Download Base isolation technique in structural design

Base isolation technique in structural design
Even as we are living in an era when science and technology is at their best, we still
cannot predict earthquakes accurately. Strong earthquakes could easily destroy
everything built in years within minutes. To critical facilities such as data centers,
Internet collocation facilities, hospitals, and air traffic control centers, the sensitivity
of expensive equipment to vibration is very significant so the earthquake hazards
present a pressing concern to these facilities. Until science comes up with a better
earthquake prediction technology, the civil industry has to rely on better building
designs against potential earthquakes. In recent years, base isolation has become an
increasingly popular structural design technique for buildings and bridges in highly
seismic areas. Many types of structures have been constructed using this approach,
and many others are in the design phase or under construction.
There are two basic types of isolation systems. The widely-adopted system in recent
years is through the use of elastomeric bearings. The elastomer is usually made of
natural rubber. In elastomeric bearings approach, the structure is decoupled from the
horizontal components of the earthquake motion by interposing a layer with low
horizontal stiffness between the ground and the structure. This layer lowers the
frequencies of isolated structures compared to fixed-base structures. More
importantly, the frequencies of isolated structures are much lower than the
predominant frequencies of the ground motion. The isolation system does not absorb
the earthquake energy, but rather deflects it through the dynamics of the system.
The diagram below provides a visual representation of two different structures: fixed
base and elastomeric bearing isolation. A base isolated structure is supported by a
series of bearing pads which are placed between the building and the building's
foundation. A variety of different types of base isolation bearing pads have now been
used, which includes rolling type bearings.
Base
isolation
bearings
Fixed-base structures
Base isolation structures
The second type of isolation system is through the use of sliding system. The
approach works by limiting the transfer of shear across the isolation interface. Many
sliding systems have been proposed and some have been used. Some of the
applications in sliding isolation systems include using specially selected sand in
sliding interface (China) and lead-bronze plate sliding on stainless steel with an
elastomeric bearing (South Africa). The friction-pendulum system is also a sliding
system using a special interfacial material sliding on stainless steel and has been used
for several projects in the United States.
Rolling type bearings application for equipment isolation
The sensitivity of expensive modern equipment to seismic vibration as discussed
above has led to consideration of equipment isolation. Manufacturing platforms,
computer servers, optical equipments, antiques and so on are high-value instruments.
The cost of these instruments might be much higher than structure itself. Adding
additional energy dissipation devices to these mechanics can’t be a reasonable
solution to resist seismic excitations. Modifying other company’s products may have
some technical and legal issues. Taking mechanics apart could lose warranties and the
right of maintenance. Most of these mechanics are protected by patents. Using base
isolation technology to protect these mechanics could be an appropriate solution to
resist seismic excitations thanks to low cost and non-modifying designs.
The principle in rolling type bearing is simple. The following sketch shows a visual
representation of the rolling type mechanism. In this sketch, the roller is laid on a
slope. If the roller leaves its equilibrium position, it can go back to the minimal
potential energy position due to the weight coming from top loading. The mechanism
of rolling type bearings will minimise the seismic excitation on the interested
instruments.
One of the commercial applications of rolling type bearings has been introduced by
WorkSafe technology. The WorkSafe base isolation technology is claimed to be the
most effective way to protect mission-critical and expensive electronic equipment.
WorkSafe uses a patented geometry-specific isolation bearing to decouple the
equipment (or any structure) from damaging ground motions. The bearing is 100%
gravity restoring which means that after the shaking stops, it will smoothly return to
its original location. The bearing also works as a white-noise filter virtually
eliminating the transfer of seismic vibrations into sensitive components.
If all other peripheral systems, (such as power), stay up and running, the isolated
equipment could continue to operate and process data right on through a major
earthquake.
Reference:
 “Engineering village” online database, Elsevier Engineering Information Inc.,
Hoboken, New Jersey, U.S.A.
 http://www.worksafetech.com