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Frontiers
Smarter Than the Average Load-Bearing Structure
July/August 1998
Progress in civil engineering today means making
not just better buildings, but smarter ones. Ask Linda Hanagan, civil
engineer at the University of Miami in Florida. Hanagan came to the aid
of a data entry office whose structurally sound but lightweight floor
vibrated significantly as people walked, causing computer screens to bounce.
To deal with the problem, Hanagan installed an experimental system that
can be mounted underneath the floor. The system, developed with NSF support,
includes an electromagnetic shaker about the size of a two-drawer filing
cabinet. A sensor on the floor sends signals to a computer, which then
causes a 66-pound mass on the shaker to move up or down, counteracting
the floor's vibrations.
The solution is based on what civil engineers call damping force in proportion
to the velocity of the system. In this case, and at other test sites,
Hanagan's shaker reduced floor vibration to less than a quarter of the
previous magnitude.
Ken Chong, program director in NSF's Directorate for Engineering, says
that the United States, Japan and Western European countries are leading
the way in research on smart materials and systems.
ACTIVE CONTROLS: A MIXED BLESSING?
Hanagan's device is an example of what is called an active control system
because it adds energy to the system. Some civil engineers worry that,
if not exactly calibrated, such devices could actually contribute to destabilization.
That's why B.F. Spencer, Jr. of the University of Notre Dame in Indiana
is experimenting with a semi-active control system.
"With these systems," says Spencer, whose work relies in part on NSF
funding, "you control the way in which the devices resist the motion of
the structure. Generation of forces is not required." One example is the
variable orifice damper, which works like an intelligent shock absorber.
In a regular shock absorber, a piston moves back and forth in a cylinder
in response to motion. This shifts fluid in the damper through a fixed
orifice in a way that provides the damping effect. Variable orifice dampers
respond to data from sensors placed along the structure by mechanically
opening or closing the orifice through which the fluid runs, thus adjusting
the rate of damping to suit the amount of motion. Such a device may extend
the life of a bridge reverberating with traffic or a skyscraper buffeted
by winds.
SMART MATERIALS TO THE RESCUE
However, mechanical valves are vulnerable to weather and wear.
So Spencer, along with Michael K. Sain, an electrical engineer at Notre
Dame, is working with a new kind of controllable damper, one that uses a
smart material called magnetorheological (MR) fluid.
An MR fluid can reversibly change from a free-flowing to a semi-solid
state within milliseconds upon exposure to a magnetic field.
In Spencer's damper, the piston passes through a compartment
containing MR fluid. An electromagnet around the piston head
responds to the motion data from the sensors by generating a
greater or lesser magnetic field, changing the viscosity of the
MR fluid and, thus, the rate at which the piston moves. These
developments are part of a joint effort with the Lord Corporation
of Cary, North Carolina.
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