Investigation of Auxiliary Damping Devices to Mitigate Dynamic Response of Tall Structures

Reported by Jon P. Wicke

Colorado State University Research Experiences for Undergraduates-Sound and Vibration-Summer 2003

Faculty Advisor: Bogusz Bienkiewicz

Special Thanks to: National Science Foundation and Army Research Office

Throughout the Twentieth Century, skyscrapers appeared throughout the planet, demonstrating both economic and technological advancements. It was quickly discovered that wind or seismic effects on these tall structures caused both comfort and safety concerns. In the latter part of the century, engineers and scientists developed methods to mitigate the dynamic response due to wind and or seismic events. Several devices were created to dampen the inherent vibrations. These damping devices range from simple, everyday items to instruments employing space aged fluid and high tech computers. This investigation includes research surrounding the state of the art of structural control, as well as an investigation into the future methods and devices used to dampen vibrations.

There are three basic classifications of control systems.

1. Passive Control- Devices such as tuned mass dampers and sloshing dampers that rely on inertial motion of the mass to counteract vibrations. These devices are self-sustained and usually cost effective.

2. Active Control- Devices that are monitored by computers, which evaluate dynamic responses and send feedback to the system, initiating damping motion. Active control systems are more expensive and efficient than passive dampers.

3. Semi-Active Control- Devices that combine characteristics of Passive and Active control systems.

What’s To Come?

The future of damping systems lies in semi-active controls. An example of a semi-active control system in use today is a magnetorheological damper used to mitigate motions of the Dongting Cable-stayed bridge in China. The device uses a space-aged “smart” fluid known as magnetorheological (MR) fluid to increase damping when necessary. The fluid is made up of millions of micron-sized iron particles suspended in special oil. When subjected to a magnetic field, the iron particles rearrange as “chains” along the field lines. The result is a fluid that’s motion is restricted by micron-sized walls within the fluid. Thus it is capable to change the consistency of the fluid from a liquid to a paste-like substance. The system requires very little voltage to exert the considerable change in the fluid.

The application to structures is quite simple. The damping device (as seen in the drawing) manufactured by the Lord Corporation, acts as a passive damper when no voltage is applied to the electromagnet. The sloshing of fluid back and forth mitigates vibrations due to sizable wind loading. However, when a severe wind or seismic event occurs, The magnetic field is applied to the device and the damping force of the fluid increases due to the ability to control the viscosity of the fluid.

Experimental setup showing swing and oscilloscope

Testing and Results

In order to investigate damping characteristics of different methods and substances, a one-degree of freedom swing was constructed. Four dishes of different substances were placed on the swing and tested one at a time. First, a dish half full of water was placed on the swing to imitate the behavior of a sloshing damper. Second, the same dish was tested with oil, a more viscous fluid. Third, a dish with oil and very small iron fillings was tested to imitate a MR fluid. Instead of investigating on a microscopic level, the iron fillings were manipulated with a magnet to arrange a wall in the center of the dish. The friction as the oil sloshed through and around the makeshift wall behaved comparable to, if not exactly like, an MR fluid. The fourth dish was empty with a weight attached that was equal to the weights of the other dishes. The swing was displaced with the same initial amplitude each time the test was run. A non-contact probe measured the distance the swing traveled back and forth after it was released. The results were then displayed on an oscilloscope and are seen in the below graphs.

Damping Device manufactured by the Lord Corporation

As can be seen, the different dampers behaved differently. When no damping device was present, the swing lost energy due to friction in the system. The sloshing water damper performed somewhat, decreasing the amplitude of each period. It is clear that the viscosity of the oil increases damping by quite a bit. The imitated MR damper slightly improved the damping performance, which illustrates that a semi-active control system is superior to passive control (sloshing damper).

For further information on MR dampers and their applications, visit www.rheonetic.com