Effects of seismic isolation bearing with sliding mechanism on the response of bridge

Transcription

1 Materials and Structures / Matériaux et Constructions, Vol. 37, July 2004, pp Effects of seismic isolation bearing with sliding mechanism on the response of bridge S. K. Park and K. B. Han Department of Civil Engineering, Sungkyunkwan University, Korea ABSTRACT In this paper, the seismic analysis and the modelling techniques have been introduced for aseismic performances assessment, when seismic isolation bearings are applied on a bridge. Nonlinear time-history analysis is carried out using finite element analysis program. In this study, El Centro earthquake (1940, N00W), Mexico earthquake (1985, N90W), and artificial earthquake are used as earthquake ground motions. The response of seismic isolated bridge is compared with that of a bridge using conventional POT bearings, after obtaining the displacements of the deck, the deformations of the piers, shear forces and moments of the bottoms of the piers. The results show that seismic isolation bearing could reduce earthquake forces. According to characteristics of input earthquake ground motions or combination method of materials, the effect of seismic isolation bearings becomes different. Especially EDF (Electricité De France) system which is seismic isolation bearing with sliding mechanism, could reduce earthquake forces well. RÉSUMÉ Dans cet exposé, l analyse sismique et les techniques de modélisation présentées évaluent les performances antisismiques par l application sur un pont de paliers d'isolement sismiques. L analyse non-linéaire de la courbe temps-histoire est effectuée en utilisant le programme d analyse d éléments finis. Dans cette étude, le tremblement de terre d EL Centro (1940, N00W), celui de Mexico (1985, N90W), et un tremblement de terre artificiel sont utilisés comme mouvements au sol de tremblements de terre. La réponse du pont isolé sismique est comparée avec celle d un pont utilisant des paliers conventionnels, paliers POT, après avoir obtenu des déplacements du pont, des déformations des piliers, des forces arrachées et des moments des bas des piliers. Les résultats montrent que le palier d isolation sismique pourrait réduire des forces de tremblement de terre. Selon des caractéristiques de mouvements au sol de tremblements de terre donnés ou la méthode de combinaison de matériaux, l effet des paliers d isolation sismiques devient différent. Particulièrement le système d EDF (Électricité De France), palier d isolation sismique avec le mécanisme du glissement, pourrait réduire bien des forces de tremblement de terre. 1. INTRODUCTION Ground motions generated from earthquakes differ from one another in magnitude, source, characteristics, distance and direction from the rupture location and local soil conditions. The ability of a structure to dissipate energy is central to controlling displacement demands, and various energy dissipation mechanisms have been proposed to enhance structural response [1, 2]. These energy dissipation mechanisms can be of various types such as viscous, friction, rigid-plastic, elastoplastic, viscoplastic, or combination of thereof. Seismic design is the most reliable and permanent method for preparing for earthquakes. Although many advanced countries, such as some European countries, The USA and Japan, have adopted earthquake resistant design using seismic isolation bearing, these countries have also been exerting their efforts to develop new seismic isolation bearing, too. For instance, France has developed an elastomer bearing pads, which is the EDF (Electricité De France) system, to which a consideration of the friction is given, to protect its nuclear power plant from earthquakes. It has adopted it as the standard design. Also, R. Guraud et al., [3] carried out theoretical research to review the safety of the structures of /04 RILEM 412

2 Materials and Structures / Matériaux et Constructions, Vol. 37, July 2004 the nuclear power plant with the use of the EDF system and to test the performance of the EDF system based on the shaking table test and numerical modelling. As a similar, Mostaghel et al. [4] studied on the R-FBI (Resilient- Friction Isolation) that is focused on the building structures. The dynamic response of a structure depends on its mechanical characteristics and the nature of the induced excitation. Mechanical properties which are efficient to mitigate the structure's response when subjected to certain inputs might have an undesirable effect during other inputs. Under seismic excitations that have relatively long durations, a structure undergoes several cycles during the forced vibration part of the response; therefore, its response depends more on the amount of energy that is dissipated during each cycle (area under the force displacement loop) than on the nature of the dissipated force that develops. Because of this, the dissipation properties of structures are averaged over a cycle of motion and are expressed in terms of dimensionless ratios which originate from the linear theory of structural dynamics. Within the context of linear viscoelasticity the effect of linear damping on the response of isolated structures subjected to stationary random excitations was addressed in depth by Inaudi and Kelly [5] Su et al. [6, 7] considered the seismic performance in various types of seismic isolation bearing, both theoretically and empirically. They modelled systems for the seismic isolation bearings and the theoretical behaviour was studied compared to various kinds of earthquakes. To improve the earthquake resistant performance, we would like to introduce the seismic response analysis, with application of seismic isolation bearing to the actual bridge, and the modelling techniques. The seismic isolation bearings to be used in this study are LRB, P-F, NZ, R-FBI and the EDF system; these were compared to the behaviour of the POT Bearing which was used before. For this, nonlinear time history analysis is executed with the use of finite element analysis program. Also, as the techniques for the modeling of seismic isolation bearing or design method are not well developed, this study introduces effective modeling techniques of various types of seismic isolation bearings to which a finite element analysis program is applied so that the study of dynamic behavior can be carried out easily at the actual work. (a) Configuration of P-F system (b) Schematic diagram Fig. 1 - P-F system. describes the background of this. One of the characteristics of this system is that no sliding occurs in the minor earthquakes, but sliding occurs on the system and the effect of seismic isolation occurs only if ground acceleration exceeds a fixed critical value. The analysis model of the P-F system can be described as Fig. 1 (b), and the equation of motion is as follows, provided that the friction plate is sliding. M x( t ) M gsign( x ) Mx g (1) where, M is the total mass of the deck of the bridge bearing for seismic isolation, g is weight acceleration, µ is the friction factor of the friction plate, x is the speed of the bridge bearing for seismic isolation and x is the ground acceleration. 2.2 LRB (Laminated Rubber Bearing) system The LRB, which is the most commonly used bearing support, is a bridge bearing for seismic isolation and mostly consists of earthquake resistant rubber. A steel plate is inserted to reinforce vertical stiffness. The major function of this system is to elongate the vibration cycle through the parallel action of a spring and damper. The form of the LRB system and mechanical analysis model are described in Figs. 2 (a) and (b), and the equation of motion is as 2. KINDS OF SEISMIC ISOLATION BEARINGS AND ANALYSIS MODELS The major purpose of the bearing for seismic isolation is to decrease the breaking energy of earthquakes that is applied to the structures. Various kinds of methods have been suggested for this. All these methods contain flexible transverse stiffness and energy dissipation capacity in common. Several types of bridge bearings for seismic isolation and analysis models that are commonly used are introduced in this section. 2.1 P-F (Pure-Friction) system The P-F system protects structural objects from earthquakes by using the friction between the floors of structures and the ground. It can be considered the simplest system of all bridge bearings for seismic isolation. Fig. 1 (a) (a) Configuration of LRB system (b) Schematic diagram Fig. 2 - LRB system. 413

3 Park, Han follows. M x( t ) Cx( t ) Kx( t ) M x g (2) where C and K are the damping coefficient and stiffness of seismic isolation bridge bearing, respectively. 2.3 NZ (New Zealand) system The NZ system is also called L.R.B., and is an improved version of the standard LRB system. By inserting a cylindrical lead in the centre of the LRB, it is used as an additional energy distribution system. The configuration of the NZ system and the dynamic analysis model are described in Figs. 3 (a) and (b) respectively, and the equation of motion is as follows. (a) Configuration of R-FBI system M x( t ) Cx( t ) Kx( t ) NQ M x g (3) where N is the number of seismic isolation bearings used, Q is the hysteretic restoring force created from the lead core. (b) Schematic diagram Fig. 4 - R-FBI system. Mx( t ) Cx( t ) Mgsign( x ) Kx( t ) M x g (4) 2.5 EDF (Électricité De France) system (a) Configuration of NZ system (b) Schematic diagram The EDF system consists of a reinforced neoprene pad which is reinforced with a steel plate and the lead-bronze alloy of the deck of the laminated plate. The lead-bronze alloy plate faces the friction side of the steel plate on the floor of the structure. This system, in contrast to the R-FBI system, connects the LRB system and the P-F system in series. Fig. 5 (a) is a configuration of the EDF system. The mechanical analysis model of the EDF system can be described as in Fig. 5 (b), and the displacement is divided into the displacement x 2 of the friction plate of the deck and the lower displacement x 1 Provided that the friction plate is Fig. 3 - NZ system. 2.4 R-FBI (Resilient-Friction Isolation) system The R-FBI system consists of plates coated with round Teflon which are kept in contact by friction, and rubber core or LRB which provides restoring force is at the centre. The R-FBI system has the characteristics of an LRB system and P-F system, and it is as similar as if the LRB system and P-F system are simultaneously used for the structures. This system, like the P-F system, is not activated if the ground acceleration is smaller than the fixed critical value, but unlike the P-F system, it is restored by the spring of the LRB and damping force if displacement occurs. Mechanically, the R-FBI system is modelled connecting in parallel with spring, visco damper and friction damper. A mechanical analysis model can be described as in Fig. 4 (b), and the equation of motion is as follows, provided that the friction plate is sliding. (a) Configuration of EDF system (b) Schematic diagram Fig. 5 - EDF system. 414

4 Materials and Structures / Matériaux et Constructions, Vol. 37, July 2004 sliding, the equation of motion of the bearing and that of the friction plate area are as follows. x 1( t ) Kx1( t ) Mgsign( x 2 x ) (5) C 1 M x ( t ) Mg ( x x ) M x 2 sign 2 1 g (6) Table 1 shows the values recommended for the various types of systems. Table 1 - Values of parameters used for various base isolators Friction Natural Damping Types Coefficient Period (sec) Ratio (%) P-F LRB NZ R-FBI EDF been set as free, and transverse direction displacements of all pier were assumed as fix. The commercial finite element analysis program SAP2000-Nonlinear [8] is used for numerical analysis. Table 2 shows the material properties that are used in this study. This bridge is of the PSC box girder type and is a 15 clear span continuous bridge which is installed by the I.L.M. (Incremental Launching Method), the width and length of the bridge are m and 725 m respectively. General pre-stressed concrete bridge has structural damping ratio of 2-3 %, so Rayleigh damping with = 2 % is used in the modelling of Dong-Jin bridge. Fig. 6 (a) is a transverse section drawing and Fig. 6 (b) is the overall modelling. The earthquakes assumed in the 3. ANALYSIS MODEL AND USED EARTHQUAKE GROUND MOTIONS A prototype bridge (Dong-Jin Bridge) that is situated in Naehang (Korea), in service, is selected as a model to be applied with an actual seismic isolation bearing in this study. The seismic isolation bearing models to compare the performance are P-F, LRB, NZ, R-FBI and EDF systems, and they are compared to the modelling to which the POT bearing is used. Also, the performance of seismic isolation bearing is analyzed through nonlinear time hysteresis analysis. For POT Bearing, the pier number 7 as fix in longitudinal direction displacements and the rest have (a) Transverse section drawing. A1 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 A2 y z x For pot bearing longitudinal boundary condition: hinge Unit: m Abut 1 Pier 1 Pier Pier Abut Fix Superstructure element Rigid link Connect element that represent various bearings Fix H2 3.0 Pier element Fix Fix (b) Finite element model Fig. 6 - Dong-Jin Bridge. 415 Fix

5 Park, Han Types Table 2 - Values of physical parameters Vertical Stffness (N/m) Horizontal Stffness (N/m) Damping Coefficient (N sec/m) P-F LRB NZ R-FBI EDF design were the N elements of the earthquake El Centro (1940, N00W), an artificial earthquake acceleration which is the same as the 1 st degree earthquake resistance in Korea and the N90W elements of earthquake Mexico that occurred in The earthquake El Centro energy is concentrated in the 1-4 Hz range, and the maximum value is in the area of 1.5 Hz. Also, they mostly comply with the distribution of strong earthquakes energy. (a) El Centro earthquake (b) Artificial earthquake (c) Mexico earthquake Fig. 7 - Earthquake ground motions consideration in this study. An artificial earthquake whose peak ground acceleration is 0.17 g is created by using the artificial earthquake generation program. It is Mexico earthquake to which energy is concentrated under low frequency, and it can be occurred at such ground conditions as the poor subsoil. For such a type of earthquake, the unique performance of the seismic isolation bearing cannot be achieved as energy is concentrated during the time of cycle when seismic isolation is planned. Therefore, it is best not to employ seismic resistant design for the area where a low frequency earthquake is possible due to such ground conditions as poor subsoil. Fig. 7 shows the acceleration time hysteresis curve of an earthquake which is used for the analysis. 4. RESULT OF ANALYSIS 4.1 Responses for El Centro earthquake In Fig. 8 (a), the maximum displacement of the deck of a bridge which is modelled by using the POT Bearing, LRB, P-F, NZ, R-FBI and EDF systems is compared to the location of the pier and abutment, provided the earthquake El Centro is affected to the longitudinal direction of the bridge. Also, Fig. 8 shows graphs which compared the maximum displacement of the deck of the pier and the maximum shear force and the maximum bending moment of the lower part of the pier each other. The result shows that the seismic isolation bearing, including the LRB system, evenly distributes the response values with respect to the pier. The reason for this is because the seismic isolation bearing evenly distributes the earthquake load. However, in the case of the modelling which employed POT Bearing, the response is concentrated on pier number 7 which is the longitudinally fixed end. The displacement of the deck of the isolated bridge (used seismic isolation bearing) is greater than that of POT Bearing. However, that is the strain within the seismic isolation bearing and the strain of the pier itself is very small as described in Fig. 8 (b). Also, the shear force and the moment of the lower part of the pier of the isolated bridge is considerably reduced compared to the value at the fixed end point of the bridge to which the POT Bearing is employed. In comparing the seismic isolation bearings, the displacements of the LRB, R-FBI and NZ systems are greater than those of the EDF and P-F systems. In case of shear force and moment, all seismic isolation bearings, except the P-F system, show differences. For the EDF system, the shear force and the value of the moment are effectively reduced due to the relative sliding on the top and lower parts of the friction plate. Fig. 9 shows a comparison of responses which are obtained by applying the earthquake El Centro in a transverse direction. For the modeling to which POT Bearing is employed, the responses toward the axial perpendicular direction of the bridge were comparatively even, unlike the responses from the axial direction of a bridge. The reason for this is because all bearings effect as fixed end and responses are evenly distributed if the earthquake is applied toward the 416

6 Materials and Structures / Matériaux et Constructions, Vol. 37, July 2004 Fig. 8 - Comparison of response for El Centro earthquake (Longitudinal). axial perpendicular direction of the bridge while the responses are concentrated on pier number 7, the fixed end, if the earthquake is applied toward the axial direction of the bridge. Fig. 9 - Comparison of response for El Centro earthquake (Transverse). For a bridge to which the LRB and R-FBI systems are employed, displacements were considerably high as in the case where an earthquake was applied in the axial direction of a bridge while the shear force and moment at the lower 417

7 Park, Han part of the pier were comparatively decreased. Comparing the behavior of the LRB system to that of the R-FBI system, the R-FBI system has considerably stronger stiffness than that of the LRB system while the values of displacement, shear force and moment of the R-FBI system is comparatively small due to the effect caused by internal friction. For a bridge to which the EDF system is employed, displacement on the deck of the bridge was small compared to that of the LRB system or the R-FBI system. Although the shear force or moment of the EDF system shows minor differences from those of the LRB system and the R-FBI system, they can be considered to be within the similar range. The behavior of an artificial earthquake which was applied toward the axial direction of a bridge was similar to that of the earthquake El Centro. 4.2 Responses for artificial earthquake Fig. 10 shows the responses for this case. Similarly, for the bridge to which the POT Bearing is used, the responses were concentrated on pier number 7 which is a fixed end, while responses were evenly distributed for the bridge to which the seismic isolation bearing was used. Displacement on the deck of a bridge to which the LRB system is used was the biggest while the displacement of the seismic isolation bearing to which a consideration of friction is given was relatively small. The result shows that the overall behaviour and distribution of values are similar except for a difference in the values compared to the case where the earthquake El Centro was used. The maximum displacement as well as the shear force and moment were considerably smaller. Fig. 11 shows the maximum displacement, the shear force and the moment of the deck provided the artificial earthquake is applied to the axial perpendicular direction of a bridge. Similar to the responses obtained by affecting the earthquake El Centro, the responses are evenly distributed to each pier. Except for the case where the P-F system is used, all seismic isolation bearings effectively performed the aseismic capacity. The maximum displacement was within the allowable scope of the seismic isolation bearing, and the shear force and moment were smaller than in the case where POT Bearing was used. 4.3 Responses for Mexico earthquake The purpose of analysis of the earthquake Mexico was not to evaluate the performance of the seismic isolation bearing, but was carried out to analyze the sensitivity of the seismic isolation bearing to an earthquake with unexpected elements of frequency. Analysis was carried out with the use of the load which is the earthquake acceleration during the 60 seconds of the earthquake Mexico. In Fig. 12, the response of a bridge which was modeled by using the POT Bearing provided the earth quake Mexico is applied toward the vertical direction is compared to the response of a bridge which is modeled by using several values for seismic isolation bearings. The results of the analysis show that the response was considerably increased, contrasting to that of the earthquake El Centro. The biggest increase of the response was in LRB, and this can be considered to be due to the result of the resonance. The original purpose of the seismic isolation Fig Comparison of response for artificial earthquake (Longitudinal). bearing is to avoid the cycle of an earthquake load by purposely elongating the term of the cycles, and the response was increased as the cycle (2 seconds) of the bridge to which the LRB is used is similar to that of the earthquake Mexico. In addition to the LRB, the 418

8 Materials and Structures / Matériaux et Constructions, Vol. 37, July 2004 Fig Comparison of response for artificial earthquake (Transverse). displacements of the bridge to which R-FBI or NZ is used were considerably large. Although displacement on the R- FBI to which friction is used was very large, the shear force Fig Comparison of response for Mexico earthquake (Longitudinal). and the moment were relatively small. And as the design cycle was 1 second in case of the EDF system, the amount of displacement was small as the effect from the increase of response caused by resonance was small. 419

9 Park, Han Fig. 13 shows the maximum displacement value of the deck and shear force and the moment of the lower part of the pier, obtained by applying the earthquakes toward the transverse direction. Similar to the case in the longitudinal direction, displacement on the deck as well as the value of shear forces and moment are increased. As in the longitudinal direction, the biggest value was in the LRB, and the values of displacement, shear force and moment of NZ whose cycle is similar to that of LRB were considerably increased. Although the maximum displacement of the deck of a bridge to which the R-FBI is used is increased, it can be considered that the aseismic capacity is effectively carried out as both the shear force and moment of the lower part of pier are smaller than those of the bridge with which the POT Bearing is used. For EDF, the amount of displacement could be effectively reduced. Generally, most of the seismic isolation bearing sensitively reacted for the Mexico earthquake, and aseismic capacity was effectively carried out generally in the case of the R-FBI or EDF system to which friction is employed. 5. CONCLUDING REMARKS In this study, the behavior of bridges modeled with the application of the EDF, LRB, NZ, P-F and R-FBI were analyzed through the earthquake responses, and the results were compared to the bridge which is modeled with the use of the existing POT Bearing. For artificial earthquake and the El Centro earthquake, the bridge to which seismic isolation bearings are used performed better aseismic capacity than the bridge which is modeled with the use of POT Bearing. However, displacement on the bridge on which LRB or NZ and R- FBI were used was greatly increased with respect to the which were isolated by the EDF system was relatively small, but compared to other systems, it can be considered that it effectively performed aseismic capacity, especially in terms of the displacement. Because earthquakes with high frequency, such as the El Centro earthquake, are expected to occur in general place, the LRB or NZ system may effectively perform the aseismic capacity. However, as a preparation for such unexpected earthquakes as earthquake Mexico, it is necessary to review whether or not to use the EDF system according to the characteristics of the subsoil. REFERENCES Fig Comparison of response for Mexico earthquake (Transverse). Therefore, the result shows that the seismic isolation bearings to which friction is used are considered to have effectively carried out the aseismic capacity. For POT Bearing, both shear force and moment are still big as responses are concentrated on the fixed end. [1] Passive Energy Dissipation and Active Control, ATC-17-1 Proceedings of Seminar on Seismic Isolation, Vols. 1 and 2 (Applied Technology Council, 1993). [2] Bixio, A.R., Dolce, M., Nigro, D., Ponzo, F.C., Braga, F. and Nicoletti, M., Repeatable dynamic release tests on a baseisolated building, Journal of Earthquake Engineering 5 (3) (2001) [3] Guéraud, R., Noël-Leroux, J.-P., Livolant, M. and Michalopoulos, A.P., Seismic isolation using slidingelastomer bearing pads, Nuclear Engineering and Design 84 (1985) [4] Mostaghel, N. and Khodaverdian, M., Dynamics of resilientfriction base isolator (R-FBI), Earthquake Engineering and Structural Dynamics 15 (1987)

10 Materials and Structures / Matériaux et Constructions, Vol. 37, July 2004 [5] Inaudi, J.A. and Kelly, J.M., Optimum damping in linear isolation systems, Earthquake Engineering and Structural Dynamics 22 (1993) [6] Su, L., Ahmadi, G. and Tadjabakhsh, I.G., A comparative study of performances of various base isolation systems, Part 1: Shear beam structures, Earthquake Engineering and Structural Dynamics 18 (1989) [7] Su, L., Ahmadi, G. and Tadjabakhsh, I.G., A comparative study of performances of various base isolation systems, Part 2: Sensitivity analysis, Earthquake Engineering and Structural Dynamics 19 (1989) [8] SAP2000-Nonlinear user manual (Computers and Structures, Inc., Berkeley, Calif., 2001). Paper received: September 2, 2002; Paper accepted: October 10,