外文翻译抗震隔离桥的基本设计

外文原文TITLE : Performance-based design of seismically isolated bridges1 INTRODUCTION1.1 Seismic Protective Systems for BridesVarious types of bearings are frequently used in bridges at the superstructure support points.These bearings provide support for superstructure vertical loads, while allowing thermal and long-term horizontal deformations to develop without transmitting significant lateral forces into the substructure. The three most widely used types of bridge bearidge bearings are pot ,disc,and curved sliding Stanton et al.,1993. These bearings are generally designed to accommodate rotations about any axis.Horizontal translation is accomodated by sliding along a standard stainless steel-PT FE interface .Examples of pot and curved sliding bridge bearings are shown in Figure 1-1.In addition to the above cited benefits of bearings in brides,such bearings may provide an ideal method to control forces and accommodate horizontal displacements imposed on a bridge during an earthquake . During an earthquake,inertial loads acting on the superstructure can induce large forces in the substructure and relative displacements between the superstructure and the ground.The displacement variation between the superstructure and foundation depends on: the characteristics of the ground motion ；the mass, stiffness,strength, and energy dissipation characteristics of the substructure; the mass and flexibility of the superstructure ;and the support conditions at the abutments and foundations.For conventional bridges,piers and abutments are designed to accommodate this displacement through a ductile mode of behavior . Properly reinforced concrete piers can be designed to achieve this required displacement ductility. However, large ductile deformations in these members spalls the concrete ,yields the reinforcement in areas of plastic hinge formation, and results in permanent lateral displacements. After a large earthquake,Damaged piers must be repaired or replaced,and the bridge and roadway may have to be realigned. Higher mode contributions, support movement,and other contributions to local stress concentrations may also cause unforceseen damage in superstructure,substructure,or foundation component. This damage may be difficult to assess and costly to repair.As an alternative to conventional bridge design,seismic isolation bearings can be employed. Such seismic protection devices are intended to control and accommodate horizontal superstructure displacements without damage to bridge components(or the bearings themselves) during an earthquake. Seismic protection devices provide engineers with additional tools to achieve performance requirements for the safety and functionality of bridges in the event of earthquakes.Seismic isolation bearings can accommodate large displacements while limiting force transmission.Isolation bearings displacing on the order of 20 to 40 in. (500 to 1000 mm） or more during an earthquake have been designed for bridges located close to major earthquake faults. Representative examples of elastomeric and sliding seismic isolation bearings are shown in Figure1-2 and 1-3.To prevent damage to these bearings and to limit damage to bridge hinges,expansion joints,and abutments,it is import to control seismic displacement and force demands.Thus, seismic isolation bearings typically provide between 8 and 30 percent effective damping during seismic excitation,depending on the bearing design and amount of bearing displacement. This nonlinear behavior, combined with hysteretic energy dissipation may effectively control peak superstructure displacements, and substantially reduce demands on the substructure and superstructure compared to an elastically designed structure.Supplemental energy dissipation devices can also be added to seismically isolated and conventional bridges to increase the total effective damping of the system and further control response. Representative examples of a supplemental viscous damping device is shown in Figure 1-4.Seismic isolation bearings and supplemental dampers are examples of innovative and practical technologies to enhance the performance of bridges during earthquakes.They provide a practical method for protecting life and property in the event of a major earthquake event.1.2 Commentary on the State of the Art Because of the potential benefits of seismic protective systems,considerable research related to their use has been completed in the past ten years.An extensive summary of this history is included in a related work by Whittaker et al.Whittaker et al.,1998 conducted as part of the Berkeley-Caltrans Seismic Protective Systems Program. As such,a review of this literature is not provided herein.Most studies to date have focused on the behavior of the isolation bearings or energy dissipation devices themselves.Relatively few studies have addressed the overall performance of seismically protected bridge systems. Even fewer studies have thoroughly examined the effects of seismically protected bridge systems.Even fewer studies have thoroughly examined the efforts of more than one component of earthquake ground motion on bridge response.Through a synthesis and analysis of these previous studies and the observation of experienced bridge engineers,important issues impeding the application of this technology to bridges have identified.Foremost among these,perhaps,is the potentially large displacement and force demands on seismically isolated bridges located near major faults. Large amplitude, long duration velocity pulses characteristic of near-fault ground motions have raised concerns regarding the displacement demands of relatively weak systems undergoing large displacements .This behavior generally gives the overall structure the appearance of having a long effective period of vibration . For this reason,the applicability of seismic isolation for bridges situates on soft soils has also been questioned.This is because site response effects for large ground motions tend to produce significant displacements in the long period range. Codification of design guidelines for the application of seismic isolation to bridge systems is also in its relative infancy.Few seismically isolated bridges have been subjected to large design level earthquakes in the field,or tested in the laboratory,to confirm the adequacy of these guidelines.Many of the concepts and procedures incorporated in these design methodologies have essentially been adapted from the building code provisionsU.B.C.,1994,and as such,may not adequately consider the differences in dynamic behavior and performance objectives betweeen isolated building and bidges. The design of highway bridges with seismic isolation and supplemental damping is specified in the American Association of State Highway and Transportation Officials Guide Specifications for Seismic Isolation DesignAASHTO,1999,referred to in this report as the Guide Specifications or Guide Specifications or Guide Spec. The Guide Specifications provide requirements for analysis of seismiclly protected bridges,isolation bearing and damper design, and testing and acceptance of bearings and damper components. The Guide Specifications incorporate a simplified linearized procedure for bridge design .This is based on the conceptual equivalence of nonlinear isolated response with that of an equivalent elastic system undergoing harmonic oscillations.In this respect , this approach is inherently not entirely realistic . Provisions for more accurate nonlinear time-history methods are included in the Guide Specifications, but not required for all systems.Further,unlike the building code ,the Guide Specifications implicitly suggest that some measure of substructure yielding is also allowed in isolated bridge system. However ,no guidance is given on how to account for the effort of this substructure yielding into the procedure.although the guide specifications are a major advance in bridge engineering,it is apparent that the limited assessment of the response of complete, seismically isolated bridge systems through analytical studies, laboratory tests or exposure to actual earthquakes, and the variety of unresolved concerns have impeded the application of protective systems technology to bridge.As such, research is needed directed towards developing a better understanding of seismically isolated bridge structures and the adequacy of current design criteria.1.3 The protective systems research program1.3.1 overall objectives of the programIn consideration of these needs,a series of studies have been undertaken under the protective systems research program by the california department of transportation.A variety of studies were develop as part of this program in order to:1. Evaluate the effect of bi-directional loading on seismic isolation bearings and develop improved analytical bearing models.2. Establish an understanding of global and localized response characteristics of simple seismically protected bridges subjected to various types of seismic input including farfield,near-fault,and soft-soil motions.3. Validate the efficacy of seismic protective system for bridge of more complex(realistic)configurations.4. Assess the applicability of requirement in the AASHTO guide specifications for seismic isolation design and recommend improvements based on the results of the research.1.3.2 Overall tasks undertakenSeven major tasks were undertaken to meet the overall objectives of the program.The tasks were organized to improve knowledge of component behavior and use component information to understand bridge system performance through integrated experimental and analytical studies.The experimental studies of seismic isolation components and model testing of seismically protected bridges made extensive use of the earthquake simulator and bearing test machines at the pacific earthquake engineering research center,university of california,berkelly.the major tasks in the research program were as follows.1. Experimentally characterize the properties of representative elastomeric and sliding isolation bearings under a wide variety of uni-directional and bi-directional displacement histories and rates.2. Use the experimental force-displacement data for isolator bearings to develop and calibrate mathematical models for elastomeric and sliding isolation bearings suitable for nonlinear response-history analysis of complete bridge systems.3. Investigate analytically and experimentally the response of a simple,isolated bridge system to uni-directional and bi-directional earthquake-history inputs,representing far-field and near-fault earthquake shaking on different soil types.4. By experiment and analysis,investigate the effectiveness of fluid viscous dampers on the earthquake response of simple bridges,including consideration of the effect of damper configuration.5. Use experimentally validated analytical models to examine the effect of substructure stiffness,substructure mass,substructure damping and varying isolation system properties on the response of seismically isolated bridge overcrossings subjected to various seismic inputs.6. Use a versatile shaking table model of a more complex bridge with one and two spans,supported on flexible piers,to investigate the effect of mass and stiffness eccentricity,pier flexibility,piermass and strength,and varying isolator properties on the bridge system subjected to one to three components of earthquake ground motion representing far-field,near-fault,and soft-soil earthquake shaking.7. Evaluate the efficacy and applicability of requirements in the AASHTO Guidge Specifications for Seismic Isolation Design and recommend improvements based on the results of the research.1.3.3 Protective system device consideredThee types of seismic isolation bearings were studied in the overall research program.These included lead-rubber(LR) and high-damping rubber (HDR) elastomeric bearings and Friction Pendulu(FP) sliding isolation bearings. At the time of writeing, LR and FP seismic isolation bearings have been used in bridge structures in the United States.Lead-rubber and HDR bearings are composed of alternating layers of elastomer and steel providing the bearing with suffcient stiffness to carry vertical loads.Energy dissipation is Provide by a central lead core in LR bearings and by a specially formulated elastomer in the HDR bearing. Photographs of the LR and HDR bearings used in the program are shown in Figure 1-2.The FP bearing is composed of a high axial load capacity,low-friction composite coated slider riding on a spherical stainless steel surface.Energy dissipation is achieved through friction as the articulated slider moves across the stainless steel surface. An example of an installed FP bearing is shown in Figure 1-3. Detailed specifications of these test bearing is shown in Figure 1-3. Detailed specifications of these test bearings are provide in Chapter 3. The fluid viscous damper has been the sole type of supplemental damping device used in a bridge in California to this date. Fluid viscous damper fluid (typically, a silicone oil) past or through the damper piston. Figure 1-4 shows a typical cross-section through the type of fluid viscous damper used in this study.1.3.4 Test ModelsIn the protective Systems Research Program, seismically isolated bridge systems were investigated experimentally with two models on the earthquake simulator. The first model, referred to as the rigid block model, is shown in Figure 1-5. This model represents a seismically isolated bridge on rigid pier or abutment supports.The second model,referred to as the elevated bridge model, is illustrated in Figure 1-6.Experimentation with this latter model allowed testing of one-and two-span bridge configurations.Piers were designed to provide a range of stiffness and strengths in the longitudinal and transverse directions of the model.Pier stiffness could also be arranged to provide stiffness eccentricity along the length of the bridge.Pier components also allowed substructure mass to be varied.The model allowed dynamic testing with one,two,and three components of ground motion.1.4 Scope of this ReportThis report addresses several key components of the above outlined Protective Systems Research Program. The scope of this report relates to aspects of items2,3,and 4 of the defined program objectives as outline in Section 1.3.1,excluding evaluations related to the use of supplemental damping devices. Specifically,Task 5,6 and 7 of the program, as outlined in Section 1.3.2,are performed herein. Analytical studies related to these tasks utilize the mathematical models developed and calibrated under program Tasks 1 through 3 (see Section 1.3.2). The specific scope of this report is defined as follows:1. System StudiesInvestigate the response of simple,isolated bridge overcrossings to a variety of earthquake time-history inputs, by analysis and experiment . Factors considered include the effect of mass and stiffness eccentricity, pier flexibility, pier mass and strength,substructure damping and isolator properties.A scale model of a bridge with one-and-two-span configurations on flexible piers was designed and constructed for testing on the earthquake simulator.These experiments facilitate validation of analytical results and provide proof-of-concept of the efficacy of seismic isolation for various bridge configurations and ground motion inputs.2. design methodology studiesThe applicability of several requirements in the AASHTO guide specifications for seismic isolation design are evaluated .specifically the efficacy of the uniform load method is investigated as it relates to the design of a broad range of seismically isolated bridge systems. Also , the effects of bi-directional ground motions and substructure yielding on the procedures are studied.An extensive parametric study is performed to investigate the response of simple isolated bridge over crossings to a database of recorded earthquake time-history inputs. specifically, the effects of varying isolator properties, pier flexibility, pier mass, substructure damping, and substructure yielding on system displacement and force response are examined.3. conclusions and recommendationsOverall conclusions of the research findings are identified. recommendations are developed for implementation of findings and/or improvement of current design specifications as indicated by the research results.1.5 organization of this reportChapter 2 contains a summary of key aspects of current design procedures for seismically isolated bridges. various system configurations used for short over crossings and viaducts are examined to develop a representative conceptual model of an isolated bridge system. preliminary analytical evaluations of the effect of basic system variations on isolated bridge response are presented to help identify key parameters and priorities for subsequent investigations.Chapter 3 presents the development and results of the experimental studies undertaken. ten configurations of a bridge model are constructed to investigate the effect on system response of mass and stiffness eccentricity ,pier flexibility, mass and strength, and various isolation system configurations. the models were subjected to various combinations of one and two components of horizontal ground motion along with vertical ground motion for earthquakes representing far-field, near-fault, and soft-soil characteristics.Chapter 4 presents evaluations of the chapter 3 experimental test results. these evaluations are organized around the key issues identified in chapter 2, where applicable.Chapter 5 presents the results of several related analytical studies. these studies assess the applicability of several design procedures contained on the AASHTO guide specifications. in particular, the e