provide vertical vibration isolation for mechanical
thereby protecting other structural elements from
equipment, and hydraulic damping devices that
damage.
utilize fluid flow through an orifice to provide shock
isolation for military hardware.
Many of these
(2) Active control systems. These systems
devices have been in use for decades and have
provide seismic protection by imposing forces on a
performed well in situations where they are subjected
structure that counter-balance the earthquake-induced
to millions of cycles of loading; many more than
forces. These systems are active in that they require
would be required for seismic resistance.
The
an energy source and computer-controlled actuators
immediate challenge is therefore not to develop new
to operate special braces or tuned-mass dampers
technologies, but to develop guidelines that will
located throughout the building. Active systems are
enable
us
to
adapt
existing
technologies
to
more complex than passive systems, since they rely
civil/structural engineering applications.
on computer control, motion sensors, feedback
mechanisms, and moving parts that may require
c.
Historical Overviews of Building
service or maintenance. In addition, these systems
Applications.
Several types of isolation and
need an emergency power source to ensure that they
supplemental damping systems have previously been
will operate during a major earthquake and any
used in building structures to solve problems related
immediate aftershocks.
to vertical vibrations or wind loading. For example, a
building in London is located on isolators in order to
(3) Hybrid control systems. These systems
damp vibrations from the London Underground; the
combine features of both passive and active control
World Trade Center Towers in New York City were
systems.
In general, they have reduced power
built with a system of viscoelastic dampers in order
demands, improved reliability, and reduced cost
to alleviate human discomfort due to wind loading.
when compared to fully active systems. In the future,
The use of passive energy dissipation systems for
these systems may include variable friction dampers,
seismic design is a relatively recent development,
variable viscous dampers, and semi-active isolation
although there are now examples of these systems
bearings.
throughout the world (EERI 1990).
b.
Mechanical Engineering Applications. It is
(1) Applications Outside the U.S. Beginning
important to note that the passive energy dissipation
in the early 1970s, a number of bridge structures in
systems described here are "new" technologies when
New Zealand were constructed using seismic-
applied to civil engineering structures, but have been
isolation systems.
The first building structure
used in mechanical engineering for many years.
constructed using lead-rubber bearings was a
There are numerous situations where dampers,
government facility completed in Wellington, New
springs, torsion bars, or elastomeric bearings have
Zealand in 1981. The most widespread use of both
been used to control vibration or alter the dynamic
seismic isolation and energy dissipation systems is in
behavior of mechanical systems. Several examples
Japan, where over a hundred structures have been
include vehicular shock absorbers, spring mounts that
8-3
Previous Page
