Base Isolation: Origins and Development James M. Kelly Professor Emeritus Civil and Environmental Environmental Engineering University of California Ber!eley In recent years base isolation has become an increasingly applied structural design technique for buildings and bridges in highly seismic areas. Many types of structures have been built using this approach, and many others are in the design phase or under construction. Most of the completed buildings and those under construction use rubber isolation bearings in some way in the isolation system. The ideas behind the concept of base isolation are quite simple. There are two basic types of isolation systems. The system that has been adopted most widely in recent years is typified by the use of elastomeric bearings, the elastomer made of either natural rubber or neoprene. In this approach, the building or structure is decoupled from the horizontal components of the earthquake ground motion by interposing a layer with low horizontal stiffness between the structure and the foundation. This layer gives the structure a fundamental frequency that is much lower than its fixedbase frequency and also much lower than the predominant frequencies of the ground motion. The first dynamic mode of the isolated structure involves deformation only in the isolation system, the structure above being to all intents and purposes rigid. The higher modes that will produce deformation in the structure are orthogonal to the first mode and consequently also to the ground motion. These higher modes do not participate in the motion, motion , so that if there is high energy en ergy in the ground motion at these the se higher frequencies, this energy cannot be transmitted into the structure. The isolation system does not absorb the earthquake energy, but rather deflects it through the dynamics of the system. This type of isolation works when the system is linear and even when undamped! however, some damping is beneficial to suppress any possible resonance at the isolation frequency. The second basic type of isolation system is typified by the sliding system. This works by limiting the transfer of shear across the isolation interface. Many sliding systems have been proposed and some so me have been used. In "hina there ther e are at least three buildings buildin gs on sliding systems that use a specially selected sand at the sliding interface. # type of isolation containing a leadbronze plate sliding on stainless steel with an elastomeric bearing has been used for a nuclear power plant in $outh #frica. The frictionpendulum system is a sliding system using a special interfacial material sliding on stainless steel and has been used for several pro%ects in the &nited $tates, both new and retrofit construction. "esear#$ at EE"C
'esearch on the development of natural rubber bearings for isolating buildings from earthquakes began in ()*+ at the arthquake ngineering 'esearch "enter -'" -now /', the /acific ngineering 'esearch "enter of the &niversity of "alifornia at 0erkeley. The initial research program was a %oint effort by '" and the Malaysian 'ubber /roducers 'esearch #ssociation -M'/'#, &.1. The program was funded by M'/'# through a number of grants over several years, with later funding provided by the 2ational $cience 3oundation and the lectric /ower 'esearch Institute. /rofessor 4ames M. 1elly directed the
research at '", which included considerable theoretical and experimental contributions by graduate students. #lthough not an entirely new idea at the time5a few methods using rollers or sliders had been proposed5the concept of base isolation was considered to be very impractical by most of the structural engineering profession. The research pro%ect began with a set of handmade bearings of extremely lowmodulus rubber used with a simple threestory, singlebay, 67ton model. $haking table tests showed that isolation bearings could bring about reductions in acceleration by factors of as much as ten when compared to those of conventional design and that, as predicted, the model would respond as a rigid body with all deformation concentrated in the isolation system. It was also clear that a certain degree of damping was needed in the system and that the scale of the model was too small to allow more practical rubber compounds to be used. In ()*8, a more convincing demonstration of the isolation concept was achieved with a more realistic fivestory, threebay model weighing 97 tons and by using dampingenhanced bearings made by commercial techniques. # strong interest throughout the '" research program was in the influence of isolation on the response of equipment and contents in a structure, which tend to sustain more damage when conventional methods of seismicresistant design are used and which, in many buildings, are much more costly than the structure itself. #n extensive series of tests on the fivestory frame demonstrated that isolation with rubber bearings could provide very substantial reductions in the accelerations experienced by internal equipment, exceeding the reductions experienced by the structure. :owever, the same tests showed that when additional elements -such as steel energyabsorbing devices, frictional systems, or lead plugs in the bearings were added to the isolation system to increase damping, the reductions in acceleration to the equipment were not achieved because the added elements also induced responses in the higher modes of the structure, affecting the equipment. It became clear that the optimum method of increasing damping was to provide it in the rubber compound itself. This method was applied later in the compound developed by M'/'# and used in the first baseisolated building in the &nited $tates, described below. 'ubber bearings are relatively easy to manufacture, have no moving parts, are unaffected by time, and are very resistant to environmental degradation. Test of bearing used in the Indonesian demonstration building. Photo: I. D. Aiken
The bearings are made by vulcanization bonding of sheets of rubber to thin steel reinforcing plates. 0ecause the bearings are very stiff in the vertical direction and very flexible in the horizontal direction, under seismic loading the bearing layer isolates the building from the horizontal components of the ground movement while the vertical components are transmitted to the structure relatively unchanged. #lthough vertical accelerations do not affect most buildings, the bearings also isolate the building from unwanted high frequency vertical vibrations produced by underground railways and local traffic. 'ubber bearings are suitable for stiff buildings up to seven stories in height. 3or this type of building, uplift on the bearings will not occur and wind load will be unimportant.
U.%. &ppli#ations
The first baseisolated building in the &nited $tates is the 3oothill "ommunities ;aw and 4ustice "enter, a <=7 million legal services center in 'ancho "ucamonga $an 0ernardino "ounty, about )* km -+7 miles east of downtown ;os #ngeles. "ompleted in ()8>, the building is four stories high with a full basement and subbasement for the isolation system, which consists of )8 isolators of multilayered natural rubber bearings reinforced with steel plates. The superstructure of the building has a structural steel frame stiffened by braced frames in some bays. Foothill Communities Law and Justice Center. Photo: I. D. Aiken.
The building is located 67 km -(6 miles from the $an #ndreas fault. $an 0ernardino "ounty, the first in the &.$. to have a thorough earthquake preparedness program, asked that the building be designed for a 'ichter magnitude 8.= earthquake, the maximum credible earthquake for that site. The design selected for the isolation system, which accounted for possible torsion, incorporated a maximum horizontal displacement demand of =87 mm -(> in. in the isolators at the corners of the building. Tests of fullscale sample bearings verified this capacity. The highly filled natural rubber from which the isolators are made, developed as part of the '" research program, has mechanical properties that make it ideal for a base isolation system. The shear stiffness of this rubber is high for small strains but decreases by a factor of about four or five as the strain increases, reaching a minimum value at a shear strain of >7 percent. 3or strains greater than (77 percent, the stiffness begins to increase again, providing a failsafe action under a very high load. The damping follows the same pattern but less dramatically, decreasing from an initial value of 67 percent to a minimum of (7 percent and then increasing again. The design of the system assumes minimum values of stiffness and damping and a linear response. The high initial stiffness is invoked only for wind load design and the large strain response only for failsafe action. This highdamping rubber system was also adopted for the 3ire ?epartment "ommand and "ontrol 3acility -3""3 of ;os #ngeles "ounty, completed in ())7. -The same type of high damping rubber bearing was also used for the Italian telephone company, $.I./., #ncona, Italy, the first modern baseisolated building in urope. The 3""3 building houses the computer systems for the emergency services of the county and is therefore required to remain functional after an extreme event. Fire Deartment Command and Control Facilit!. Photo: I.D. Aiken
The decision to use base isolation for this pro%ect was reached by comparing conventional and isolation schemes designed to provide the same degree of protection. In most pro%ects, the isolation design costs five percent more. 2ot only was the isolation design estimate + percent less in this case but is less for any building when equivalent levels of
protection are considered. 3urthermore, these costs are first costs. ;ifecycle costs are even more favorable. #lso noteworthy is that the conventional code design requires only a minimal level of protection, that the structure not collapse! whereas isolation design provides a higher level of protection. The &niversity of $outhern "alifornia Teaching :ospital in eastern ;os #ngeles is an eight story concentrically braced steel frame supported on +8 lead rubber isolators and 8( elastomeric isolators. The building was instrumented by the "alifornia $trong Motion Instrumentation /rogram soon after its completion in ())(. The foundation system consists of spread footings and grade beams on rock. 0ecause of functional requirements, both the building plan and elevation are highly irregular with numerous setbacks over the height. Two wings at either side of the building are connected through what is referred to as the @necked down@ portion of the building, and in the original fixedbase design the irregular configuration led to both coupling between the lateral and torsional vibration modes and very large shear force demands in the slender region between the two rings. -ven in the isolated design steel trusses are required to carry the shears in the neckeddown region. These were two of the main reasons that seismic isolation was eventually chosen for this structure. "ni#ersit! of $outhern California "ni#ersit! %osital. Photo: P. &. Clark.
The &niversity of $outhern "alifornia -&$" Teaching hospital was =+ km -6= miles from the epicenter of the Mw +.8 ())9 2orthridge earthquake. The peak ground acceleration outside the building was 7.9) g, and the accelerations inside the building were around 7.(7 to 7.(= g. In this earthquake the structure was effectively isolated from ground motions strong enough to cause significant damage to other buildings in the medical center. The records obtained from the &$" hospital are particularly encouraging in that they represent the most severe test of an isolated building to date. 'u#lear &ppli#ations
Isolation used in conventional nuclear plants greatly simplifies the expensive and time consuming design and qualification of the equipment, piping, and supports for seismic loading. In addition, when seismic design criteria are increased due to the discovery of nearby faults, for example, the plant need not be redesigned! upgrading the isolation system is sufficient. In an experimental program at '" isolation bearings were designed, produced, and tested for two types of liquid metal reactor designs. The first, called /'I$M, uses highshape factor isolation bearings designed to provide horizontal isolation only. In the other design, $#3', the reactor is supported on lowshape bearings that provide both horizontal and vertical isolation. The results of this test series extended the range of the isolator types with well understood characteristics. Base Isolation in Japan
#fter a slow start, base isolation research and development in 4apan increased rapidly. The first large baseisolated building was completed in ()8+. #lthough such buildings in 4apan
require special approval from the Ministry of "onstruction, as of 4une =7, ())8, >>7 base isolated buildings had been approved. 0ase isolation has advanced rapidly in 4apan for several reasons. The expenditure for research and development in engineering is high with a significant amount designated specifically for base isolation! the large construction companies aggressively market the technology! the approval process for constructing a baseisolated building is a straightforward and standardized process! and the high seismicity of 4apan encourages the 4apanese to favor the longterm benefits of life safety and building lifecycle costs when making seismic design decisions. The system most commonly used in the past has been natural rubber bearings with mechanical dampers or leadrubber bearings. 'ecently, however, there has been an increasing use of highdamping natural rubber isolators. There are now several large buildings that use these highdamping bearingsA an outstanding example is the computer center for the Tohoku lectric /ower "ompany in $endai, Miyako /rovince. Tohoku 'lectric Power Coman!( Jaan. Photo: P. &. Clark
"urrently the largest baseisolated building in the world is the Best 4apan /ostal "omputer "enter, located in $anda, 1obe /refecture. This sixstory, 9*,777 m square ->77,777 ft square structure is supported on (67 elastomeric isolators with a number of additional steel and lead dampers. The building, which has an isolated period of =.) sec, is located approximately =7 km -() miles from the epicenter of the ())> :yogoken 2anbu -1obe earthquake, and experienced severe ground motion. The peak ground acceleration under the isolators was 977 cmCsec square -7.9( g but was reduced by the isolation system to (6* cmCsec square -7.(= g at the sixth floor. The estimate of the displacement of the isolators is around (6 cm -9.8 in.. # fixedbase building ad%acent to the computer center experienced some damage, but there was no damage to the isolated building. The use of isolation in 4apan continues to increase, especially in the aftermath of the 1obe earthquake. #s a result of superior performance of the Best 4apan /ostal "omputer "enter, there has been a rapid increase in the number of permits for baseisolated buildings, including many apartments and condominiums. %ummary
Dngoing research has improved the effectiveness of isolators in decreasing problems of stability, rollout, failure of the isolators, or unexpected response and the trend away from addon mechanical dampers. #dditionally, the difficulties of manufacturing large isolators have diminished. It is now possible to make bearings as large as +7 in. -(.> m in diameter. The *7 natural rubber bearings built for the M.;. 1ingC".'. ?iagnostics Trauma "enter in Billowbrook, "alifornia, were at the time of their manufacture the largest isolation bearings in the &.$. The isolators are (.7 m -97 in. in diameter. The combination of increased size with lowmodulus rubber has resulted in highly reliable isolation systems.
There are several local applications of seismic isolation systems. The Dakland "ity :all was retrofitted after the ()8) ;oma /rieta, "alifornia, earthquake with about ((7 large isolators. # new public safety building for 0erkeley is now under construction and will use seismic isolators. The Martin ;uther 1ing 4r. "ivic "enter 0uilding in 0erkeley will be retrofitted using isolation, as will the :earst Memorial Mining 0uilding on the &niversity of "alifornia at 0erkeley campus. The classic 0eaux #rts architecture and interior fixtures will be untouched by the retrofitting process, while the seismic resistance will be substantially enhanced. To date 9> baseisolated buildings in the &.$. are planned, under construction, or completed 5for new construction and for retrofitting. The use of base isolation applications up to this time in the &.$. has been for structures with critical or expensive contents, but there is increasing interest in applying this technology to public housing, schools, and hospitals in developing countries, where the replacement cost due to earthquake damage can be a significant part of the gross national product. The cooperation between '" and M'/'# led to a new %oint effort supported by the &nited 2ations Industrial ?evelopment Drganization -&2I?D that developed lowcost isolation systems for such countries, and several demonstration pro%ects are in place in Indonesia, the /eopleEs 'epublic of "hina, and #rmenia. The research program at '", initially supported by M'/'#, was instrumental in making the base isolation approach to earthquakeresistant design a reality. This aer is an udate of )*ase Isolation: +rigins and De#eloment(, '" 2ews, -ol. /( 0o. ( Januar! 11. "eferen#es
"lark, /eter B., Masahiko :igashino, and 4ames M. 1elly. ())+. F/erformance of $eismically Isolated $tructures in the 4anuary (*, ())9 2orthridge arthquake.G Proceedings of the $i2th ".$.3Jaan &orksho on the Imro#ement of *uilding $tructural Design and Construction Practices in the "nited $tates and Jaan. Hictoria, 0."., "anadaA #pplied Technology "ouncil and 4apan $tructural "onsultants #ssociation. #T"(>>. 1elly, 4ames. M. ())*. 'arth4uake35esistant Design with 5ubber. 6nd ed. 0erlin and 2ew orkA $pringer Herlag. Taniwangsa, Bendy, and 4ames M. 1elly. ())+. '2erimental and anal!tical studies of base isolation alications for low3cost housing . 0erkeley, "alif.A arthquake ngineering 'esearch "enter, &niversity of "alifornia. &"0C'")+C79.
