Fluid viscous dampers are used to control story drifts and member forces in structures during earthquake events. These elements provide satisfactory performance at the design-level or maximum considered earthquake. However, buildings using fluid viscous dampers have not been subjected to very large earthquakes with intensities greater than the design and maximum considered events. Furthermore, an extensive database of viscous damper performance during large seismic events does not exist. To address these issues, a comprehensive analytical and experimental investigation was conducted to determine the performance of damped structures subjected to large earthquakes. A critical component of this research was the development and verification of a detailed viscous damper mathematical model that incorporates limit states. The development of this model and the laboratory and simulation results conclude good correlation with the new model and the damper limit states and provide superior results compared with the typical damper model when considering near collapse evaluation of structures. 1281 Figure 4. Dampers used in the retrofit of a concrete-frame building.Figure 5. Physical parts of a viscous damper.the unit through small orifices. The mechanical construction and orifice properties can be varied to obtain the desirable damper properties. Figure 5 presents a line drawing of viscous dampers, identifying the pertinent components. Following is a description of each component [2].1. Piston rod: Solid 17-4 PH (precipitation hardening) stainless steel, billet machined, through hardened, and then hand-polished to a mirrored finish of less than 100 m (4 in).

Static and earthquake-simulator testing of two 230-kV, 3000A, Type-U transformer bushings manufactured in the mid 1980s by the General Electric (GE) Company was undertaken to evaluate both the seismic performance of bushings known to be vulnerable to damage from moderate earthquake shaking, and the efficacy of simple retrofit details. Both bushings survived earthquake shaking compatible with the IEEE spectrum for High Level qualification when mounted on a rigid frame. When mounted on a flexible frame, one 230-kV bushing was unable to sustain the High Level qualification shaking without oil leakage and slip of the porcelain units. Two retrofit details utilizing two semicircular ring plates and epoxy filler were prepared by utility representatives with the objective of limiting slip of the UPPER-1 porcelain unit over the gasket connection. The second detail, Ring-2, permitted the 230-kV bushing to sustain substantially larger accelerations and displacements than those of the unretrofitted bushing and eliminated the permanent slip of the UPPER-1 porcelain unit over the gasket connection for extreme earthquake shaking.

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The earthquake performance of 196-kV, 230-kV, and 550-kV porcelain transformer bushings was studied by earthquake-simulator experimentation and analysis. The fundamental frequencies of the 196-kV, 230-kV, and 550-kV bushings were identified to be approximately 15 Hz, 18 Hz, and 8 Hz, respectively, using resonance-search testing on the simulator. The corresponding damping ratios were between 2 and 4 percent of critical. The 196-kV and 230-kV bushings survived severe earthquake shaking and were qualified to the High Level per the IEEE 693-1997 standard. The discrepancy between excellent behavior in the laboratory and poor behavior in the field of these types of bushings calls into question the procedures used for qualification of substation equipment. The 550-kV bushing survived shaking at the moderate performance level with limited damage and minor oil leakage. Current procedures for fragility testing of transformer bushings were evaluated and found to be neither appropriate nor conservative.

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