| What are typical
design displacements? |
In high seismic
zones such San Francisco, Tokyo and Istanbul the
isolator displacements are up to 30 inches (750
mm). For structures located further from faults
or on better soil the isolator displacements are
up to 20 inches (500mm). In low seismic zones
such as the eastern United States, movements are
in the range of 2 to 6 inches (50 to 150 mm).
DIS has tested isolators to 47 inches of lateral
displacement and provides isolators for all seismic
zones worldwide. |
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| How
is the period of the structure shifted? |
The fundamental
period of the structure is shifted by the addition
of flexible isolators. The isolated period is
generally more than 2 seconds. The dominant frequencies
of an earthquake are in the 0.2 to 0.6 second
range. The severe accelerations of an earthquake
are avoided due to the period shift provided by
isolation (See diagram below). |
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| How is displacement
controlled? |
| The isolator
displacement is decreased by increasing its stiffness
or damping. The design trade-off is that forces
and accelerations increase as the displacement
is decreased. |
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| How does added
damping benefit the structure? |
Damping absorbs
earthquake energy. The addition of damping reduces
the displacements and forces in the superstructure
by as much as 50% (See Diagram
below). |
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| What is the
suggested level of damping in an isolation system? |
Most structures
have 2-5% inherent damping. Isolation systems
for bridges typically provide damping levels from
15 to 30%. Isolation systems for buildings have
damping in the 10 to 20% range. The building damping
levels are optimized to provide low accelerations
in the structure which maximize content protection. |
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| What is the
difference of the lead core yield strength at creep
loads and earthquake loads? |
The hysteretic
behavior of lead is dependent on the rate of loading.
The yield strength is lower at creep velocities
than at earthquake velocities. This is beneficial
especially in bridges where the isolator moves
over a range of velocities. During the high velocity
seismic motion, the yeild stress ranges from 10
to 14 MPa. providing significant levels of damping.
For thermal movements, the yield stress is in
the range of 4 to 6 MPa, which imposes small forces
on the structure. Intermediate values of lead
stress resist service loads such as wind and braking. |
|
Plotted here are the responses
of an isolator over a range of velocities
from one cycle in a day to
one cycle in twelve seconds. The lead
force at low velocity is 60% of that at
high velocity.
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| Can a tall building be
isolated? |
Tall
buildings such as the 18 story Oakland
City Hall in California have
benefited from isolation. Buildings
normally require the isolated period
to be 2.5 to 3 times that of the non-isolated
building. There are many tall buildings
isolated in Japan that have an isolated
period in the range of 4 to 6 seconds.
The designers chose isolation for
the better performance that it provides. |
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| Does the structure
re-center after an earthquake? |
A structure re-centers
after an earthquake because a restoring force
is provided by the rubber. The shaking characteristics
also make the structure oscillate at ever-decreasing
displacements about its original position as the
earthquake motion subsides. The Eel River Bridge
in California re-centered after a magnitude 7.0
earthquake to within 1/4 inch of its original
position. |
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| What is the
response to the vertical component of earthquake? |
Isolators are
stiff in the vertical direction and do not change
the vertical seismic response. The vertical component
of the earthquake results in axial load variations
which can be accommodated in the design of columns
and the isolators. Shake table tests have been
conducted with and without the vertical component
of the earthquake motion. The results indicate
that there is very little difference in the performance
of the isolators. |
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| What is the
design life of the bearings? |
| The normal design life of the bearing
is over 50 years. Elastomeric pads in highway bridges
have been in use for over four decades exhibiting
good durability. Isolators with modern rubber formulations
surrounded by a protective cover rubber are expected
to be more durable and stable in their long-term
performance. |
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| Can more than one bearing
be used under a column? |
Multiple
isolators have been used on San Francisco
City Hall and the Tan Tzu Medical
Center in Taiwan. Multiple isolators
are used when they are more economical
than one larger, single isolator. |
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| Groups of four isolators
are located under heavily-loaded column
in the Tan Tzu Medical Center. |
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| Can an isolator
resist tension forces? |
An allowable tensile
stress of up to 50 psi can be applied to an isolator.
The actual allowable stress depends on the displacement
of the isolator and the rubber modulus. In general,
tension is avoided in design. |
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| Does the lead
core fatigue? |
Lead is in its
elasto-plastic phase at ambient temperature. As
with other metals in this phase, lead re-crystallizes
rapidly after being deformed without fatigue. |
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| How
do utilities accommodate movement across
the isolation plane in the buildings? |
Utilities
that cross the seismic plane must
be detailed to move horizontally.
They often are made to be flexible
or are fitted with universal joints. |
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| How are stairways
detailed? |
Stairways and
access points are detailed to be fixed to the
superstructure and be "simply supported"
on the structure below the isolators. Small sliders
are sometimes used to support stairs and accommodate
lateral movements. |
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| How are elevators
accommodated? |
The bottom section
of the elevator is suspended from the superstructure
of the building. The framing cantilevers down
and is not supported by the substructure. Alternately
the plane of isolation can be lowered several
feet locally to allow the elevator pit to be isolated
as part of the superstructure. |
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| What
type of fire protection is needed for seismic
isolators? |
Fire protection
is dictated by the requirements for the
fire-space, not by the materials from which
the isolator is constructed. When isolators
are located in areas of the structure with
no fire load, fire protection is often not
required. When fire protection has been
required, then sprinklers, spray-on mineral
fiber, fire blankets and fire board enclosures
have been used. |
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Fire
board protects this
isolator under the
Kamikuzawa Condominiums. |
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Isolators
in the Long Beach 911 Center
required no fire protection. |
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Fire
blankets were used in Channing House. |
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