Some Frequently Asked Questions


Is Performance Velocity Dependent?

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.

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.
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.
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.
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.