James M. Kelly

Aseismic base isolation: review and bibliography

Abstract The idea that a building can be uncoupled from the damaging effects of the ground movement produced by a strong earthquake has appealed to inventors and engineers for more than a century. Many ingenious devices have been proposed to achieve this result, but very few have been tried and the concept now generally referred to as base isolation or seismic isolation has yet to become acceptable to the engineering profession as a whole. Although most of the proposed systems are unacceptably complicated, in recent years a few practical systems have emerged and have been implemented. While some of these systems have been tested on large-scale shaking tables, none has to date been tested as built by a strong earth tremor. The shake testing and related static testing of full-scale components such as isolation bearings, however, has led to a certain degree of acceptance by the profession and it is possible that the number of practical implementations of base isolation will increase quite dramatically in the next few years. This review summarizes much of the literature on theoretical aspects of seismic isolation, describes testing programmes and enumerates those isolation systems which have been used in buildings completed or under construction. It describes the characteristics of the various implemented systems with an indication of their range of applicability and some assessment of their development as backed by research. A bibliography of all papers published on the topic from 1900 to 1984 is included. The bibliography is as complete as possible, but, due to the rapid increase in research interest in the topic in the past few years, there may be a substantial degree of omission in the later years.

The role of damping in seismic isolation

In the current code requirements for the design of base isolation systems for buildings located at near-fault sites, the design engineer is faced with very large design displacements for the isolators. To reduce these displacements, supplementary dampers are often prescribed. These dampers reduce displacements, but at the expense of significant increases in interstorey drifts and floor accelerations in the superstructure. An elementary analysis based on a simple model of an isolated structure is used to demonstrate this dilemma. The model is linear and is based on modal analysis, but includes the modal coupling terms caused by high levels of damping in the isolation system. The equations are solved by a method that avoids complex modal analysis. Estimates of the important response quantities are obtained by the response spectrum method. It is shown that as the damping in the isolation system increases, the contribution of the modal coupling terms due to isolator damping in response to the superstructure becomes the dominant term. The isolator displacement and structural base shear may be reduced, but the floor accelerations and interstorey drift are increased. The results show that the use of supplemental dampers in seismic isolation is a misplaced effort and alternative strategies to solve the problem are suggested. Copyright

Robust control of base-isolated structures under earthquake excitation

We propose the use of robust control in conjunction with base isolation in order to assure arbitrarily small motion of a seismically excited structure. The proposed method requires control force application only at the base (first) floor. The efficacy of the scheme is illustrated by extensive simulations for a prototype six-story building.

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.

Base Isolation: Linear Theory and Design

The idea that a building can be uncoupled from the damaging effects of the ground movement produced by a strong earthquake has appealed to inventors and engineers for more than a century. Many ingenious devices have been proposed to achieve this result, but very few have been implemented and the concept now referred to as base isolation or seismic isolation has yet to be generally accepted by the engineering profession. Although most of the proposed systems are unacceptably complicated, in recent years a few practical systems have been developed and implemented. While some of these systems have been tested on large-scale shaking tables, none have to date been tested as-built by a strong earth tremor. The shake table testing and related static testing of full-scale components such as isolation bearings, however, has led to a certain degree of acceptance by the profession and it is possible that the number of practical implementations of base isolation will increase quite dramatically in the next few years. This paper describes recent implementations of base isolation and describes an approximate linear theory of isolation which can be used for the design of base isolation systems that use multilayer elastomeric isolators.

Seismic analysis of internal equipment and components in structures

Abstract An analytical method is developed whereby a simple estimate can be obtained of the maximum dynamic response of light equipment attached to a structure subjected to ground motion. The natural frequency of the equipment, modelled as a single-degree-of-freedom system, is considered to be close or equal to one of the natural frequencies of the N- degree-of-freedom structure. This estimate provides a convenient, rational basis for the structural design of the equipment and its installation. The approach is based on the transient analysis of lightly damped tuned or slightly detuned equipment-structure systems in which the mass of the equipment is much smaller than that of the structure. It is assumed that the information available to the designer is a design spectrum for the ground motion, fixed-base modal properties of the structure, and fixed-base properties of the equipment. The results obtained are simple estimates of the maximum acceleration and displacement of the equipment. The method can also be used to treat closely spaced modes in structural systems, where the square root of the sum of squares procedure is known to be invalid. This analytical method is also applied to untuned equipment-structure systems for which the conventional floor spectrum method is mathematically valid. A closed-form solution is obtained which permits an estimate of the maximum equipment response to be obtained without the necessity of computing time histories, as required by the conventional floor spectrum method.

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.

Seismic Isolation Systems for Developing Countries

This paper describes an experimental and theoretical study of the feasibility of using fiber reinforcement to produce lightweight low-cost elastomeric isolators for application to housing, schools and other public buildings in highly seismic areas of the developing world. The theoretical analysis covers the mechanical characteristics of multi-layer elastomeric isolation bearings where the reinforcing elements, normally steel plates, are replaced by a fiber reinforcement. The fiber in the fiber-reinforced isolator, in contrast to the steel in the conventional isolator (which is assumed to be rigid both in extension and flexure), is assumed to be flexible in extension, but completely without flexure rigidity. This leads to an extension of the theoretical analysis on which the design of steel-reinforced isolators is which accommodates the stretching of the fiber-reinforcement. Several examples of isolators in the form of long strips were tested at the Earthquake Engineering Research Center Laboratory. The tested isolators had significantly large shape factors, large enough that for conventional isolators the effects of material compressibility would need to be included. The theoretical analysis is extended to include compressibility and the competing influences of reinforcement flexibility and compressibility are studied. The theoretical analysis suggests and the test results confirm that it is possible to produce a fiber-reinforced strip isolator that matches the behavior of a steel-reinforced isolator. The fiber-reinforced isolator is significantly lighter and can be made by a much less labor-intensive manufacturing process. The advantage of the strip isolator is that it can be easily used in buildings with masonry walls. The intention of this research is to provide a low-cost lightweight isolation system for housing and public buildings in developing countries.

A phenomenological penetration model of plates

Abstract An analysis has been developed to describe phenomenologically the plugging process occurring in thin plates or those of intermediate thickness when struck by blunt projectiles at normal incidence. The present model is concerned with an underformable striker and a target whose mechanical behaviour is characterized by a strain-rate independent rigid/plastic constitutive relation. Plastic wave theory is employed to construct a five-stage penetration process that consists sequentially of indentation, plug formation, separation and slipping, and of post-perforation deformation. The dynamic response of the target is included in terms of the action of a plastic hinge resulting from shear applied at the projectile periphery. The equations of motion are obtained for the small number of bodies defined by the system components, the wave fronts and the hinges. The problem was coded in Fortran IV and run on a PDP-11/60 computer. Experimental results for residual projectile velocity obtained by other investigators were found to be in excellent correspondence with the calculations of the model. The axial shear deformation mode does not predict the total target deflection and contributes only within a narrow zone beyond the projectile radius, while bending is a more dominant mechanism. The inclusion of shear was found to provide closer correspondence with experimental data near the ballistic limit.

Effect of bulk compressibility on the stiffness of cylindrical base isolation bearings

Abstract The seismic design technique based on mounting building structures on horizontally flexible foundations is becoming popular. The horizontal flexibility accompanied by a very high vertical stiffness is well realized by multilayered elastomeric bearings made of interleaved steel plates and rubber slices. The rubber is vulcanized to the steel to ensure bond. Several design expressions for such pads were already proposed. However, these expressions either consider the rubber incompressible or they account for compressibility by an ad hoc modified formula. In this paper, the governing equations for the pressure in a rubber slice of arbitrary cross section are presented. The solution is carried out for the circular shape and compared to experimental results. It was found that the formula for the compressive stiffness of those bearings developed here by including the effect of bulk compressibility predicts very accurately the measured stiffness. The classically used formula that ignores the rubber compressibility yielded exaggerated over-estimation of the compression modulus. The ad hoc modified formula gave closer results but was still inaccurate. An expression derived from the exact solution and a simplified form are herein provided to replace the ad hoc modified formula and the expression that ignores rubber compressibility.