Ian D. Aiken

Testing of Passive Energy Dissipation Systems

Over the period 1986 to 1991, seven different passive energy dissipation systems were studied in experimental research programs at the Earthquake Engineering Research Center of the University of California at Berkeley. This paper presents an overview of these studies, describing the different types of devices, the results of the shake table experiments, and associated analytical work. Four of the systems studied are friction systems, and of these, three (Sumitomo, Pall, and Friction-Slip) are based on Coulomb friction. The fourth is the Fluor-Daniel Energy Dissipating Restraint, which is a device capable of providing self-centering friction resistance that is proportional to displacement. The three other systems all have different energy dissipation mechanisms: ADAS elements, which utilize the yielding of mild-steel X-plates; viscoelastic shear dampers using a 3M acrylic copolymer as the dissipative element; and Nickel-Titanium alloy shape-memory devices that take advantage of reversible, stress-induced phase changes in the alloy to dissipate energy. The effectiveness of the various systems is evaluated by comparing the response of the test structures without and with the energy dissipators. In some cases, where devices were studied using the same test structure, they are compared directly. All of the systems investigated exhibited characteristics beneficial to improved structural response to earthquake loading.

AN ANALYTICAL HYSTERESIS MODEL FOR ELASTOMERIC SEISMIC ISOLATION BEARINGS

For the purpose of accurately predicting the seismic response of base-isolated structures, an analytical hysteresis model for elastomeric seismic isolation bearings is proposed. An extensive series of experimental tests of four types of seismic isolation bearings—two types of high-damping rubber bearings, one type of lead-rubber bearing and one type of silicon rubber bearing—was carried out with the objective of fully identifying their mechanical characteristics. The proposed model is capable of well-predicting the mechanical properties of each type of elastomeric bearing into the large strain range. Earthquake simulator tests were also conducted after the loading tests of the individual bearings. In order to show the validity of the proposed model, non-linear dynamic analyses were conducted to simulate the earthquake simulator test results. Good agreement between the experimental and analytical results shows that the model can be an effective numerical tool to predict not only the peak response value but also the force–displacement relationship of the isolators and floor response spectra for isolated structures.

Experimental and analytical studies of shape-memory alloy dampers for structural control

In the wake of damaging earthquakes in both the United States and Japan over the past year, the performance of structures, in addition to traditional life-safety concerns, has become an important issue for designers and owners. Many possible approaches to enhancing the seismic performance of structures have been proposed, and one promising family of solutions which is receiving attention today is passive damping devices. The work presented here is part of an ongoing experimental and analytical study of the applicability of one particular type of damping device for controlling the response of civil structures. Two different types of reduced-scale dampers using shape memory alloys have been tested over a range of strain amplitudes, loading frequencies, and temperatures. The purpose of the tests was to thoroughly characterize an alloy and examine variations in device design and installation configurations that could lead to a number of different hysteretic shapes. The ultimate behavior of the devices was also examined. Parallel to the device development and testing, a series of analyses of a steel frame building incorporating shape memory alloys has been undertaken to quantify the benefits of using these devices in an actual structure. Preliminary results of these analyses are presented.

Large-Scale Testing of Steel Unbonded Braces for Energy Dissipation

This paper outlines large-scale tests of tension/compression yielding braces (also called “unbonded braces”) in support of their first applications in the United States. The core steel in these braces provides stable energy dissipation by yielding under reversed axial loading, while the surrounding concrete-filled steel tube resists compression buckling. The paper summarizes a series of tests on large-scale unbonded braces, having yield forces of 1200, 1600, and 2100 kN (270, 360, and 470 kips). Each brace was subjected to a cyclic loading pattern consistent with that used widely for testing steel beam-column connections. Additional tests explored the behavior of the braces under a near-field loading history, a displacement time history derived from a seismic analysis of an idealized 5-story building, and a low-cycle fatigue test.

Seismic Isolation for Advanced Nuclear Power Stations

Seismic isolation offers an attractive approach for reducing seismic loads in nuclear structures, and more significantly, in reactor components. Isolation will lead to a simplification of designs, facilitate standardization, enhance safety margins, and may potentially reduce cost. To date, six large Pressurized Water Reactor units have been isolated in France and South Africa and several advanced nuclear concepts in the U.S., Japan, and Europe have incorporated this approach. It is recognized that to qualify and license an isolation system in the U.S. and in Japan, a comprehensive testing program of isolation components and systems would be required. A major seven year program was initiated in Japan in 1987 with the objective of establishing a qualified seismic isolation design for a large fast breeder reactor to be constructed at the end of this decade. In the U.S., two concepts which use steel laminated elastomeric bearings for seismic isolation have been developed. One of these concepts is a novel system which provides three-dimensional isolation. An extensive test program of scaled prototype bearings to demonstrate their feasibility and effectiveness has been carried out.

Experimental studies of the seismic response of structures incorporating base-isolation systems

Abstract Whereas the concept of base-isolating structures from the damaging effects of earthquake motions is not new, implementation of the technique is a relatively new occurrence. This has mainly been due to the need for several important developments in materials science and experimental and analytical modeling before base isolation could evolve into a practical approach for seismic design. One of these developments has been the ability to test large-scale isolation systems using simulated seismic loads. These tests have not only proven the performance and reliability of the isolation systems and hardware, but have enabled correlation studies to be undertaken which have confirmed the accuracy of analytical methods and the acceptability of current design procedures. The Earthquake Engineering Research Center (EERC) at the University of California at Berkeley has been an active participant in this work, and this paper reviews some of the achievements of the Center in the last few years. Component tests on single isolators are described. Tests on plain and high-damping natural-rubber bearings, lead—rubber bearings, sliding bearings, and bearings incorporating uplift resistance mechanisms have been performed. High-shear strain tests on large (up to full scale) elastomeric bearings have been conducted to determine the stability characteristics and limit states of the isolators. Performance evaluation studies using the earthquake simulator to test large-scale model isolated structures have been carried out for a variety of isolation systems and structures. Uplift studies of slender base-isolated buildings and investigation of the behavior of base-isolated skew bridge decks have been studied. This paper aims to highlight those areas where progress has been made.

Comparison of earthquake simulator test results with the SEAONC tentative seismic isolation design requirements

Earthquake simulator tests were performed on a 1/5-scale, 6-story reinforced concrete shear-wall structure and a 1/4-scale, 9-story braced steel frame structure. The structures were supported by five different base isolation systems which consisted of various types and combinations of elastomeric bearings. The main objective of this study was to compare the peak experimental displacements of the base isolation systems tested with values given by the tentative base isolation design provisions proposed by the Seismology Committee of the Structural Engineers Association of Northern California (SEAONC). Comparisons of experimental results and values from the SEAONC base isolation design formula for displacements indicated that the formula is generally conservative, even for predominantly low frequency earthquake motions, provided the ground motion coefficient A v (based on the effective peak velocity as defined by ATC 3-06) is used in the design equation for base-isolated structures with periods greater than 1 second.