F.A.Q.
Some Frequently Asked Questions
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Is Performance Velocity Dependent?
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The performance of friction damper is displacement
dependent and independent of velocity. (FEMA specification 356/2000,
item 9.3.3). Similar explanations are made in many Guidelines
and textbooks (Passive Energy Dissipation Systems in Structural
Engineering by Professors Soong and Dargush, John Wileys Publication,
1997). However, friction for certain materials having low coefficients
of friction, such as Teflon, is velocity dependent. The fact that
Pall Friction Dampers are independent of velocity was confirmed
in the shake table studies in which several earthquake records
of various velocities were used. Scale tests were also conducted
at wide range of frequencies (velocity) to confirm that the slip
load is not significantly influenced.
In Pall Friction Dampers, the forces remain constant for all
future earthquakes at DBE and MCE level. A friction-damped structure
is an engineered structure in which forces exerted are decided
by the engineer and not by future earthquake. This is a great
technical and economic advantage over other dampers in which forces
vary with velocity and are much higher at
MCE level.
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| Creep / Relaxation of Bolts |
High-strength bolts
(ASTM A325/ A490) are being used in structural steel connections
for more than 50 years. Lehigh University has done considerable
research on the relaxation properties of these bolts. According
to their findings, the creep in the bolts is 7-8% over a period
of 80 years, 90% of which takes place in the first 10 years. In
practice, the future loss due to creep is partially compensated
by initially setting the slip load higher by 3-5%.
Paramatric studies have shown that variations in slip load up to
±25% of optimum does not affect the response significantly.
Hence, no adjustment to damper is necessary over the life of building.
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| Reliability of Friction |
| The common problems identified in friction
are: degradation due to wear; stick-slip and cold welding phenomenon;
environment and change in slip coefficient over long period of inactivity.
Developing a reliable friction is very difficult and tricky. Over
a period of more than a decade of research and development, these
problems were successfully overcome by using specially treated friction
surfaces and a unique manufacturing process. Pall Friction Dampers
slip smoothly without stick-slip and cold welding phenomenon. |
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| Testing
of Pall Friction Dampers |
Prototype testing and production testing is carried out for 20 cycles
in accordance with FEMA specifications. Each Pall Friction Damper
is load tested for a minimum of two cycles of reversals. Before
shipment, 10% of dampers from each batch are randomly selected and
retested. Our dampers meet or exceed the test criteria of FEMA specifications.
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| Periodic
Testing |
In 2000, a Pall Friction Damper was removed from a building after
10 years of installation and load tested by an independent authority.
The movement of the damper was smooth and chatter free. The variation
in slip load was -7.2%. The FEMA specifications allow variations
of ±15% at the time of manufacturing. The performance was
very satisfactory. |
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