Testing protocols for seismic isolation systems

Transcription

1 Testing protocols for seismic isolation systems Gianmario Benzoni Department of Structural Engineering, University of California San Diego, La Jolla, Ca , U.S.A. Giuseppe Lomiento Department of Structural Engineering, University of California San Diego, La Jolla, Ca , U.S.A. Noemi Bonessio Department of Structural Engineering, University of California San Diego, La Jolla, Ca , U.S.A. Keywords: Seismic isolation, testing protocols, elastomeric bearings, dampers, friction pendulum ABSTRACT Recent seismic events like Chile in 21 and Japan in 211 confirmed the seismic isolation technology as capable to provide structures with a high level of protection in case of earthquakes of exceptional intensity. This relatively young protection technique continues to experience a steady increase of applications beyond particular structures like bridges and buildings of strategic importance. Even though the use of seismic isolators and energy dissipators is still surrounded, in some countries, by a certain level of diffidence, the confidence level in its potential and reliability was significantly improved, in the last decade, by extensive experimental campaigns completed at newly designed and dedicated testing facilities. The availability of testing rigs of large capabilities, able to perform testing programs of full scale devices under realistic loading conditions allowed both the certification of unprecedented devices as well as the improvement of the knowledge of the behavior of existing solutions. The experimental activity contributed as well to the development of ad hoc codes that regulate the testing protocols, procedures and acceptance criteria for all the device s typologies. In this paper, recent observations generated by the completion of many tests on a large variety of seismic isolators and energy dissipators, are presented and discussed. In particular the effects of applied load and velocity on the performance parameters of lead-rubber bearings and friction pendulum systems, the complex behavior of double concave sliding devices, the urgent issue of the assessment of the durability of material and assemblies, will be discussed. The evidence from past experience points towards the need for a more pro-active role for the structural engineer A deeper understanding of the device s performance, beyond the specific design requirements established by the codes, is indeed a new crucial duty of the designer that is supported in this effort by the industry, the academia as well as the experimental facilities. 1 INTRODUCTION The importance of the experimental validation of anti-seismic devices is emphasized when associated to the current design procedure of structures equipped with this technology. An optimized design requires, indeed, concentrating the large majority of the non-linear behavior of the structure within the seismic protection devices, demanding only a limited ductility capacity for the rest of the structure. For this reason, it is of paramount importance that the expected performance of the devices be delivered, as designed, during the service life of the structure as well as during seismic events. The required performance needs also to be guaranteed in the long term with minimum deviance from the optimum baseline. These requirements are also complicated by the fact that seismic isolation deals with quite complex mechanical products, with new advanced materials often in service under adverse environmental conditions. In this scenario, shared with other fields like mechanical and aeronautical engineering, the experimental activity plays a fundamental role, not only for the qualification of isolators and energy dissipators, but also for the credibility and acceptance of the whole technology. In many earthquake prone areas, the implementations of

2 the technology are still limited by overly conservative codes, by the low probability of occurrence of a large earthquake over the economic life of the building, by the wrong estimate of the economic benefit of the intervention when compared only with the first construction costs etc. These factors, and others, foster diffidence in this seismic protection technique. However, in recent years, the availability of experimental facilities for the validation of the products that allow implementing the principles of seismic isolation, contributed to significantly improve the credibility of the technology by supporting the industry, the designers and the agencies involved, by providing documentation of the performance of devices under realistic loading conditions. The experience of the last ten years of activity at the University of California Laboratories, which have pioneered the experimental activity on large scale devices, has resulted in a consistent body of observations and reflections on testing facilities, testing protocols and performance characterization of specific classes of devices, that are presented and discussed in the following sections. 2 TESTING FACILITIES During the last decade several testing rigs were specifically designed and dedicated to the qualification of devices for seismic isolation applications. The availability of these equipments, limited before to the manufacturing plants, extended to universities and other independent agencies. Some of these facilities are now able to provide services beyond the research phase, as they have the capabilities to perform tests on devices unprecedented in size and performance to provide as in service characterization. An example of the motivations that generated one of these facility is the Toll Bridge program, initiated by the California Department of Transportation (Caltrans) after the 1989 Loma Prieta earthquake. After the completion of a seismic retrofit program of nearly 25, ordinary bridges in California, the Department of Transportation allocated 2.6 billion dollars to seismically retrofit five bridge structures in northern California plus the Vincent Thomas bridge in Los Angeles and the Coronado bridge in San Diego. These toll bridges are the largest and most complex bridge structures in the State. Nowhere in the world have bridges as complex as these been seismically retrofitted. These long-span bridges presented new engineering challenges that found a cost effective solution in the use of isolation devices, viscous dampers and lock-up devices. The safety of the retrofitted bridges relies heavily on the well defined and experimentally verified performance characteristics of the installed devices but isolators and energy dissipators of the size and capacity required to support long-span bridges were at the time neither manufactured nor tested to ensure compliance with design specifications. For this reason the California Department of Transportation commissioned a dedicated testing facility (Caltrans SRMD) for the prototype and production validation of Seismic Response Modification Devices (SRMD) at the Department of Structural Engineering of the University of California San Diego (Benzoni & Seible, 1998). The unique performance characteristics of the San Diego testing rig completed in year 2, are reported in Table 1. The ranges of displacement, velocity and vertical load are clearly motivated by the intention to test full scale isolation and energy dissipation devices. Table 1. Caltrans SRMD testing facility characteristics. Accuracy of readout Vert. Force 53,4 kn.5% full range Long. Force 8,9 kn 1.% full range Lateral force 4,45 kn 1.% full range Vert. displ..127 m 1.% full range Long. displ m 1.% full range Lateral displ..61 m 1.% full range Vert. velocity 254 mm/s Long. Velocity 1,778 mm/s Lateral velocity Relative Platen rotation 762 mm/s 2 Figure 1. Overview of the Caltrans SRMD Facility and bottom portion of a large friction pendulum device. Facilities dedicated to the tests of anti-seismic devices are also available at NCREE in Taiwan (MATS) and at the EUCENTRE of Pavia, Italy, in addition, of course, to several installations at manufacturing plants. The availability of experimental resources not only contributed to the development of new technologies but also to the improvement of the knowledge on the

3 performance of more traditional devices. At the same time, it supported the formulation of new specific codes for the qualification of isolators and dissipators, contributing to the definition of coherent testing protocols, procedures and acceptance criteria. The experimental activity continues to provide feedbacks to the community that should maintain updated the regulations and procedures for the characterization and qualification of anti-seismic devices. With this goal, results and observations from recent experimental campaigns are presented in what follows. 3 TESTING PROTOCOLS Current regulations (AASHTO, 21) (EN 15129, 29) (NISTIR 58, 1996) consider two types of tests for seismic isolation devices: Prototype (Type) and Production (Proof) tests. The first type of test is intended to verify the device performance under a wide range of loads and displacements. These tests are typically completed during the transition between the Research and Development (R&D) phase and the Production and Commercialization (P&C) phase. The design parameters that will qualify the device need to be established as being either associated to a specific project or to a level of prequalification requested by the manufacturer or the end user. The Production tests are instead designed to validate the average performance of a batch of produced units, with reference to the prototype response. The level of applied actions is quite limited for these tests in order to maintain the device in working conditions and ready for installation. In many code requirements, the details for the correct execution of a test are incomplete. For instance, the request of a fixed number of fully reversed cycles at specified maximum displacement and peak velocity without indicating the shape of the displacement time history to be applied (sinusoidal, constant velocity etc.), is quite common. This condition is critical for devices with many mechanical joints involved in the overall motion that could experience a response sensitive to acceleration peaks. The typical request of the repetition of a number of cycles at design displacement is also broadly applied to devices of the same family without consideration for the supplemental phenomena involved in a device s performance for large ranges of displacements. In this respect, a positive example is instead offered by the new European Code EN (EN 15129, 29) in which, for instance, it is stated that for viscous dampers The Structural Engineer shall prescribe the acceptable variation of the output force due to changes in ambient or internal temperature, or due to causes as ageing, wearing, etc.. Modifications of the device performance due to thermal effects are, in this case, taken into account and the structural engineer s role is forced out of a passive acceptance of the performance observed in the laboratory. It is well understood that codes cannot comprehensively regulate every aspect of complex phenomena. This observation should further support a more pro-active role for the structural engineer that is a relatively new function for this professional figure. Indeed, use of new technologies within a project requires an active participation of the designer in the phase of collection and interpretation of the documentation of the performance of a mechanical device to be included in the design. This phase should ideally be initiated during the selection process of the most appropriate technologies to be used, in close collaboration with manufacturers. Too often, unfortunately, this role is devolved to the manufacturer, based on the reason of lack of expertise on the part of the designer. Introducing a separation between the detailed knowledge of the overall structural concept and the specific device s performance is however extremely dangerous in the design of seismically isolated structures. In this respect, the codes tend to be used as missing link between the manufacturer s and the designer s specific expertise, with the assumption that the satisfaction of code requirements guarantees a minimum safety level. It is the author s opinion that the existing codes should always be regarded as guidelines but that a deeper understanding of the device s performance, beyond the specific design requirements is a crucial duty of the structural engineer in relation to the specific application to the structure he/she is designing or retrofitting. The extrapolation of performance results, obtained during laboratory tests on certain devices, to units of quite different dimensions and performance characteristics, is a highly debated subject. The European code (EN 15129, 29) prescribes some limitation to this process, indicating, for instance, the modifications to an

4 elastomeric isolator requiring a new set of prototype tests. It is however unquestionable the tendency, in practice, to propose this extrapolation process, disguised under the name of pre-qualification. Reasons of concern for this approach are justified when, for instance, dealing with friction-based devices. The father of friction research, Leonardo da Vinci, was probably the first who consciously investigated pairs of materials. From that time, the unfortunate misunderstanding about a friction coefficient as a material property has grown, and it is still deeply rooted in the mind of engineers,. [However] the friction coefficient is just a convenience, describing a friction system and not a material property. G.Salomon (Salomon, 1964). Hence, the assessment of a given performance based on the similitude of employed material, cannot be proposed when devices represent different systems. Several tests performed at the SRMD laboratory indicated, for instance, that friction coefficients obtained by tests on small sample of materials were not reproduced by tests on full scale devices using identical materials. Testing protocols only rarely contains specifications about the testing equipment. The existence of a current calibration of displacement transducers, load cells, pressure transducers etc, is generally implied, but neither procedures nor ranges of accuracy versus capacity, are provided for guidance. It is however desirable the initiation of a process of accreditation of dedicated facilities that could include, moreover, conditions of repeatability and traceability, procedures for data acquisition and data reduction etc. The development of specific regulations on this matter is considered a crucial step in view of an improved service toward the seismic isolation technology. The capability, robustness and reliability of testing equipment are directly linked with the requirements of specific testing protocols, such as the completion of tests with simultaneous application of components of motion in multiple directions (2D or 3D tests) which has recently generated an interesting debate, in Italy. It must be noted that the European code suggests multidirectional tests but considers the limited availability of facilities with this performance. For this reason it allows substituting the multidirectional tests by 1D tests repeated in orthogonal directions of motion (EN 15129, ). However, nowhere in the European code, and in any other existing code, is this simplification intended to question the significance of tests using multi-directional motions. Unfortunately, the importance of these tests was recently doubted by experts. On the contrary, the activity at the San Diego Laboratory, can document not only the increasing interest of the community in the use of realistic type of motions, but also an extensive history of criticalities highlighted by multi-directional tests on every family of devices. As mentioned above, it should be the designer responsibility to verify if the selected products were extensively tested under real conditions independently of what the code prescribes as minimum requirements. To illustrate the issues introduced above, some observations derived from extended prototype tests on the most common isolation and energy dissipation devices are presented by device typology. 4 ELASTOMERIC BEARINGS An example of a testing campaign extended beyond the requirements of current testing protocols was carried out in relation to the seismic retrofit project of a bridge in California. The bridge is equipped with lead-rubber bearings, a very common anti-seismic provision for bridge applications. The device was approximately 15 mm in diameter, 5 mm high and with a lead core diameter of 28 mm. Tests were performed at the constant shear strain of 1% equal to a displacement of 35 mm, under vertical loads between 2224 kn and 5783 kn. The goal of these tests was to complete the investigation of the device performance including the effects of the axial load variation as well as the sensitivity of response parameters to a large range of velocities. The original testing requirements for this specific application did not impose a verification of the response outside the design range of velocities and axial load. Figure 2 shows the shear force across the bearing as a function of the applied vertical load and maximum test velocity. It is visible the quite constant response over the vertical load range. The effect of the vertical load appears to contribute, for these tests, only for a maximum variation of 1%, 9% and 6.5% for first, second and third cycle, respectively. To evaluate the effects of the velocity (strain rate) the high speed

5 Characteristic Strength Qd (kn) Force (kn) Characteristic Strength Qd (kn) Force (kn) Characteristic Strength Qd (kn) tests results were compared with the bearing response at the minimum speed of.76 mm/s, assumed here as a reference. In terms of peak shear force, the effects of the velocity are higher than those introduced by the vertical load amplitude. It is visible, in Figure 2 (right plot), for a vertical load of 44 kn, that the maximum increase of shear force with velocity is equal to approximately 74% for the first cycle. For the second and third cycle the variation reduces to a consistent value of 45% and 3%, respectively. The performance appears very symmetric between positive and negative forces, except for the first cycle, where the positive results exceed the forces in the reversed direction of motion. 15 1st cycle 2nd cycle 3rd cycle different vertical load levels, was recorded, as illustrated in Figure 3. The variation is reduced for the second cycle to an average increment of 67% and of 46% for the third one. The peak of characteristic strength is reached at different test velocities, for different vertical loads. It is noticeable the negligible difference between results for slow speed tests. Comparison between slow and fast motions indicates also a general increase of post-yield and effective stiffness with test speed, particularly evident for the first cycle vertical load = 2224 kn st cycle 2nd cycle 3rd cycle Vertical load (kn) vertical load = 44 kn Peak velocity (mm/s) vertical load = 44 kn st cycle 2nd cycle 3rd cycle negative force st cycle 2nd cycle 3rd cycle Peak velocity (mm/s) Figure 2. Effect of vertical load and test velocity on peak shear force Among all performance parameters, the one that appears to be more sensitive to the variation of axial load is the post-yield stiffness ( K d ). The increase of vertical load resulted in a reduction of K d. A peak reduction of 16% was noticed for tests at 957 mm/s maximum velocity. The effect of the strain rate is instead quite significant for all performance parameters. The force at zero displacement (characteristic strength Q d ) shows the larger variations for increasing motion velocity. For the first cycle, a maximum increase of Q of 76%, 13% and 89% at the three d Peak velocity (mm/s) vertical load = 5783 kn 1st cycle 2nd cycle 3rd cycle Peak velocity (mm/s) Figure 3. Characteristic strength for different vertical load and test velocity The response in terms of damping ratio indicates a maximum increase with vertical load equal to 1%, 12% and 13% for the first three cycles. The more visible effect was noticed for the tests at very slow velocity. The influence of

6 the high testing velocity was responsible for an increase of the damping ratio equal to 29%, at the first cycle. Additional information about these tests are reported in Benzoni, & Casarotti (29). The designer should be able to incorporate such results, or others similar obtained from manufacturers, in a simplified analytical model or through the correct use of hysteretic rules in commercial Finite Element software. For these specific bearings the experimental observations were implemented in an analytical model dependent only on the mechanical and geometric characteristics of the device and able to reproduce the effects of velocity and repetition of cycles (Benzoni, & Casarotti, 29). Recent tests on lead-rubber isolators indicated premature degradation of performance, as well as failure, under bi-directional motion, torsional component of motion, combination of shear and relative rotation between top and bottom plates and melting temperatures developed at the leadcore. The interpretation of these results is still in progress but suggests, once again, that greater attention should be paid to more realistic loading conditions. 5 VISCOUS DAMPERS Viscous dampers represent a family of devices apparently easier to validate through testing once the required levels of force and velocity are set and available. However the demand for larger devices is increasing and rapidly approaching the peak capacities of many experimental facilities. For this technology the issue of durability has recently come at the forefront of the scientific and manufacturer s community and will influence both design and testing activities in the future. Moreover this subject is expected to extend, in the near future, to other anti-seismic products. In some major bridges in California, the degradation of viscous dampers was detected after a short time in service (2 to 1 years). This scenario clearly motivated a reasonable concern about the widely used technology for the uncertainty of the remaining life of the devices as well as for the lack of immediate alternative solutions. Procedures to continuously monitor the device performance were developed and implemented for a critical bridge in Los Angeles (Vincent Thomas bridge) (Bonessio et al., 211) and research efforts were initiated in order to study the source of the early degradation. Dampers removed from bridges were extensively tested and submitted to a detailed forensic investigation. Even though critical construction details were identified, the major source of degradation was attributed to the unexpected extensive and continuous motion experienced by the devices during service conditions (Sikorsky et al., 29). The laboratory tests led to the conclusion that the slow and continuous motion of the device in service requires material characteristics and construction techniques unlikely to be found in devices designed for the rare event of an earthquake. Even though this separation of functions of a viscous device should be clearly established at the design stage, this requirement was not sufficiently investigated during the design of the overall structural intervention and supported by experimental validations. This example underlines the consequences of a design vision that does not include the early involvement of the manufacturer s and experimental facility s expertise. Extensive considerations about the durability of devices are beyond the scope of this paper and certainly not limited to viscous devices. It must be noted that while existing codes are extremely specific on the requirements that guarantee the long term performance of each single material used in device fabrication, very little guidance is provided for the assessment of long-term performance of the device itself, under service conditions during which their pristine configurations could have been modified by a number of factors such as environmental conditions, undesired structural performance and aging of materials used in the components and assemblies. 6 FRICTION PENDULUM DEVICES Curved sliding bearings for the protection of bridge and building structures recently achieved a significant level of popularity due to their peculiar and well known characteristics (Zayas et al., 1987). Similarly to what reported in the elastomeric bearings section, the effects of the variations of vertical loads and the peak velocity on the characteristic response parameters of a friction pendulum device are presented here, in order to make the designer aware of the changes in performance due to loading conditions not necessarily prescribed by code regulations. Even

7 though the presented results cannot be considered representative of all the existing devices of this family, they suggest ranges of performance variation to be taken into account by designers. The tested unit consisted of a single pendulum device, fabricated with a radius of curvature of 2,5 mm and an un-lubricated polymer composite liner with about 4 MPa compressive yield strength. Tests were performed for three applied vertical loads, maintained constant during the application of motion and corresponding to pressure values of 15, 3 and 6 MPa. The intermediate value of pressure represented the design parameter. For each set of loads, several peak velocities v were selected, ranging from 1.27 mm/s to 8 mm/s. A sinusoidal input, with displacement amplitude D equal to 2 mm was used for all the tests. In Figure 4 the friction coefficients, as function of the applied pressure (left plot) and peak velocity (right plot), are reported. The average friction coefficients associated to low, medium and high pressure are 8.7%, 5.7% and 3.4%, respectively. The large scatter of results, visible at constant vertical pressure, indicates a maximum variation of friction coefficient, equal to 18%, 17% and 28% during the first cycle for low, medium and high load, respectively, increasing to 18%, 45% and 63% during the second cycle. very similar for all the vertical load cases and is approximately equal to 14%. The experimental restoring stiffness K r appears well correlated to the theoretical values obtained as: K r W (1) R where W represents the applied vertical load and R is the radius of curvature of the concave surface. The peak force and the dissipated energy (EDC) appears instead poorly represented by the theoretical values, obtained as: F max W W R D (2) EDC 4WD av (3) where D and D av are the maximum and the average displacement, respectively. The experimental results in terms of Energy Dissipated per Cycle (EDC) are reported in Figure 5, together with the theoretical values. The maximum increase of EDC with respect to the slowest test is in the order of 5.3%, 21.4% and 16% for the three vertical load cases. The amount of energy dissipated appears also to reduce significantly for velocities higher than 8 mm/s, except for the low vertical load case. Figure 4. Variation of Friction Coefficient with Pressure and Testing Peak Velocities. The effect of the test velocity is more relevant for higher values of testing pressure and for the second cycle. The friction coefficient increases with velocity up to approximately 1 mm/s for medium and high vertical loads. For the lowest applied load an increasing trend was noticed for the whole range of velocities for the first cycle only. A significant reduction from the first to the second cycle is evident for all cases. The maximum increment of friction coefficient, with respect to the slow speed tests (1.27 mm/s), is Figure 5. EDC at different vertical loads and testing speed for two cycles A significant reduction of dissipation is also associated with the repetition of cycles. It was observed that both peak force and energy dissipated results, normalized to the applied vertical load W, are well correlated with friction coefficients indicating that most of the variance of these two parameters is justified by the variation of the frictional performance (Figure 6). Further details are reported in (Benzoni et. al., 211). The original concept of the single friction pendulum evolved, quite recently, in two additional devices, the double and the triple

8 Normalized peak force Fmax / W Normalized energy EDC / W (mm) pendulum. A description of the triple pendulum can be found on the manufacturer web page ( ) Fmax.9.8 W 2 R MPa 3MPa 6Mpa 15MPa 3MPa 6Mpa 15 cycle MPa #1 15 MPa 15 3 MPa 3 MPa 63 MPa 6 M 15 cycle MPa #115 MPa 15 3 MPa 3 MPa 63 MPa 6 MP cycle MPa #215 MPa 15 3 MPa 3 MPa 63 MPa 6 M cycle MPa #215 MPa 15 3 MPa 3 MPa 63 MPa 6 MP 15 MPa15 MPa 15 3 MPa 3 MPa 63 MPa MPa 6 M 15 MPa15 MPa 15 3 MPa 3 MPa 63 MPa MPa 6 MP Friction coefficient (%) Friction coefficient (%) 8 (a) 8 (b) Figure 6. 6 Normalized peak force (a) and dissipated 6 energy (b) versus friction coefficients EDC W 2 R. 995 The more common double concave pendulum system (DCFP) was motivated by its capacity to accommodate substantially larger displacements compared to a traditional friction pendulum bearing of identical plan dimensions. Analytical studies of the double pendulum performance are presented in Tsai et al., (Tsai et al., 25) and Fenz & Constantinou (Fenz & Constantinou, 26). Fenz and Constantinou propose also the possibility to use sliding surfaces with varying radii of curvature and coefficients of friction, offering the designer greater flexibility to optimize performance. The advantages of this additional flexibility in the configuration of the DCFP are far from being demonstrated and will not be discussed in this paper. Some experimental observations suggest instead that the analytical formulations and derived simplified models are quite inaccurate in describing the experimental results. A comparison between numerical and experimental performance reported by Tsai (Tsai et al., 25) indicates a large discrepancy, motivated by vertical load variations as well as by the assumption of the friction coefficient not dependent on the sliding velocity and the applied load. The impact of the following implicit simplification in the analytical formulations deserves further investigation. The horizontal stiffness of the DCFP isolator is expressed as: W K b (4) R 1 R 2 were R 1 and R 2 are the radii of curvature of the lower and upper concave surfaces. This formulation, however, assumes that the horizontal displacements of the slider, relative to the center of the lower and upper concave surfaces, are simultaneous. However, the complex frictional phenomena on the sliding surfaces do not guarantee that this condition is achieved. Even a small difference between the static friction coefficient at the upper and lower interface can, for instance, activate a non simultaneous motion. From Eq. 4 it is also clear that when only one surface is sliding, the DCFP reverts back to the performance of a single pendulum, with an horizontal stiffness that is double the expected value (if R 1 = R 2 ). Figure 7. Sequence of Motion of the double concave friction pendulum components The photograms of Figure 7 were taken during recent tests of DCFPs with R 1 = R 2 and 1 = 2. The white lines show the position of the upper surface and of the slider in the previous step of motion. During the tests, the displacements of the slider with respect to the upper and the lower surface were not always simultaneous. In photograms b) and e), a relative displacement between the upper surface and the slider is occurring, while the slider is not moving relatively to the lower surface. In c) and f), a relative displacement between the lower surface and the slider is visible, while the upper surface is not moving relatively to the slider (same displacement of the slider and of the upper surface).

9 u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-) u/r (-).2 Figure 8. Hysteresis loops of the tested DCPF isolator: (1 st column) global force-displacement curve, (2 nd column) forcedisplacement curve for the upper surface; (3 rd column) force-displacement curve for the lower surface The simultaneous sliding of the two interfaces is noticeable only in d) (the displacement of the upper surface is twice the displacement of the slider relatively to the lower surface). Assuming the displacement history described in Figure 7, a simulated hysteretic performance of the device was generated. The static friction coefficients of the upper and lower interface were assumed as slightly different ( 1s =.9 and 2s=.1). The dynamic friction coefficients were assumed as 1= 2=.5 and radii of curvature R 1 =R 2 =1m were adopted. The numerical forcedisplacement curves relative to the whole isolator, the upper surface and the lower surface are reported in Figure 8. The first 6 rows correspond to the photograms of Figure 7, from a) to f). The last row contains the complete hysteresis loops with the dotted line indicating the theoretical behavior. Forces F have been normalized to the vertical load W, and displacements u have been divided by the radius of curvature R. The potential significant deviation from the theoretical response, due to nonsimultaneous motion, is visible. It is worth noting that the condition of simultaneous sliding of the

10 two interfaces appears to be function of the motion velocity 7 CONCLUSIONS The technology of seismic isolation is experiencing a large interest for applications not limited to structures of complex performance or strategic importance. This increasing popularity is motivated by a higher level of confidence in this solution achieved with extensive experimental campaigns of characterization and qualification of the response of anti-seismic devices as well as by the excellent performance shown by isolated buildings during recent seismic events (Chile 21, Japan-211). Attracted by the simplicity of the fundamental principles of seismic isolation, the designer could be tempted, however, to shift a substantial portion of his responsibility to the device manufacturer, the building inspector, the certifying laboratory and the code. Even though all these other entities play a critical role for the successful completion of a seismic isolation project, it is the designer that should lead a coordinated process of documentation, interpretation and selection of the technologies better fitting his/her project. In this paper some observations from an extensive experimental activity on anti-seismic devices were presented to stress the importance of the acquisition of a device performance knowledge that extends beyond what regulated by existing codes. Open issues like durability of materials and devices used by this technology as well as plans for efficient maintenance of installed devices should maintain the industry and the scientific community fully involved in the continuation of the development of an extremely valuable technology. Lead_Rubber Seismic Isolators Journal Of Earthquake Engineering, 13(4). Benzoni, G., Bonessio, N., Lomiento, G., 211 Experimental Performance and Modeling of Sliding Anti-Seismic Devices, 7 th World Conference on Joints, Bearings and Seismic Systems for Concrete Structures, Las Vegas, Nevada, October 2-6. Bonessio, N., Lomiento, G., Benzoni, G., 211 Damage Identification Procedure for Seismically Isolated Bridges, Structural Control and Health Monitoring, Early View Online, March, 3. European Committee for Standardization, 29 Anti- Seismic Devices, EN Fenz, D.M., Constantinou, M.C., 26 Behaviour of the double concave Friction Pendulum bearing, Earthquake Engineering & Structural Dynamics, 35, Salomon, G., (1964). Introduction. In Mechanisms of Solid Friction, P. J. Bryant and M. Lavik (eds.), Elsevier, Amsterdam, pp Shenton, H. W., III, 1996 Guidelines for Pre-Qualification, Prototype and Quality Control Testing of Seismic Isolation Systems, National Institute of Standards and Technology, NISTIR 58, January. Sikorsky, C., Benzoni, G., Karbhari, V., M., 29 Forensic Study of Viscous Dampers from the Vincent Thomas Bridge, Structural Systems Research Project, University of California San Diego, La Jolla, Ca, U.S.A., December. Tsai, C.S., Chiang, T.C., Chen, B.J., 25 Experimental evaluation of piecewise exact solution for predicting seismic responses of spherical sliding type isolated structures, Earthquake Engineering & Structural Dynamics, 34, Zayas, V., Low, S., and Mahin, S The FPS earthquake resisting system, Report No. CB/EERC-87/1, Earthquake Engineering Research Center, University of California at Berkeley, REFERENCES AASHTO, American Association of State Highway and Transportation Officials, 21 Guide Specifications for Seismic Isolation Design, Third Edition, July. Benzoni, G., Seible F., 1998 Design Of The Caltrans Seismic Response Modification Device (SRMD) Test Facility Proceeding of the USA ITALY Workshop on Protective Systems for Bridges, New York City, April, Report No. MCEER-98-15, Multidisciplinary Center for Earthquake Engineering Research, Buffalo, New York. Benzoni, G., Casarotti, C., 29 Effects Of Vertical Load, Strain Rate And Cycling On The Response Of