systems are much larger in scale, designed to manage vibrations and absorb the intense shocks caused by earthquakes. The size of these dampers depends on the building's dimensions. There are three primary classifications for damping systems:
- Passive -- These dampers operate without external power, requiring no control to function. They're simple, cost-effective, but they lack the ability to adjust to changing conditions.
- Active -- Active dampers are force generators that directly apply force to the structure, counteracting disturbances. They are fully controllable but consume significant power.
- Semi-Active -- A hybrid of passive and active systems, semi-active dampers don't apply force directly but use a controlled resistive force to reduce movement. They are controllable, require minimal power, and unlike active dampers, they don't risk destabilizing the structure. MR fluid dampers are a form of semi-active device, adjusting their damping by varying the current supplied to an electromagnet that regulates the flow of MR fluid.
A full-sized MR fluid damper, measuring 1 meter in length and weighing 250 kilograms, can exert a force of 20 tons (200,000 N) on a building.
Photo courtesy of Lord Corp.How An MR Fluid Damper Works
Inside the MR fluid damper, an electromagnetic coil is wrapped around three sections of the piston. The main chamber of the damper is filled with about 5 liters of MR fluid. During an earthquake, sensors installed in the building trigger a signal to the computer, which then sends an electrical charge to the dampers. This charge magnetizes the coil, causing the MR fluid to transition from a liquid to a near-solid state.
As the building experiences vibrations, the electromagnet within the damper may pulse in response. This causes the MR fluid to shift between liquid and solid states thousands of times each second. This rapid switching may also lead to a rise in the fluid's temperature. To prevent dangerous pressure buildup, a thermal expansion accumulator is mounted at the top of the damper housing, allowing the fluid to expand safely as it heats up.
Buildings equipped with MR fluid dampers are designed to reduce vibrations during an earthquake.Depending on the building's size, it may feature an array of dampers, potentially numbering in the hundreds. Each damper is placed on the floor and connected to chevron braces that are welded into a steel cross beam.
As the building starts to tremble, the dampers will sway back and forth, counteracting the vibrations caused by the shock. When magnetized, the MR fluid increases the force the dampers can exert.
