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Nonlinear natural rubber bearings for seismic isolation
Nuclear Engineering and Design 84 (1985) 417-428 North-Holland, Amsterdam
417
N O N L I N E A R NATURAL RUBBER BEARINGS FOR SEISMIC ISOLATION C.J. D E R H A M
1, J . M . K E L L Y 2 a n d A . G . T H O M A S
1
1 The Malaysian Rubber Producers" Research Association, Hertford, England 2 College of Engineerin~ University of California, Berkeley, California, USA Received 25 April 1984
This paper summarizes the results of a ten-year joint research program on base isolation carried out by the Malaysian Rubber Producers' Research Association in England and the University of California at Berkeley. Special rubber bearings have been developed which protect buildings and their contents from earthquake damage without requiring additional mechanical devices to enhance damping or to avoid problems of wind movement. The lack of complexity in the system makes prediction of response more certain and reduces cost so that in many designs the additional protection will be achieved at a cost lower than that for conventional protection. The principles of design are explained and results given for the many experimental investigations of the system which have included measurements of the response of both the building and its internal equipment. Practical aspects of the use of the system are discussed with reference to a large structure at present being constructed on nonlinear natural rubber bearings in a highly seismic area.
1. Introduction The primary objective of any strategy for earthquake protection must be the saving of lives - but in terms of potential human suffering the breakdown of the infrastructure of society in the aftermath of an earthquake is very nearly as important a consideration. Within this infrastructure, the necessity for the uninterrupted operation of hospitals, rescue services, and communications is obvious - but less obvious is the importance of protection for computer records in connection with social services, health and insurance [1]. Nuclear power plants have now become an important part of this infrastructure and, at least in the public imagination, represent a serious potential hazard if damaged in an earthquake. Traditional methods of earthquake protection which rely on strengthening and ductility are mainly aimed at preventing the collapse of structures and not at reducing the forces within the structure. To provide protection for the contents - which is the key to sustaining the services mentioned and to the effective containment of radioactive or toxic materials - it is necessary to take a different approach. The earthquake forces transmitted to the structure must be reduced, and they can be reduced by exactly the same approach used in every
other field of vibration engineering - by isolation. The concept of earthquake isolation has been discussed for many years but the problem has always been that of controlling effectively the very large amplitudes of vibration associated with earthquakes. It has now been shown that these amplitudes can be sustained by specially designed laminated rubber and steel mountings, generally known as bearings. The function of the bearings is to reduce the translational natural frequency of the mounted structure to a region well below that of most significant earthquake energy. In regions of good soil or rock it has been shown that a natural frequency of around 0.5 Hz for the mounted structure will result in greatly reduced forces on both the structure and its contents. Bearings can be satisfactorily designed to support a structure at this frequency.
2. The basic system 2.1. General description For twenty-five years at least laminated rubber/steel structural bearings have been used in civil engineering. Initially their mean function was as bridge bearings where their high vertical stiffness combined with their
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C.J. Derham et aL / Nonfinear natural rubber bearings
relatively low horizontal stiffness enabled them to be used instead of mechanical bearings. Because they require no maintenance and are easy to install, they are now used worldwide to support new bridge decks. Since the mid-1960s, similar bearings have also been used to mount buildings where there is a problem of groundborne vibration. There are now many buildings constructed in this way, usually over or adjacent to railway lines, and the techniques has proved successful both in effectiveness and longevity. All indications are that the bearings are at least as durable as conventional building materials. Recently, the technique of building on laminated rubber and steel bearings has been extended to provide earthquake protection to structures. A small number of buildings have been constructed in this way - possibly the largest is still under construction in San Bernardino, Califorma. Some aspects of the design of this building are discussed in more detail below. The basic requirements for earthquake protection are that; - The bearings must support the dead load of the structure and must have a high vertical stiffness. - The horizontal stiffness of the bearings must be such as to confer on the mounted structure a low horizontal natural frequency so that the building will not respond to the destructive components of the ground motion. F r o m the response spectra of Seed [2] it is clear that under a wide range of conditions a horizontal natural frequency of 0.5 Hz is suitable. - Some earthquake energy will always occur at or near the horizontal natural frequency so the system must contain sufficient damping to limit translational movement to an acceptable level. 2.2. E l a s t o m e r characteristics
Isolation bearing systems must prevent excessive movement of structures under wind loading. It must be evident that this requirement is not on the grounds of safety - the system is designed to prevent damage due to much more severe effects - but primarily for the comfort of the occupants as even a slight swaying motion might be disconcerting. Also, there might be a problem of fatigue in the service connections. The magnitude of the wind movement, d, for a bearing system with a linear force/shear deflection relation given in terms of the natural horizontal frequency, f, is: d = a g / ( 2 ~ ' f ) 2,
(1)
where a is the ratio of the wind force to the weight of
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the building. For f = 0.5 Hz and a = 0.05 (typical values for wind loading requirements), this relation yields d = 5 cm (2 in). In order to prevent this perhaps unacceptable movement, various devices have been proposed. For example, it has been suggested that mechanical devices can be incorporated which will be strong enough to resist wind loads but which would break if an earthquake of appreciable magnitude occurred. While it is quite possible to design such a system [3] it is an added complication, and it now appears that an adequate solution to the problem can be obtained by utilizing the elastic nonlinearity of filled rubber compounds. It has been known for many years that rubber vulcanizates containing filler show markedly nonlinear stress-strain behavior (see for example refs. [4] and [5]). The form of the nonlinearity in shear is such that for small deformation, of the order of a few per cent, the material has a substantially higher effective modulus than for high deformation. For a filled material similar to that used in the earthquake bearings for the San Bernardino project, the simple shear stress-strain relation is as shown in fig. 1. The nonlinearity is clear. Under cyclic deformation at various maximum amplitudes, the relation is as shown in fig. 2. This again shows the nonlinearity, the effective modulus at small amplitudes being high, but it also illustrates the substantial mechanical hysteresis found when the material is taken around a stress cycle. Indeed, the compound was specifically designed to have a large hysteresis for strain amplitudes of about 50% in order to give the bearings an acceptable response
CJ. Derham et aL / Nonlinear natural rubber bearings
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under earthquake conditions. At these large deformations, the rubber is less nonlinear and the linear viscoelastic model used to predict the response of the mounted building to an earthquake is adequate [8]. The hysteresis loss of this rubber at 50% strain is equivalent to that for a linear material with a loss angle of about 11 °, giving 10% of critical damping. The curves of fig. 2 show no sharp corners and this implies that a building mounted on such a material will not have its higher modes strongly excited, in contrast to systems giving an instantaneous yield or break point. Also shown in the inset to fig. 2 is the change in the stress-strain curve when the shear strain exceeds 100%. The material hardens and the effective modulus begins to increase. The shear stress on the bearing under wind loading can be written as: d
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where d is the displacement under wind loading for a linear bearing, given by expression (1), t is the total rubber thickness in the bearing, and Gl is the shear modulus of the rubber at relatively large stress, when the stress-strain relation is assumed to be linear. The actual strain, eI , at this fairly low stress, o, can be found from the measured stress-strain relation of fig. 1, and
the true displacement under wind loading will be given by: d 1 = tel,
(3)
which for the nonlinear materials considered here will be substantially less than d. Alternatively, d can be written in terms of the effective modulus, Gt, at stress o: d,
=
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This form shows the effect of nonlinearity more clearly as it is well known that the small strain modulus G1 may be several times greater than G I. For example, with the special compounds used in the San Bernardino project bearings, the ratio G J G 1 at the relevant stresses is almost 20% so that the displacement d a is about 1 cm (0.4 in), which is considered to be sufficiently small so that no special arrangements need be made to accommodate it. For comparison, a building 50 m long will exhibit a change in length of about this amount for a temperature change of 20 ° C. Thus, by exploiting the nonlinear behavior of the compounded natural rubber materials the necessity for mechanical fuses or equivalent restraints can be avoided. These materials have the added advantage that their characteristic stress-strain relation is very quickly
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c J . Derham et al. / Nonlinear natural rubber bearings
recovered after cycling so that even after an earthquake no repair or replacement should be required and there will be no permanent offset.
3. Details of investigations 3.1. Laboratory investigations
A computer analysis of a seven-story building a representative of a standard type of hospital construction, carried out by Atkins Research and Development at the request of the Malaysian Rubber Producers' Research Association (MRPRA) [7], indicated that the structure when isolated would experience a reduction in peak acceleration of around tenfold as compared to the peak acceleration experienced when carried on a conventional foundation. This and later computer studies [6] predicted that the building on rubber isolators would behave as a rigid body and that the forces transmitted to the upper levels would be greatly reduced. An experimental testing program to verify this approach to seismic protection was begun at the Earthquake Simulator Laboratory of the Earthquake Engineering Research Center of the University of California, Berkeley. This research work was initially sponsored by the MRPRA and later by the National Science Foundation. It has included much dynamic testing of different forms of isolation system on the shaking table at the Earthquake Simulator Laboratory and also static testing of full-scale isolation devices on the large-scale testing facility at the Earthquake Engineering Research Center. Since dynamic testing began in 1976 several designs of rubber bearing have been tested and several models have been used. The shaking table at the laboratory is 6.1 m (20 ft) square and can test models up to 50 tonnes total weight. It can reproduce one horizontal component and the vertical component of any historical earthquake for which a strong motion record is available and any artificial record. The earthquake intensity can be increased to a maximum of around 1.5g peak acceleration in the horizontal component and around 0.5g vertically. Time scaling of the earthquake input to correspond to geometrical scaling of the model is also possible. In early tests in the program handmade natural rubber bearings were used with a 20-tonne, three-story, single bay model. These tests, reported in ref. [8], confirmed the computer predictions and demonstrated that the approach was practicable. Later, commercially made bearings were used with a larger model, a 40-tonne, five-story, three bay frame, a one-third scale portion of
a typical small structure. The testing program included the testing of wind restraints [3] and hysteretic energyabsorbing devices in conjunction with the rubber bearings [9,10]. The use of sliding friction with isolation bearings was also tested. In this system the frictional element was used to dissipate energy and control displacement and also functioned as a fail-safe system in the sense that if the rubber bearing system was overextended the structure would be carried on the sliding surfaces [11]. The response of sensitive internal equipment has been very important aspect of this research program. In many cases the cost of internal equipment exceeds that of the structure housing it. In several dynamic tests instrumented models of internal equipment were attached to the structural models and the response of the equipment recorded when the structure was isolated and when not. The reductions in peak acceleration achieved when rubber bearings were used was even greater for the equipment than for the structure, demonstrating that isolation of a building protects not only the building but the equipment and contents. This portion of the research program was sponsored by the Electric Power Research Institute (EPRI) and has been reported in refs. [12] and [13]. The Institute also sponsored a study of the influence of isolation on the response of large equipment items in power plants. A model steam generator in a model of a structural frame was tested on the shaking table and it was shown that isolation can also be an effective strategy for the seismic protection of large components. This is reported in refs. [14] and [15]. In all, during the various aspects of the base isolation testing program, several hundred dynamic tests utilizing horizontal and vertical excitations and up to high levels of intensity have been carried out. They have demonstrated that base isolation using rubber bearings is an effective anti-seismic strategy for many types of building and structure. 3.2. Field investigations
A field test of a base isolation system was undertaken as one of the experiments of the SIMQUAKE II test program conducted in 1978 by EPRI at the McCormick Test Site. In this experiment an explosive source consisting of two parallel arrays of explosive, 40 tonnes and 30 tonnes of ANFO explosive, were detonated with the intention of producing a ground motion similar to that of a large earthquake. Several experiments were included in this test in addition to the isolation experiment. This involved natural rubber bearings and a fail-safe system, carrying a one-twenty-fourth
C.J. Derham et al. / Nonlinear natural rubber bearings
scale model of a containment structure. The results of the test of the isolation equipment are reported in refs. [161. In the initial design of the SIMQUAKE II experiment a certain design spectrum and peak acceleration were assumed and the bearings were designed to produce a natural frequency of 1 Hz in the isolated structure which would put the model out of the range of the anticipated horizontal ground acceleration. In the actual SIMQUAKE II event, the ground motions at the location of the isolation model were quite different. The ground motions took the form of two distinct earthquakes, the first having a peak horizontal acceleration o f + 0.75g and peak vertical accelerations o f + 1.25g,-0.94g, the second having a peak horizontal acceleration of + 3.0g and peak vertical accelerations of + 4 . 4 g , - 5 . 6 g . The two earthquakes resulted from the fact that the two arrays of explosive were fired with a time separation of 1.2 s and at the location of the model, 68.6 m (225 ft) from the first array (fired last), the two motions remained distinct. T h a t the array producing the less intense ground motion was fired first proved fortunate for the isolation system. The system easily overrode this earthquake, attenuating the horizontal acceleration transmitted to it as expected and slightly amplifying the vertical acceleration. In the second earthquake the upward acceleration reached 4.4g followed immediately by a downward acceleration of 5.6g. This produced tension in the bearings and two delaminated as a result. The fail-safe system then took over from these two bearings and collapse of the model was prevented with, of course, substantially increased acceleration in the superstructure. The high horizontal accelerations in this earthquake with a peak of 3.0g preceding the failure were successfully attenuated by the system. The isolation system survived an earthquake with peak horizontal and vertical accelerations of 0.75g and 1.25g, respectively, and was only overcome by an earthquake motion with a peak horizontal acceleration of 3.0g and a peak vertical acceleration of 5.6g, at which the fail-safe system came into operation and prevented the collapse of the structure. Vertical accelerations of the magnitude experienced by the model in the SIMQUAKE II test are not possible seismic events and tension failure such as occurred in this experiment would not occur in a fullscale system. 4. Example installations 4.1. Implemented systems
The particular features of the system outlined here are the use of laminated natural rubber bearings with no
421
additional wind restraint and no hysteretic energy-absorbing device. A system of this kind albeit using conventional natural rubber polymer was installed in an elementary school in Lambesc, Marseilles, France, completed in 1978 [17]. This school was built on 152 isolators, is 80 m (254 ft) by 30 m (100 ft) and three stories high. The isolators are 30 cm (1 ft) in diameter with a total rubber thickness of 5 cm (2 in) and the system was designed for an VIII M.M. earthquake. The school building was constructed using a prefabricated concrete building system that without the use of isolators would not have satisfied the local seismic code requirements. Modifying the design to conform to the code would have increased construction costs significantly. The seismic requirements for the school were not stringent and the displacement capacity of the bearings is consequently not great. The system has been modified and is now being used for a nuclear waste storage facility for the French Navy. The bearings in this modified system are much larger and the displacement capacity very much greater, but the design is the same and conventional natural rubber is used. The first use of the isolation system which utilizes high-damping rubber formulations specifically designed for seismic isolation bearings is a large local government building now under construction in San Bernardino County, California. This building, the thirty million dollar Communities Law and Justice Center in Rancho Cucamonga, will be mounted on 98 isolators (fig. 3). The isolators must be able to carry a wide range of vertical load from 100 tonnes to around 400 tonnes and thus several types of isolator are used. They are all however 76 cm (30 in) in diameter and around 38 cm (15 in) thick. There are two high damping rubber compounds and four rubber thickness-steel shim combinations. The building is 127 m (417 ft) by 33.5 m (110 ft), has four stories and a full basement. It is located within 21 km (13 miles) of the San Andreas Fault and has been designed to remain elastic under a Richter magnitude 8.3 earthquake on that fault. The design spectrum (5% damping) selected as appropriate for such an event at the building site is a constant velocity spectrum of 127 c m / s (50 in/s) over a range of period from 0.8 s to 4.0 s. The peak spectral acceleration is lg. In this range the design spectrum is more severe than the Newmark-Hall spectrum given in Regulatory Guide 1.60 [18]. The building had been designed with a moment-resisting frame and for this spectrum the roof displacement would be of the order of 38 cm (15 in), clearly unacceptable in a four-story building. By reducing the displacement to an acceptable level using a braced
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C.J. Derham et al. / Nonlinear natural rubber bearings
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frame, very high forces in the structure and on the contents, equipment and occupants would be produced. In fact it is uneconomic to design any structure with a conventional structural system to be elastic for such a spectrum. However, it can be done using rubber bearings. At a two-second period the spectrum requires 40.5 cm (16 in) relative deflection in the bearings based on 5% damping. In the final design 10% damping was selected and a two-second period. Rotation produced by eccentricity was accounted for and including rotation the maximum relative displacement at the corner bearings was estimated to be 38 cm (15 in). The beatings for the building were constructed by Oil States Industries, Athens, Texas and have been installed. The erection of the superstructure is underway. 4.2. Verification testing
Prior to the decision to proceed with construction of the building on rubber, four prototype bearings were constructed in the new rubber formulation and were tested at the EERC to verify that they were capable of
the displacement demands. The bearings tested had nineteen 1.60 cm (0.63 in) layers of rubber and eighteen 6.4 mm (0.25 in) thick steel plates with two 31.8 mm (1.25 in) thick end plates. The end plates are dowelled into sole plates which connect the beatings to the foundation and to the superstructure. The dowels transmit only shear load and thus reduce tension loads on the beatings and allow for ease of removal if this should be necessary. The bearings were tested in a special test rig that allowed the application of simultaneous vertical load and horizontal displacement. The vertical load which ranged up to a maximum of 5.35 NM (1.2M lbs) per bearing was applied by the 18 MN (4M lb) Southwork and Energy testing machine at the EERC and horizontal load was produced by large hydraulic jacks between the bearings. The set-up is shown in fig. 4. The bearings were loaded to preset vertical loads and with the vertical load held constant were loaded through horizontal displacement cycles that increased in steps of 2.54 cm (1 in) to a maximum of 38 cm (15 in). They were also cycled thirty times at a maximum displacement of 28 cm (11 in) to verify their fatigue resistance. After the
C.J. Derham et al. / Nonlinear natural rubber bearings
423
Fig. 4. Configuration of bearings at 38 cm displacement.
test program was completed, a bearing was cut in half to see if any internal damage had been produced. No damage was evident and it is clear that 38 cm (15 in) displacement which represents a shear strain of 125% is not the limit of the bearing response. The horizontal and vertical stiffness of the bearings as measured during the test were exactly as predicted from the tests on the rubber from which they were fabricated. The damping was lower than that predicted by the tests on the rubber samples due to the fact that the samples were tested at 2 s per cycle whereas the bearings were tested at 10 rain per cycle. These tests represent a very severe test of the material. Failure processes in rubber are time dependent and in a severe earthquake the bearings will be at high strains ( > 100%) for only fractions of a second while in the tests the maximum strain was held for some minutes. The configuration of the bearings at a displacement of 38 cm (15 in) is shown in fig. 4.
The major criticism of the use of rubber bearings alone with no hysteretic system to control displacement is the possibility of very large displacements. The critical factor in seismic bearings is not the maximum shear strain in the rubber but the ratio of the maximum displacement to the plan dimension of the bearing. In dynamic tests on model isolation systems using the shake table circular bearings have been loaded to 80% of their diameter without failure and square bearings to a slightly higher ratio. It is clear that 38 cm (15 in) for the prototype bearings is quite conservative yet they have been designed to provide elastic response under design spectra more severe than anything for which buildings are conventionally designed. If larger displacements are anticipated, larger bearings can be used and there is no practical limit to the size of bearing that can be manufactured.
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5. Reliability The reliability of natural rubber bearings has been demonstrated over the past three decades through experience with bridge bearings which have the same general structure as isolation bearings. High-precision rubber bearings are used in helicopters to replace journal bearings in locations where the motion is cyclic rather than rotary and a similar form of bearing is used for fenders on docks and wharves. In some cases bearings have been removed and retested after decades of use and have been found to have the same mechanical characteristics as when installed. Natural rubber bearings have been used to isolate buildings from ground-borne acoustic vibration and in at least one instance the reliability of the bearings has been continually monitored for more than twenty years [19]. Machinery has been isolated for many years but the requirements for seismic isolation are quite different. In machinery isolation the frequency range to be isolated against is generally well defined and the amplitudes are small. In seismic isolation neither the frequency content of the input nor its amplitude can be prescribed in advance and the dynamic response of the system will be transient rather than steady state. For these reasons the design problem is quite different although the general principle remains the same. The displacements to be designed for are also an order of magnitude greater than in isolation for machinery vibration. Thus such isolation concepts can be transferred to seismic isolation only with care. Further differences are that bearings for machinery isolation are generally small but lightly loaded whereas for buildings the vertical loads may be extremely large and although the plan dimensions of a bearing will be large the average bearing stress will be much higher. However, it is a fact that the design of a bearing for a set isolation frequency is easier since buckling of the bearing becomes less important as the isolated mass carried by the bearing increases. It is thus more convenient to isolate a complete structure than to isolate separate items of equipment. Small items of equipment should be grouped together and isolated as a single unit on a connecting slab. This facilitates bearing design and eliminates the need to consider coupling of isolated and nonisolated components. Uneven reactions and settlements will present problems in isolated buildings but no more so than in conventional buildings since the vertical stiffness of the bearings is comparable to that of standard columns. Severe differential settlements could damage the diaphragm immediately above the bearing system but the
potential for retrofit always exists and compensating for unexpected differential settlement would be relatively easy as compared to conventional construction. In the system described here hysteretic energy-absorbing devices are not used and it is felt that their use should be avoided if at all possible. The damping that they can provide is higher than that which can be provided by the elastomer but it is purchased at the cost of mechanical complexity, uncertainties in performance and the need for nonlinear analysis in the design phase. They must also inevitably lead to the accumulation of nonreversible displacement and the consequent need to devise a procedure for the elimination of these displacements. In the plain rubber system permanent sets do not occur. One question left unanswered by the laboratory testing program is as to the response of the system to travelling wave effects. While both horizontal and vertical shake table motions were used and this provides some indication of the response no shake table presently can simulate this phenomenon and recourse must be made to numerical analysis. A further seismic phenomenon, namely the effect of distant earthquake, is not felt to be important since resonance at low amplitude is dealt with by the intrinsic damping of the material. This intrinsic damping also has the effect of reducing the influence of imperfections in the response of the system which can induce unwanted response in dynamic systems. The most prominent such effect in isolation systems is that of modal coupling of torsional and lateral motions due to the noncoincidence of the centers of mass and rigidity. Since it is likely that an isolated building will have closely spaced natural frequencies in the torsional and the two lateral modes, modal coupling may be anticipated. The phenomenon has been studied in detail, however, and it has been shown [20] that at the levels of damping envisaged here the coupling generated by the imperfections has very little effect on the overall response of the system.
6. Cost 6.1. Cost of installed system
In assessing the effectiveness of base isolation the remaining unanswered question is that of cost since designs with and without isolators and offering the same degree of seismic protection are not readily available. The bearings themselves are not expensive items particularly if many are made. The bearings for the San Bernardino County building, for example, cost less than
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C.J. Derham et al. / Nonlinear natural rubber bearings PROTOTYPE MEDICAL BUILDING COST COMPARISON ADD ROOF 5
46
BASE ISOLATORS
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$3840o0
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BASE ISOLATION
$ 107000 PLUS COST OF BRACING LOOSE EQUIPMENT
Fig. 5. Cost of medical building when conventionally constructed and when base isolated.
1% of total building cost. There were additional costs in this example since the building had originally been designed according to the UBC and architectural modifications had to be made to allow for the seismic gap around the building. The savings on the other hand in the construction of the superstructure of a building could offset these increased costs. Seismic shear walls will be diminished and other structural elements reduced in size. Bracing of components suspended from the ceiling and of mechanical and electrical components will be reduced. The results of a study carried out by Reid&Tarics Associates of San Francisco on the comparative cost of an isolated and conventionally founded six-story medical building with roughly 16000 sq m (170000 sq ft) are summarized in fig. 5 [21]. A potential savings of 107 000
dollars had base isolation been used was estimated for the structural system alone. Further savings would be possible since the major portion of the cost of a medical facility is in sensitive equipment. If such equipment must be braced or attached to the walls so as to protect it from damage in the event of an earthquake its mobility will be greatly reduced. Equipment that cannot conveniently be moved from one location to another within a building must be replicated, perhaps many times. Clearly, where the protection of equipment is of paramount importance, base isolation must reduce cost. 6.2. Savings as a function of input motion
As the intensity of input required by the regulatory authorities increase it is clear that conventional design
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methods will become impractical. The transition seems to be at a peak ground acceleration of 0.5g. To design for continued structural integrity above this input some new structural systems must be used particularity if elastic response is mandated. Cost comparisons are impossible to make but it is clear that isolation will be more cost effective as the input level increases. As a case in point we cite the example of a large nuclear plant built conventionally for a peak acceleration of 0.4g. On discovery of an adjacent possibly active fault the design level peak acceleration was increased to 0.75g. The retrofit strategy of modifying the various structural and piping systems has proved extremely difficult and costly. Had the system been isolated and designed for a peak acceleration of 0.4g, and had the structure then to be redesigned for a 0.75g peak acceleration it is clear that the modification of an isolation system - in order to maintain the same seismic levels in the superstructure, the piping systems and other equipment - either by changing the bearings or by increasing the damping in existing bearings through add-on devices would have been an order of magnitude less difficult and much less costly than has been the case. 7. Special issues relevant to base isolation
The foregomg discussion has shown that the use of rubber bearings for seismic isolation is both practical and cost effective and can lead to simple design techniques. Although the system is nonlinear, the degree of nonlinearity is not great and for most structures and particularly non-nuclear applications the system can be designed using a linear approach assuming for the stiffness and damping of the bearings the minimum values. This will allow the use of a design spectrum directly avoiding the use of time history analysis with spectrum compatible earthquake input. A design spectrum incorporates probabilistic concepts whereas the expense of nonlinear analysis precludes the use of more than a few spectrum-compatible earthquake inputs. The nonlinear aspects of the present system are invoked only at small strain where the high initial stiffness and damping are assumed to carry the wind load and the increasing stiffness as the strain increases beyond 100% (inset in fig. 2) can be relied on to control response should an earthquake greater than the maximum credible occur. While the use of a design spectrum simplifies the design procedures it is not without disadvantages. For example, a spectrum such as that used for the building quoted here with a constant spectral velocity over the period range 0.8 to 4.0 s will underestimate the acceleration at shorter periods and overestimate the displace-
ment at longer periods. Generally, buildings with long periods are tall and must be designed for high wind loads and this will usually control the lateral stiffness design. Thus the conservatism in the seismic design is not important and in any case excessively large displacements can be spread over many floors to produce interstory relative displacements which are lower than code drift limitations. However, base-isolated buildings with long periods will not be tall and the spectral displacement will be concentrated at the isolation layer particularly for rigid structures such as nuclear plants. The conservatism at long periods in a design spectrum therefore involves a penalty in contrast to the underdesign permitted at short periods for rigid structures. If a constant displacement spectrum could be used in the period range 2.0 to 4.0 s rather than constant velocity, designers of isolation systems would be provided an incentive to aim for longer period systems. As it stands a 2.0 s period is the best compromise between force reduction and displacement demand, e.g. increasing the period to 3.0 s only reduces the forces by 33% but increases the displacement demand by 50%. If constant displacement design spectra were permitted in this range, a very substantial reduction in the design forces in the structure could be realized with no penalty in displacement demand. The reason for the conservatism in all design spectra at longer periods is the substantial uncertainty that exists in the determination of displacement from strong motion records. In the past this has been done by double integration of the acceleration trace and this has tended to produce errors in the low-frequency range that distort the displacements. However, new methods of processing strong motion data are now available (see for example ref. [22]) and in the future more reliable displacement calculations at low frequencies will be possible. An example is given in ref. [22] of the use of a new digitization and processing system applied to a particular record where the peak velocity was increased by 6% and the maximum displacement decreased by 76% over those produced by the standard method. If more realistic design spectra based on these recent improvements in strong motion record processing are used for the design of base-isolated systems then this radically new approach to seismic protection will be able to achieve its full potential in reducing structural cost and increasing safety. 8. Conclusions
The design of practicable base isolation systems for many types of structure is now possible. Very simple
C.J. Derham et al. / Nonlinear natural rubber bearings
systems will be used to isolate relatively small buildings with limited occupancy - buildings such as structures housing transformers or pump stations. Somewhat more elaborate systems will be designed for buildings such as hospitals, schools, and similar public structures that must be designed for severe earthquake loading. Because such buildings are now designed by dynamic analysis, the design costs will not be greater when base isolation is used, and if nonlinear analysis is not necessary these costs can be substantially reduced. More elaborate and costly systems are appropriate for nuclear power plants. The seismic qualification of equipment has always been of concern in the design of nuclear power plants and this concern has been extended to non-nuclear installations as well. The cost of internal equipment in, for example, a telephone exchange or computer complex is potentially several times that of the building in which it is housed. A compelling argument for base isolation is thus the protection afforded internal equipment and piping. Although the main structure of a building or power plant can be protected from the damaging effects of an earthquake relatively easily, the necessary strengthening of the main structure increases the seismic loads transmitted to nonstructural components and equipment. The response of nonstructural components has been shown [23,24] to be determined primarily by the response of the primary structure to earthquake ground motion and not by the ground motion itself. The design process for components to be housed in conventionally founded structures is particularly difficult. The process is complicated by uncertainties in the specification of ground motion and by uncertainties about the properties of the primary structure. Such uncertainties can be avoided and safer designs realized by the alternative approach of constructing buildings on base isolation systems. The major benefits of base isolation to equipment and piping design are that considerations of equipment-structure interaction and inelastic response are unnecessary. In addition, because the primary structure above the isolation system moves as a rigid body, the displacement time histories of all support points of a piping system will be identical. Multiple support response analysis need not be employed. Clearly, this is of primary importance to the aseismic design of equipment and piping installations. Although base isolation has generally been proposed for new construction, the concept is readily adaptable to the rehabilitation of buildings or power plants that do not comply with.code requirements. The technology to insert rubber bearings under existing buildings with no
427
damage to the structure is available. Rehabilitation by base isolation will be a much less costly and disruptive procedure than current practice.
References [1] R.B. Rigney, Local authority problems in planning for earthquake disaster, Proc. Intern. Conf. on Natural Rubber for Earthquake Protection of Buildings and Vibration Isolation, Kuala Lumpur, Malaysia (1983). [2] H.B. Seed, C. Ugas and J. Lysmer, Site-dependent spectra for earthquake-resistant design, Bulletin of the Seismological Society of America 66 (1976) 221-243. [3] J.M. Kelly and D.E. Chitty, Testing of a wind restraint for aseismic base isolation, Report No. UCB/EERC-78/29, Earthquake Engineering Research Center, University of California, Berkeley (1978). [4] W.P. Fletcher and A.N. Gent, Nonlinearity in the dynamic properties of vulcanizedrubber compounds, Rubber Chemistry and Technology 27 (1954) 209-222. [5] A.R. Payne, Study of carbon black structures in rubber, Rubber Chemistry and Technology 38 (1965) 382-399. [6] C.J. Derham and A.G. Thomas, The design and use of rubber bearings for vibration isolation and seismic protection of structures, Preprint 79-PVP-58, Pressure Vessels and Piping Conference, American Society of Civil Engineers, San Francisco, California (1979). [7] C.J. Derham, L.R. Wootton and S.B.B. Learoyd, Vibration isolation and earthquake protection of buildings by natural rubber mountings, Proc., The Rubber in Engineering Conference, Kuala Lumpur, Malaysia (1974). [8] C.J. Derham et al., Natural rubber foundation bearings for earthquake protection: experimental results, Natural Rubber Technology,Vol. 8, Part 3 (1977) 41-61; reprinted in Rubber and Chemistry Technology 53 (1980). [9] J.M. Kelly, J.M. Eidinger and C.J. Derham, A practical soft story earthquake isolation system, Report No. UCB/EERC-77/27, Earthquake Engineering Research Center, University of California, Berkeley (1977). [10] J.M. Kelly, M.S. Skinner and K.E. Beucke, Experimental testing of an energy absorbing base isolation system, Report No. UCB/EERC-80/35, Earthquake Engineering Research Center, University of California, Berkeley(1980). [11] J.M. Kelly and K.E. Beucke, A friction damped base isolation system with fail-safe characteristics, Intern. J. of Earthquake Engineering and Structural Dynamics 11 (1983) 33-56. [12] J.M. Kelly and D.E. Chitty, Control of seismicresponse of piping systems and components in power plants by base isolation, Engineering Structures 2 (1980) 187-198. [13] J.M. Kelly, The influence of base isolation on the seismic response of light secondary equipment, EPRI NP-2919, Electric Power Research Institute, Palo Alto, California (1983).
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[14] J.M. Kelly, The use of base isolation and energy-absorbing restrainers for the seismic protection of a large power plant component, EPR1 NP-2918, Electric Power Research Institute, Palo Alto, California (1983). [15] M.A. Bhatti, V. Ciampi, J.M. Kelly and K.S. Pister, An earthquake isolation system for steam generators in nuclear power plants, Nucl. Engrg. Des. 73 (1982) 229-252. [16] J.M. Kelly, Testing of a natural rubber base isolation system by an explosively simulated earthquake, EPRI NP-2917, Electric Power Research Institute, Palo Alto, California (1983). [17] G.C. Delfosse, The GAPEC system: a new highly effective aseismic system, Proc., Sixth World Conf. on Earthquake Engineering, New Delhi, India (1972). [18] US Atomic Energy Commission, Regulatory Guide 1.60, Design Response Spectra for Seismic Design of Nuclear Power Plants, Revision 1 (1973). [19] C.J. Derham and R.A. Waller, Luxury without rumble, Consulting Eng. 39 (1975).
[20] T.-C. Pan and J.M. Kelly, Seismic response of torsionally coupled base-isolated structures, Intern. J. of Earthquake Engineering and Structural Dynamics (1984) to appear. [21] A.G. Tarics, Cost considerations of base isolation, Proc. Intern. Conf. on Natural Rubber for Earthquake Protection of Buildings and Vibration Isolation, Kuala Lumpur. Malaysia (1983). [22] S.B. Hodder, Computer processing of New Zealand strong motion accelerograms, Bulletin, New Zealand National Society for Earthquake Engineering 16 (1983) 234-246. [23] J.M. Kelly and J.L. Sackman, Shock spectra design methods for equipment-structure systems, The Shock and Vibration Bulletin 49 (1979) 171-176. [24] J.L. Sackman and J.M. Kelly, Equipment response spectra for nuclear power plant systems, Nucl. Engrg. Des. 57 (1980) 277-294.
417
N O N L I N E A R NATURAL RUBBER BEARINGS FOR SEISMIC ISOLATION C.J. D E R H A M
1, J . M . K E L L Y 2 a n d A . G . T H O M A S
1
1 The Malaysian Rubber Producers" Research Association, Hertford, England 2 College of Engineerin~ University of California, Berkeley, California, USA Received 25 April 1984
This paper summarizes the results of a ten-year joint research program on base isolation carried out by the Malaysian Rubber Producers' Research Association in England and the University of California at Berkeley. Special rubber bearings have been developed which protect buildings and their contents from earthquake damage without requiring additional mechanical devices to enhance damping or to avoid problems of wind movement. The lack of complexity in the system makes prediction of response more certain and reduces cost so that in many designs the additional protection will be achieved at a cost lower than that for conventional protection. The principles of design are explained and results given for the many experimental investigations of the system which have included measurements of the response of both the building and its internal equipment. Practical aspects of the use of the system are discussed with reference to a large structure at present being constructed on nonlinear natural rubber bearings in a highly seismic area.
1. Introduction The primary objective of any strategy for earthquake protection must be the saving of lives - but in terms of potential human suffering the breakdown of the infrastructure of society in the aftermath of an earthquake is very nearly as important a consideration. Within this infrastructure, the necessity for the uninterrupted operation of hospitals, rescue services, and communications is obvious - but less obvious is the importance of protection for computer records in connection with social services, health and insurance [1]. Nuclear power plants have now become an important part of this infrastructure and, at least in the public imagination, represent a serious potential hazard if damaged in an earthquake. Traditional methods of earthquake protection which rely on strengthening and ductility are mainly aimed at preventing the collapse of structures and not at reducing the forces within the structure. To provide protection for the contents - which is the key to sustaining the services mentioned and to the effective containment of radioactive or toxic materials - it is necessary to take a different approach. The earthquake forces transmitted to the structure must be reduced, and they can be reduced by exactly the same approach used in every
other field of vibration engineering - by isolation. The concept of earthquake isolation has been discussed for many years but the problem has always been that of controlling effectively the very large amplitudes of vibration associated with earthquakes. It has now been shown that these amplitudes can be sustained by specially designed laminated rubber and steel mountings, generally known as bearings. The function of the bearings is to reduce the translational natural frequency of the mounted structure to a region well below that of most significant earthquake energy. In regions of good soil or rock it has been shown that a natural frequency of around 0.5 Hz for the mounted structure will result in greatly reduced forces on both the structure and its contents. Bearings can be satisfactorily designed to support a structure at this frequency.
2. The basic system 2.1. General description For twenty-five years at least laminated rubber/steel structural bearings have been used in civil engineering. Initially their mean function was as bridge bearings where their high vertical stiffness combined with their
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C.J. Derham et aL / Nonfinear natural rubber bearings
relatively low horizontal stiffness enabled them to be used instead of mechanical bearings. Because they require no maintenance and are easy to install, they are now used worldwide to support new bridge decks. Since the mid-1960s, similar bearings have also been used to mount buildings where there is a problem of groundborne vibration. There are now many buildings constructed in this way, usually over or adjacent to railway lines, and the techniques has proved successful both in effectiveness and longevity. All indications are that the bearings are at least as durable as conventional building materials. Recently, the technique of building on laminated rubber and steel bearings has been extended to provide earthquake protection to structures. A small number of buildings have been constructed in this way - possibly the largest is still under construction in San Bernardino, Califorma. Some aspects of the design of this building are discussed in more detail below. The basic requirements for earthquake protection are that; - The bearings must support the dead load of the structure and must have a high vertical stiffness. - The horizontal stiffness of the bearings must be such as to confer on the mounted structure a low horizontal natural frequency so that the building will not respond to the destructive components of the ground motion. F r o m the response spectra of Seed [2] it is clear that under a wide range of conditions a horizontal natural frequency of 0.5 Hz is suitable. - Some earthquake energy will always occur at or near the horizontal natural frequency so the system must contain sufficient damping to limit translational movement to an acceptable level. 2.2. E l a s t o m e r characteristics
Isolation bearing systems must prevent excessive movement of structures under wind loading. It must be evident that this requirement is not on the grounds of safety - the system is designed to prevent damage due to much more severe effects - but primarily for the comfort of the occupants as even a slight swaying motion might be disconcerting. Also, there might be a problem of fatigue in the service connections. The magnitude of the wind movement, d, for a bearing system with a linear force/shear deflection relation given in terms of the natural horizontal frequency, f, is: d = a g / ( 2 ~ ' f ) 2,
(1)
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the building. For f = 0.5 Hz and a = 0.05 (typical values for wind loading requirements), this relation yields d = 5 cm (2 in). In order to prevent this perhaps unacceptable movement, various devices have been proposed. For example, it has been suggested that mechanical devices can be incorporated which will be strong enough to resist wind loads but which would break if an earthquake of appreciable magnitude occurred. While it is quite possible to design such a system [3] it is an added complication, and it now appears that an adequate solution to the problem can be obtained by utilizing the elastic nonlinearity of filled rubber compounds. It has been known for many years that rubber vulcanizates containing filler show markedly nonlinear stress-strain behavior (see for example refs. [4] and [5]). The form of the nonlinearity in shear is such that for small deformation, of the order of a few per cent, the material has a substantially higher effective modulus than for high deformation. For a filled material similar to that used in the earthquake bearings for the San Bernardino project, the simple shear stress-strain relation is as shown in fig. 1. The nonlinearity is clear. Under cyclic deformation at various maximum amplitudes, the relation is as shown in fig. 2. This again shows the nonlinearity, the effective modulus at small amplitudes being high, but it also illustrates the substantial mechanical hysteresis found when the material is taken around a stress cycle. Indeed, the compound was specifically designed to have a large hysteresis for strain amplitudes of about 50% in order to give the bearings an acceptable response
CJ. Derham et aL / Nonlinear natural rubber bearings
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under earthquake conditions. At these large deformations, the rubber is less nonlinear and the linear viscoelastic model used to predict the response of the mounted building to an earthquake is adequate [8]. The hysteresis loss of this rubber at 50% strain is equivalent to that for a linear material with a loss angle of about 11 °, giving 10% of critical damping. The curves of fig. 2 show no sharp corners and this implies that a building mounted on such a material will not have its higher modes strongly excited, in contrast to systems giving an instantaneous yield or break point. Also shown in the inset to fig. 2 is the change in the stress-strain curve when the shear strain exceeds 100%. The material hardens and the effective modulus begins to increase. The shear stress on the bearing under wind loading can be written as: d
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This form shows the effect of nonlinearity more clearly as it is well known that the small strain modulus G1 may be several times greater than G I. For example, with the special compounds used in the San Bernardino project bearings, the ratio G J G 1 at the relevant stresses is almost 20% so that the displacement d a is about 1 cm (0.4 in), which is considered to be sufficiently small so that no special arrangements need be made to accommodate it. For comparison, a building 50 m long will exhibit a change in length of about this amount for a temperature change of 20 ° C. Thus, by exploiting the nonlinear behavior of the compounded natural rubber materials the necessity for mechanical fuses or equivalent restraints can be avoided. These materials have the added advantage that their characteristic stress-strain relation is very quickly
420
c J . Derham et al. / Nonlinear natural rubber bearings
recovered after cycling so that even after an earthquake no repair or replacement should be required and there will be no permanent offset.
3. Details of investigations 3.1. Laboratory investigations
A computer analysis of a seven-story building a representative of a standard type of hospital construction, carried out by Atkins Research and Development at the request of the Malaysian Rubber Producers' Research Association (MRPRA) [7], indicated that the structure when isolated would experience a reduction in peak acceleration of around tenfold as compared to the peak acceleration experienced when carried on a conventional foundation. This and later computer studies [6] predicted that the building on rubber isolators would behave as a rigid body and that the forces transmitted to the upper levels would be greatly reduced. An experimental testing program to verify this approach to seismic protection was begun at the Earthquake Simulator Laboratory of the Earthquake Engineering Research Center of the University of California, Berkeley. This research work was initially sponsored by the MRPRA and later by the National Science Foundation. It has included much dynamic testing of different forms of isolation system on the shaking table at the Earthquake Simulator Laboratory and also static testing of full-scale isolation devices on the large-scale testing facility at the Earthquake Engineering Research Center. Since dynamic testing began in 1976 several designs of rubber bearing have been tested and several models have been used. The shaking table at the laboratory is 6.1 m (20 ft) square and can test models up to 50 tonnes total weight. It can reproduce one horizontal component and the vertical component of any historical earthquake for which a strong motion record is available and any artificial record. The earthquake intensity can be increased to a maximum of around 1.5g peak acceleration in the horizontal component and around 0.5g vertically. Time scaling of the earthquake input to correspond to geometrical scaling of the model is also possible. In early tests in the program handmade natural rubber bearings were used with a 20-tonne, three-story, single bay model. These tests, reported in ref. [8], confirmed the computer predictions and demonstrated that the approach was practicable. Later, commercially made bearings were used with a larger model, a 40-tonne, five-story, three bay frame, a one-third scale portion of
a typical small structure. The testing program included the testing of wind restraints [3] and hysteretic energyabsorbing devices in conjunction with the rubber bearings [9,10]. The use of sliding friction with isolation bearings was also tested. In this system the frictional element was used to dissipate energy and control displacement and also functioned as a fail-safe system in the sense that if the rubber bearing system was overextended the structure would be carried on the sliding surfaces [11]. The response of sensitive internal equipment has been very important aspect of this research program. In many cases the cost of internal equipment exceeds that of the structure housing it. In several dynamic tests instrumented models of internal equipment were attached to the structural models and the response of the equipment recorded when the structure was isolated and when not. The reductions in peak acceleration achieved when rubber bearings were used was even greater for the equipment than for the structure, demonstrating that isolation of a building protects not only the building but the equipment and contents. This portion of the research program was sponsored by the Electric Power Research Institute (EPRI) and has been reported in refs. [12] and [13]. The Institute also sponsored a study of the influence of isolation on the response of large equipment items in power plants. A model steam generator in a model of a structural frame was tested on the shaking table and it was shown that isolation can also be an effective strategy for the seismic protection of large components. This is reported in refs. [14] and [15]. In all, during the various aspects of the base isolation testing program, several hundred dynamic tests utilizing horizontal and vertical excitations and up to high levels of intensity have been carried out. They have demonstrated that base isolation using rubber bearings is an effective anti-seismic strategy for many types of building and structure. 3.2. Field investigations
A field test of a base isolation system was undertaken as one of the experiments of the SIMQUAKE II test program conducted in 1978 by EPRI at the McCormick Test Site. In this experiment an explosive source consisting of two parallel arrays of explosive, 40 tonnes and 30 tonnes of ANFO explosive, were detonated with the intention of producing a ground motion similar to that of a large earthquake. Several experiments were included in this test in addition to the isolation experiment. This involved natural rubber bearings and a fail-safe system, carrying a one-twenty-fourth
C.J. Derham et al. / Nonlinear natural rubber bearings
scale model of a containment structure. The results of the test of the isolation equipment are reported in refs. [161. In the initial design of the SIMQUAKE II experiment a certain design spectrum and peak acceleration were assumed and the bearings were designed to produce a natural frequency of 1 Hz in the isolated structure which would put the model out of the range of the anticipated horizontal ground acceleration. In the actual SIMQUAKE II event, the ground motions at the location of the isolation model were quite different. The ground motions took the form of two distinct earthquakes, the first having a peak horizontal acceleration o f + 0.75g and peak vertical accelerations o f + 1.25g,-0.94g, the second having a peak horizontal acceleration of + 3.0g and peak vertical accelerations of + 4 . 4 g , - 5 . 6 g . The two earthquakes resulted from the fact that the two arrays of explosive were fired with a time separation of 1.2 s and at the location of the model, 68.6 m (225 ft) from the first array (fired last), the two motions remained distinct. T h a t the array producing the less intense ground motion was fired first proved fortunate for the isolation system. The system easily overrode this earthquake, attenuating the horizontal acceleration transmitted to it as expected and slightly amplifying the vertical acceleration. In the second earthquake the upward acceleration reached 4.4g followed immediately by a downward acceleration of 5.6g. This produced tension in the bearings and two delaminated as a result. The fail-safe system then took over from these two bearings and collapse of the model was prevented with, of course, substantially increased acceleration in the superstructure. The high horizontal accelerations in this earthquake with a peak of 3.0g preceding the failure were successfully attenuated by the system. The isolation system survived an earthquake with peak horizontal and vertical accelerations of 0.75g and 1.25g, respectively, and was only overcome by an earthquake motion with a peak horizontal acceleration of 3.0g and a peak vertical acceleration of 5.6g, at which the fail-safe system came into operation and prevented the collapse of the structure. Vertical accelerations of the magnitude experienced by the model in the SIMQUAKE II test are not possible seismic events and tension failure such as occurred in this experiment would not occur in a fullscale system. 4. Example installations 4.1. Implemented systems
The particular features of the system outlined here are the use of laminated natural rubber bearings with no
421
additional wind restraint and no hysteretic energy-absorbing device. A system of this kind albeit using conventional natural rubber polymer was installed in an elementary school in Lambesc, Marseilles, France, completed in 1978 [17]. This school was built on 152 isolators, is 80 m (254 ft) by 30 m (100 ft) and three stories high. The isolators are 30 cm (1 ft) in diameter with a total rubber thickness of 5 cm (2 in) and the system was designed for an VIII M.M. earthquake. The school building was constructed using a prefabricated concrete building system that without the use of isolators would not have satisfied the local seismic code requirements. Modifying the design to conform to the code would have increased construction costs significantly. The seismic requirements for the school were not stringent and the displacement capacity of the bearings is consequently not great. The system has been modified and is now being used for a nuclear waste storage facility for the French Navy. The bearings in this modified system are much larger and the displacement capacity very much greater, but the design is the same and conventional natural rubber is used. The first use of the isolation system which utilizes high-damping rubber formulations specifically designed for seismic isolation bearings is a large local government building now under construction in San Bernardino County, California. This building, the thirty million dollar Communities Law and Justice Center in Rancho Cucamonga, will be mounted on 98 isolators (fig. 3). The isolators must be able to carry a wide range of vertical load from 100 tonnes to around 400 tonnes and thus several types of isolator are used. They are all however 76 cm (30 in) in diameter and around 38 cm (15 in) thick. There are two high damping rubber compounds and four rubber thickness-steel shim combinations. The building is 127 m (417 ft) by 33.5 m (110 ft), has four stories and a full basement. It is located within 21 km (13 miles) of the San Andreas Fault and has been designed to remain elastic under a Richter magnitude 8.3 earthquake on that fault. The design spectrum (5% damping) selected as appropriate for such an event at the building site is a constant velocity spectrum of 127 c m / s (50 in/s) over a range of period from 0.8 s to 4.0 s. The peak spectral acceleration is lg. In this range the design spectrum is more severe than the Newmark-Hall spectrum given in Regulatory Guide 1.60 [18]. The building had been designed with a moment-resisting frame and for this spectrum the roof displacement would be of the order of 38 cm (15 in), clearly unacceptable in a four-story building. By reducing the displacement to an acceptable level using a braced
422
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frame, very high forces in the structure and on the contents, equipment and occupants would be produced. In fact it is uneconomic to design any structure with a conventional structural system to be elastic for such a spectrum. However, it can be done using rubber bearings. At a two-second period the spectrum requires 40.5 cm (16 in) relative deflection in the bearings based on 5% damping. In the final design 10% damping was selected and a two-second period. Rotation produced by eccentricity was accounted for and including rotation the maximum relative displacement at the corner bearings was estimated to be 38 cm (15 in). The beatings for the building were constructed by Oil States Industries, Athens, Texas and have been installed. The erection of the superstructure is underway. 4.2. Verification testing
Prior to the decision to proceed with construction of the building on rubber, four prototype bearings were constructed in the new rubber formulation and were tested at the EERC to verify that they were capable of
the displacement demands. The bearings tested had nineteen 1.60 cm (0.63 in) layers of rubber and eighteen 6.4 mm (0.25 in) thick steel plates with two 31.8 mm (1.25 in) thick end plates. The end plates are dowelled into sole plates which connect the beatings to the foundation and to the superstructure. The dowels transmit only shear load and thus reduce tension loads on the beatings and allow for ease of removal if this should be necessary. The bearings were tested in a special test rig that allowed the application of simultaneous vertical load and horizontal displacement. The vertical load which ranged up to a maximum of 5.35 NM (1.2M lbs) per bearing was applied by the 18 MN (4M lb) Southwork and Energy testing machine at the EERC and horizontal load was produced by large hydraulic jacks between the bearings. The set-up is shown in fig. 4. The bearings were loaded to preset vertical loads and with the vertical load held constant were loaded through horizontal displacement cycles that increased in steps of 2.54 cm (1 in) to a maximum of 38 cm (15 in). They were also cycled thirty times at a maximum displacement of 28 cm (11 in) to verify their fatigue resistance. After the
C.J. Derham et al. / Nonlinear natural rubber bearings
423
Fig. 4. Configuration of bearings at 38 cm displacement.
test program was completed, a bearing was cut in half to see if any internal damage had been produced. No damage was evident and it is clear that 38 cm (15 in) displacement which represents a shear strain of 125% is not the limit of the bearing response. The horizontal and vertical stiffness of the bearings as measured during the test were exactly as predicted from the tests on the rubber from which they were fabricated. The damping was lower than that predicted by the tests on the rubber samples due to the fact that the samples were tested at 2 s per cycle whereas the bearings were tested at 10 rain per cycle. These tests represent a very severe test of the material. Failure processes in rubber are time dependent and in a severe earthquake the bearings will be at high strains ( > 100%) for only fractions of a second while in the tests the maximum strain was held for some minutes. The configuration of the bearings at a displacement of 38 cm (15 in) is shown in fig. 4.
The major criticism of the use of rubber bearings alone with no hysteretic system to control displacement is the possibility of very large displacements. The critical factor in seismic bearings is not the maximum shear strain in the rubber but the ratio of the maximum displacement to the plan dimension of the bearing. In dynamic tests on model isolation systems using the shake table circular bearings have been loaded to 80% of their diameter without failure and square bearings to a slightly higher ratio. It is clear that 38 cm (15 in) for the prototype bearings is quite conservative yet they have been designed to provide elastic response under design spectra more severe than anything for which buildings are conventionally designed. If larger displacements are anticipated, larger bearings can be used and there is no practical limit to the size of bearing that can be manufactured.
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5. Reliability The reliability of natural rubber bearings has been demonstrated over the past three decades through experience with bridge bearings which have the same general structure as isolation bearings. High-precision rubber bearings are used in helicopters to replace journal bearings in locations where the motion is cyclic rather than rotary and a similar form of bearing is used for fenders on docks and wharves. In some cases bearings have been removed and retested after decades of use and have been found to have the same mechanical characteristics as when installed. Natural rubber bearings have been used to isolate buildings from ground-borne acoustic vibration and in at least one instance the reliability of the bearings has been continually monitored for more than twenty years [19]. Machinery has been isolated for many years but the requirements for seismic isolation are quite different. In machinery isolation the frequency range to be isolated against is generally well defined and the amplitudes are small. In seismic isolation neither the frequency content of the input nor its amplitude can be prescribed in advance and the dynamic response of the system will be transient rather than steady state. For these reasons the design problem is quite different although the general principle remains the same. The displacements to be designed for are also an order of magnitude greater than in isolation for machinery vibration. Thus such isolation concepts can be transferred to seismic isolation only with care. Further differences are that bearings for machinery isolation are generally small but lightly loaded whereas for buildings the vertical loads may be extremely large and although the plan dimensions of a bearing will be large the average bearing stress will be much higher. However, it is a fact that the design of a bearing for a set isolation frequency is easier since buckling of the bearing becomes less important as the isolated mass carried by the bearing increases. It is thus more convenient to isolate a complete structure than to isolate separate items of equipment. Small items of equipment should be grouped together and isolated as a single unit on a connecting slab. This facilitates bearing design and eliminates the need to consider coupling of isolated and nonisolated components. Uneven reactions and settlements will present problems in isolated buildings but no more so than in conventional buildings since the vertical stiffness of the bearings is comparable to that of standard columns. Severe differential settlements could damage the diaphragm immediately above the bearing system but the
potential for retrofit always exists and compensating for unexpected differential settlement would be relatively easy as compared to conventional construction. In the system described here hysteretic energy-absorbing devices are not used and it is felt that their use should be avoided if at all possible. The damping that they can provide is higher than that which can be provided by the elastomer but it is purchased at the cost of mechanical complexity, uncertainties in performance and the need for nonlinear analysis in the design phase. They must also inevitably lead to the accumulation of nonreversible displacement and the consequent need to devise a procedure for the elimination of these displacements. In the plain rubber system permanent sets do not occur. One question left unanswered by the laboratory testing program is as to the response of the system to travelling wave effects. While both horizontal and vertical shake table motions were used and this provides some indication of the response no shake table presently can simulate this phenomenon and recourse must be made to numerical analysis. A further seismic phenomenon, namely the effect of distant earthquake, is not felt to be important since resonance at low amplitude is dealt with by the intrinsic damping of the material. This intrinsic damping also has the effect of reducing the influence of imperfections in the response of the system which can induce unwanted response in dynamic systems. The most prominent such effect in isolation systems is that of modal coupling of torsional and lateral motions due to the noncoincidence of the centers of mass and rigidity. Since it is likely that an isolated building will have closely spaced natural frequencies in the torsional and the two lateral modes, modal coupling may be anticipated. The phenomenon has been studied in detail, however, and it has been shown [20] that at the levels of damping envisaged here the coupling generated by the imperfections has very little effect on the overall response of the system.
6. Cost 6.1. Cost of installed system
In assessing the effectiveness of base isolation the remaining unanswered question is that of cost since designs with and without isolators and offering the same degree of seismic protection are not readily available. The bearings themselves are not expensive items particularly if many are made. The bearings for the San Bernardino County building, for example, cost less than
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C.J. Derham et al. / Nonlinear natural rubber bearings PROTOTYPE MEDICAL BUILDING COST COMPARISON ADD ROOF 5
46
BASE ISOLATORS
RETAINING WALL 720'x10'x$20
DEDUCT
$46OOO $14400O
4
3 2
BASEMENT SLAB SEPARATION 240x120 x$2
$67000
MISC. OTHER
$300oo
1 f, -CHANIC~,L ROOM PIPEBA: EMENT
SEISMIC BRAC~G SHEAR WALLS 720'x73'x$3
/ BASE ISOLATOR
12"ck
SECTION 240' ,¢ 'RETAININGWALL ¢
$168000
¢
'w
t
FLOOR AND ROOF DIAPHRAMS, CONNECTIONS 240x 120x7x$0.3
$80000
CEIL~G BRAC~G 240x120x7x$O.B
$8B000
BRACING MECHANICAL AND ELECTRICAL COMPONENT8
Sa0o0o
t
$277000
$3840o0
$ 384000 - $ 277000 SAVING :
BASE ISOLATION
$ 107000 PLUS COST OF BRACING LOOSE EQUIPMENT
Fig. 5. Cost of medical building when conventionally constructed and when base isolated.
1% of total building cost. There were additional costs in this example since the building had originally been designed according to the UBC and architectural modifications had to be made to allow for the seismic gap around the building. The savings on the other hand in the construction of the superstructure of a building could offset these increased costs. Seismic shear walls will be diminished and other structural elements reduced in size. Bracing of components suspended from the ceiling and of mechanical and electrical components will be reduced. The results of a study carried out by Reid&Tarics Associates of San Francisco on the comparative cost of an isolated and conventionally founded six-story medical building with roughly 16000 sq m (170000 sq ft) are summarized in fig. 5 [21]. A potential savings of 107 000
dollars had base isolation been used was estimated for the structural system alone. Further savings would be possible since the major portion of the cost of a medical facility is in sensitive equipment. If such equipment must be braced or attached to the walls so as to protect it from damage in the event of an earthquake its mobility will be greatly reduced. Equipment that cannot conveniently be moved from one location to another within a building must be replicated, perhaps many times. Clearly, where the protection of equipment is of paramount importance, base isolation must reduce cost. 6.2. Savings as a function of input motion
As the intensity of input required by the regulatory authorities increase it is clear that conventional design
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CJ. Derham et al. / Nonlinear natural rubber bearings
methods will become impractical. The transition seems to be at a peak ground acceleration of 0.5g. To design for continued structural integrity above this input some new structural systems must be used particularity if elastic response is mandated. Cost comparisons are impossible to make but it is clear that isolation will be more cost effective as the input level increases. As a case in point we cite the example of a large nuclear plant built conventionally for a peak acceleration of 0.4g. On discovery of an adjacent possibly active fault the design level peak acceleration was increased to 0.75g. The retrofit strategy of modifying the various structural and piping systems has proved extremely difficult and costly. Had the system been isolated and designed for a peak acceleration of 0.4g, and had the structure then to be redesigned for a 0.75g peak acceleration it is clear that the modification of an isolation system - in order to maintain the same seismic levels in the superstructure, the piping systems and other equipment - either by changing the bearings or by increasing the damping in existing bearings through add-on devices would have been an order of magnitude less difficult and much less costly than has been the case. 7. Special issues relevant to base isolation
The foregomg discussion has shown that the use of rubber bearings for seismic isolation is both practical and cost effective and can lead to simple design techniques. Although the system is nonlinear, the degree of nonlinearity is not great and for most structures and particularly non-nuclear applications the system can be designed using a linear approach assuming for the stiffness and damping of the bearings the minimum values. This will allow the use of a design spectrum directly avoiding the use of time history analysis with spectrum compatible earthquake input. A design spectrum incorporates probabilistic concepts whereas the expense of nonlinear analysis precludes the use of more than a few spectrum-compatible earthquake inputs. The nonlinear aspects of the present system are invoked only at small strain where the high initial stiffness and damping are assumed to carry the wind load and the increasing stiffness as the strain increases beyond 100% (inset in fig. 2) can be relied on to control response should an earthquake greater than the maximum credible occur. While the use of a design spectrum simplifies the design procedures it is not without disadvantages. For example, a spectrum such as that used for the building quoted here with a constant spectral velocity over the period range 0.8 to 4.0 s will underestimate the acceleration at shorter periods and overestimate the displace-
ment at longer periods. Generally, buildings with long periods are tall and must be designed for high wind loads and this will usually control the lateral stiffness design. Thus the conservatism in the seismic design is not important and in any case excessively large displacements can be spread over many floors to produce interstory relative displacements which are lower than code drift limitations. However, base-isolated buildings with long periods will not be tall and the spectral displacement will be concentrated at the isolation layer particularly for rigid structures such as nuclear plants. The conservatism at long periods in a design spectrum therefore involves a penalty in contrast to the underdesign permitted at short periods for rigid structures. If a constant displacement spectrum could be used in the period range 2.0 to 4.0 s rather than constant velocity, designers of isolation systems would be provided an incentive to aim for longer period systems. As it stands a 2.0 s period is the best compromise between force reduction and displacement demand, e.g. increasing the period to 3.0 s only reduces the forces by 33% but increases the displacement demand by 50%. If constant displacement design spectra were permitted in this range, a very substantial reduction in the design forces in the structure could be realized with no penalty in displacement demand. The reason for the conservatism in all design spectra at longer periods is the substantial uncertainty that exists in the determination of displacement from strong motion records. In the past this has been done by double integration of the acceleration trace and this has tended to produce errors in the low-frequency range that distort the displacements. However, new methods of processing strong motion data are now available (see for example ref. [22]) and in the future more reliable displacement calculations at low frequencies will be possible. An example is given in ref. [22] of the use of a new digitization and processing system applied to a particular record where the peak velocity was increased by 6% and the maximum displacement decreased by 76% over those produced by the standard method. If more realistic design spectra based on these recent improvements in strong motion record processing are used for the design of base-isolated systems then this radically new approach to seismic protection will be able to achieve its full potential in reducing structural cost and increasing safety. 8. Conclusions
The design of practicable base isolation systems for many types of structure is now possible. Very simple
C.J. Derham et al. / Nonlinear natural rubber bearings
systems will be used to isolate relatively small buildings with limited occupancy - buildings such as structures housing transformers or pump stations. Somewhat more elaborate systems will be designed for buildings such as hospitals, schools, and similar public structures that must be designed for severe earthquake loading. Because such buildings are now designed by dynamic analysis, the design costs will not be greater when base isolation is used, and if nonlinear analysis is not necessary these costs can be substantially reduced. More elaborate and costly systems are appropriate for nuclear power plants. The seismic qualification of equipment has always been of concern in the design of nuclear power plants and this concern has been extended to non-nuclear installations as well. The cost of internal equipment in, for example, a telephone exchange or computer complex is potentially several times that of the building in which it is housed. A compelling argument for base isolation is thus the protection afforded internal equipment and piping. Although the main structure of a building or power plant can be protected from the damaging effects of an earthquake relatively easily, the necessary strengthening of the main structure increases the seismic loads transmitted to nonstructural components and equipment. The response of nonstructural components has been shown [23,24] to be determined primarily by the response of the primary structure to earthquake ground motion and not by the ground motion itself. The design process for components to be housed in conventionally founded structures is particularly difficult. The process is complicated by uncertainties in the specification of ground motion and by uncertainties about the properties of the primary structure. Such uncertainties can be avoided and safer designs realized by the alternative approach of constructing buildings on base isolation systems. The major benefits of base isolation to equipment and piping design are that considerations of equipment-structure interaction and inelastic response are unnecessary. In addition, because the primary structure above the isolation system moves as a rigid body, the displacement time histories of all support points of a piping system will be identical. Multiple support response analysis need not be employed. Clearly, this is of primary importance to the aseismic design of equipment and piping installations. Although base isolation has generally been proposed for new construction, the concept is readily adaptable to the rehabilitation of buildings or power plants that do not comply with.code requirements. The technology to insert rubber bearings under existing buildings with no
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damage to the structure is available. Rehabilitation by base isolation will be a much less costly and disruptive procedure than current practice.
References [1] R.B. Rigney, Local authority problems in planning for earthquake disaster, Proc. Intern. Conf. on Natural Rubber for Earthquake Protection of Buildings and Vibration Isolation, Kuala Lumpur, Malaysia (1983). [2] H.B. Seed, C. Ugas and J. Lysmer, Site-dependent spectra for earthquake-resistant design, Bulletin of the Seismological Society of America 66 (1976) 221-243. [3] J.M. Kelly and D.E. Chitty, Testing of a wind restraint for aseismic base isolation, Report No. UCB/EERC-78/29, Earthquake Engineering Research Center, University of California, Berkeley (1978). [4] W.P. Fletcher and A.N. Gent, Nonlinearity in the dynamic properties of vulcanizedrubber compounds, Rubber Chemistry and Technology 27 (1954) 209-222. [5] A.R. Payne, Study of carbon black structures in rubber, Rubber Chemistry and Technology 38 (1965) 382-399. [6] C.J. Derham and A.G. Thomas, The design and use of rubber bearings for vibration isolation and seismic protection of structures, Preprint 79-PVP-58, Pressure Vessels and Piping Conference, American Society of Civil Engineers, San Francisco, California (1979). [7] C.J. Derham, L.R. Wootton and S.B.B. Learoyd, Vibration isolation and earthquake protection of buildings by natural rubber mountings, Proc., The Rubber in Engineering Conference, Kuala Lumpur, Malaysia (1974). [8] C.J. Derham et al., Natural rubber foundation bearings for earthquake protection: experimental results, Natural Rubber Technology,Vol. 8, Part 3 (1977) 41-61; reprinted in Rubber and Chemistry Technology 53 (1980). [9] J.M. Kelly, J.M. Eidinger and C.J. Derham, A practical soft story earthquake isolation system, Report No. UCB/EERC-77/27, Earthquake Engineering Research Center, University of California, Berkeley (1977). [10] J.M. Kelly, M.S. Skinner and K.E. Beucke, Experimental testing of an energy absorbing base isolation system, Report No. UCB/EERC-80/35, Earthquake Engineering Research Center, University of California, Berkeley(1980). [11] J.M. Kelly and K.E. Beucke, A friction damped base isolation system with fail-safe characteristics, Intern. J. of Earthquake Engineering and Structural Dynamics 11 (1983) 33-56. [12] J.M. Kelly and D.E. Chitty, Control of seismicresponse of piping systems and components in power plants by base isolation, Engineering Structures 2 (1980) 187-198. [13] J.M. Kelly, The influence of base isolation on the seismic response of light secondary equipment, EPRI NP-2919, Electric Power Research Institute, Palo Alto, California (1983).
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[14] J.M. Kelly, The use of base isolation and energy-absorbing restrainers for the seismic protection of a large power plant component, EPR1 NP-2918, Electric Power Research Institute, Palo Alto, California (1983). [15] M.A. Bhatti, V. Ciampi, J.M. Kelly and K.S. Pister, An earthquake isolation system for steam generators in nuclear power plants, Nucl. Engrg. Des. 73 (1982) 229-252. [16] J.M. Kelly, Testing of a natural rubber base isolation system by an explosively simulated earthquake, EPRI NP-2917, Electric Power Research Institute, Palo Alto, California (1983). [17] G.C. Delfosse, The GAPEC system: a new highly effective aseismic system, Proc., Sixth World Conf. on Earthquake Engineering, New Delhi, India (1972). [18] US Atomic Energy Commission, Regulatory Guide 1.60, Design Response Spectra for Seismic Design of Nuclear Power Plants, Revision 1 (1973). [19] C.J. Derham and R.A. Waller, Luxury without rumble, Consulting Eng. 39 (1975).
[20] T.-C. Pan and J.M. Kelly, Seismic response of torsionally coupled base-isolated structures, Intern. J. of Earthquake Engineering and Structural Dynamics (1984) to appear. [21] A.G. Tarics, Cost considerations of base isolation, Proc. Intern. Conf. on Natural Rubber for Earthquake Protection of Buildings and Vibration Isolation, Kuala Lumpur. Malaysia (1983). [22] S.B. Hodder, Computer processing of New Zealand strong motion accelerograms, Bulletin, New Zealand National Society for Earthquake Engineering 16 (1983) 234-246. [23] J.M. Kelly and J.L. Sackman, Shock spectra design methods for equipment-structure systems, The Shock and Vibration Bulletin 49 (1979) 171-176. [24] J.L. Sackman and J.M. Kelly, Equipment response spectra for nuclear power plant systems, Nucl. Engrg. Des. 57 (1980) 277-294.