35312 Friction Dampers

Friction dampers utilize the mechanism of solid friction that develops between two solid bodies sliding relative to one another to provide the desired energy dissipation. Several types of friction dampers have been developed for the purpose of improving seismic response of structures. In some cases, additional research on the long-term behavior of frictional systems may be appropriate.

A simple brake lining frictional system was studied by Pall et al. (1980); however, a special damper mechanism, devised by Pall and Marsh (1982), and depicted in Figure 35.14, permits much more effective operation. During cyclic loading, the mechanism tends to straighten buckled braces and also enforces slippage in both tensile and compressive directions.

Several alternate friction damper designs have also been proposed in the literature. For example, Roik et al. (1988) discuss the use of three-stage friction-grip elements. A simple conceptual design, the slotted bolted connection (SBC), was investigated by FitzGerald et al. (1989) and Grigorian et al. (1993). Another design of a friction damper is the energy dissipating restraint (EDR) manufactured by Fluor Daniel, Inc. There are several novel aspects of the EDR that combine to produce very different response characteristics. A detailed presentation of the design and its performance is provided in Nims et al. (1993).

In the last decade, there have been several commercial applications of friction dampers aimed at providing enhanced seismic protection of new and retrofitted structures. This activity in North America is primarily associated with the use of Pall friction devices in Canada. For example, the applications of friction dampers to the McConnel Library of the Concordia University in Montreal, Canada, is discussed in Pall and Pall (1993). A total of 143 dampers were employed in this case. Interestingly, the architects chose to expose 60 of the dampers to view due to their esthetic appeal. A series of nonlinear DRAIN-TABS (Guendeman-Israel and Powell 1977) analyses were utilized to establish the optimum slip load for the devices, which ranges from 600 to 700 kN depending upon the location within the structure. For the three-dimensional time-history analyses, artificial aseismic signals were generated with a wide range of frequency contents and a peak ground acceleration scaled to 0.18g to represent expected ground motion in Montreal. Under this level of excitation, an estimate of the equivalent damping ratio for the structure with frictional devices is approximately 50%. In addition, for this library complex, the use of the friction dampers resulted in a net savings of 1.5% of the total building cost.

Slip joint with ^ friction pad

FIGURE 35.14 X braced friction damper (Pall and Marsh 1982).

Friction Damper

Slip joint with ^ friction pad

FIGURE 35.14 X braced friction damper (Pall and Marsh 1982).

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