What is the response to the vertical component of an 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 their performance of the isolators.
Can a tall building be isolated?
Tall buildings such as the 18-story Oakland City Hall in California (left) 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.
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.
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.
Can more than one bearing be use 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.
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.
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.
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 (right).
What are the fire protection requirements for base isolators?
Out of more than 300 projects only three have required passive fire protection
due to being in a space with a fire load. The need for fire protect-ion
is determined by either the Fire Marshall or the Fire
Engineer.
Ninety-nine
percent of
our projects have not required fire protection as
shown in this
photo of The
Berkeley Public Safety Building in California (left).
In cases where fire protection was required the most cost-effective method was to
use fire-rated board materials over metal framing (middle right).
The cost of this type of fire-rated enclosure is in the cost range of a few hundred dollars.
The joint between the two sections is sealed with a simple intumescent seal.
Fire blankets (left) have also been used. DIS recommends the board-type solution as it is
generally more economical and is installed by on-site carpenters as part of the regular framing work.
Are isolators "fire-rated"?
Some isolation manufacturers have made claims that their product is "fire-rated". Such statements are
not supported by any building code and would need to be proven by testing in accordance with
ASTM E119.
DIS engaged Holmes Fire LP to assess such claims. Their findings are summarized below:
The code requires no rating of isolation systems in certain circumstances. Generally this is
due to the absence of any fire load, as is typical in a basement.
An isolator needs to be tested by a Nationally Recognized Testing Laboratory (NRTL) to determine its
compliance with a code. (DIS's understanding is that no isolation product meets
or can meet this
requirement).
A fire engineering assessment can rationalize or eliminate the need for fire protection.
Holmes' full report can be found in the PDF attached to this
link. We recommend that the Architect or the
project's Fire Consultant review Holmes Fire's assessment of code compliance.