Hydraulic Displacement Amplification System for Energy Dissipation

Transcription

1 Hydraulic Displacement Amplification System for Energy Dissipation Tracy S.K. Chung and Eddie S.S. Lam ABSTRACT An energy dissipation system with displacement amplification is presented. Displacement amplification is achieved through a hydraulic system. Energy is dissipated by combining the characteristics of hydraulic cylinders and viscous fluid dampers. A 1/3 scale prototype was constructed, and performance of the hydraulic displacement amplification system was tested experimentally. The hydraulic system has provided significant contribution to the energy dissipation and has improved the performance of viscous fluid dampers. However, the displacement amplification achieved by the system is far from satisfactory. Further studies are in progress to improve the efficiency of the amplification system. An application of the displacement amplification system was demonstrated numerically by carrying out nonlinear time-history analysis on a reinforced concrete frame Introduction It is in recent years that Hong Kong has been recognized as a region of moderate seismic risk (Lee et al, 1996). Buildings are traditionally designed without seismic provisions. Therefore, there is a need to retrofit the existing buildings to cater for possible seismic attacks. In considering the remoteness of the seismic risk, the use of passive dampers is both viable and economical. Buildings in Hong Kong are mostly reinforcement concrete structures. As compared with the steel structures, reinforced concrete structures are relatively stiff, exhibiting comparatively smaller deformation. This may adversely affect the performance of passive dampers. It is the objective of this study to use the passive dampers in typical local beam-column joints. In doing so, there is in need of a mechanism to amplify the joint deformation for better energy absorption. There exist numerous applications on the use of damping devices to improve the performance of structures under earthquake or other forms of vibration (Hanson and Soong, 2001). In particular, viscous fluid dampers have been successfully used to improve the structural response to earthquakes (Constantinou, 1994). The energy dissipated by the viscous fluid damper depends on the physical properties of its fluid and the fluid velocity or piston s velocity. However, it may not be feasible to improve the performance of a viscous fluid damper by varying the physical properties of its fluid. It is because low compressibility fluids often bear some undesirable characteristics such as toxic, flammable, having less than favorable temperature characteristics or longevity, and relatively expensive (Berton, 2004). Therefore, optimizing the piston s velocity is a common means to control the characteristics of a viscous fluid damper. Tracy S. K. Chung, Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong Eddie S.S. Lam, Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong

2 Conventional displacement amplification systems such as the toggle bracings, for instance see Berton (2004) and Hanson and Soong (2001), are invasive and are not appropriate in situations constrained by the limited space. To enhance the seismic resistance of a beam-column joint, it is desirable to have the damping devices installed next to the joint and with minimal effect to the occupants. In considering all the above-mentioned constraints, a damping system with displacement amplification capability is introduced in this study. It comprises a large bore size hydraulic cylinder and two small bore size hydraulic cylinders (the amplifying system) each connected to a viscous fluid damper. 2. Hydraulic Displacement Amplification System The hydraulic displacement amplification system consists of a large bore size hydraulic cylinder (Unit 1) and two small bore size hydraulic cylinders (Unit 2). As shown in Figure 1, Unit 1 is attached to a beam and a column via pinned connections and in the vicinity of a beam-column joint. Deformation of the beam-column joint is transmitted through Unit 1 to Unit 2. Figure 1. Installation of the hydraulic displacement amplification system. Figure 2 shows details of the hydraulic displacement amplification system. The prototype as shown in the figure is in 1/3 scale. The hydraulic cylinders are connected by high pressure hoses and the hydraulic oil used in this study is Shell TELLUS 37. The two viscous fluid dampers are model AC-2540 manufactured by AirTAC. Each damper comprises a helical spring and a viscous fluid damper. Bore sizes of the large and small hydraulic cylinders are 40 mm and 20 mm respectively. The deformation perceived by the large bore size hydraulic cylinder is amplified due to the difference in the bore sizes between the large and small bore size hydraulic cylinders. Considering the ratio of the cross-sectional areas between the two bore sizes, the displacement amplification factor is approximately equal to 3.

3 Figure 3 shows the connection details between the hydraulic cylinders. The cylinders are connected in such a way that the two small bore size hydraulic cylinders are in opposite motion. Figure 2. Hydraulic amplification system. Figure 3. Connections of hydraulic cylinders. Figure 4. Lever arm type displacement amplification system.

4 As an alternative to the hydraulic amplification system, a lever arm system can also be used to amplify the displacement as shown in Figure 4. Full details of this system will be reported elsewhere. 3. Test Setup The hydraulic cylinders and viscous fluid dampers were tested using a MTS universal machine, as shown in Figure 5. Fixing Part A was attached to the lower grip of the universal machine. Fixing Part, B, was connected to a load cell and remained stationary. Similarly, properties of the hydraulic displacement amplification system were estimated by fixing the large bore size hydraulic cylinder to the universal machine and with all the high pressure hoses duly connected to the small bore size hydraulic cylinders. Figure 5. Test setup and the sinusoidal displacement. Loading histories were applied in the form of sinusoidal displacements and in the range of ±2mm to ±4mm for the large bore size hydraulic cylinder and ±4mm to ±12mm for the small bore

5 size hydraulic cylinders and the viscous fluid dampers. A range of frequencies, from 0.5Hz to 1.4Hz, was considered. In particular, the frequencies at 1.3Hz and 1.4Hz are close to the natural frequencies of a reinforced concrete frame to be examined later in this study. 4. Experimental Results Figure 6 shows the hysteresis loops of the hydraulic cylinders, viscous fluid damper and the hydraulic displacement amplification system at 1.4Hz. The hysteresis loops were produced by taking the average of 20 complete cycles of the sinusoidal displacements. As shown in the hysteretic responses, damping characteristics of the hydraulic cylinders are effectively similar to a friction damper, and are able to dissipate energy. Frictional forces of the large bore size and small bore size hydraulic cylinders are 0.2kN and 0.1kN respectively. Figure 6. Plots of the hysteresis loops at 1.4 Hz. Hysteretic responses of the viscous fluid damper are non-symmetrical with respect to the positive and negative motions. This is due to the presence of a spring in the viscous fluid damper. In the tests, the viscous fluid damper was pre-compressed at the zero displacement.

6 Hysteretic behavior of the hydraulic displacement amplification system is similar to a viscous fluid damper, but is distorted due to the presence of springs in the viscous fluid dampers. Figure 7 shows variations of the averaged energy dissipated against the displacement amplitude of the hydraulic cylinders, viscous fluid damper and the hydraulic displacement amplification system (identified as combined in the figures) at 1.3Hz and 1.4Hz. The energy dissipated per cycle is estimated by integrating the enclosed area of the respective hysteresis loop (Hanson and Soong, 2001). The plots in the Figure 7 are obtained by taking the average of the energies dissipated from 20 complete cycles. The energies dissipated from the hydraulic cylinders are virtually linearly proportional to the displacement amplitude, whereas the energy dissipated from the viscous fluid dampers is nonlinear. Unfortunately, the energy dissipated by the viscous fluid dampers is comparative small and immaterial to the combined system. Therefore, the energy dissipation capability of the combined system is dominant by the characteristics of the large and small bore size hydraulic cylinders. Figure 7. Variations of averaged energy dissipated against displacement amplitude. The energy dissipated by the hydraulic displacement amplification system was less than the sum of the energies dissipated by the respective components. For instance, when a 4mm displacement was applied, the corresponding displacement of the small bore size hydraulic cylinder was only amplified by a factor of 1.7. Further studies are in progress to improve the efficiency of the hydraulic amplification system.

7 5. Numerical application Figure 8 shows a two-story reinforced concrete frame. It is consisted of 400mm by 550mm beams and 400mm by 400mm columns. The frame was designed to BS8110 (1985) without any seismic provisions, and represented a strong beam weak column structure. C30 concrete was assumed with f cu =30MPa, and yield strength of the reinforcements was 460MPa. The reinforced concrete frame was at 5m centre-to-centre, or a 5m loading width. It was subjected to a 6.8kPa dead load and 2.5kPa imposed load. Numerical model was constructed using the analysis programme SAP2000. Time history analyses were conducted using a Holliste-1 earthquake record. Holliste-1 was used because its dominant frequency is close to the 1 st mode of vibration of the reinforced concrete frame. Pursuant to GN , maximum acceleration for strong and frequent earthquake attacks are 0.316g and 0.056g respectively, and were adopted in the time history analyses. In this application, hydraulic displacement amplification systems were installed at all the beam-column joints as shown Figure 8, and were approximately modeled by viscous fluid dampers with C=83kN/m. Two displacement amplification factors of 10 and 30 were considered. Figure 8. A reinforced concrete frame. Table 1 and 2 summarize results obtained from the analyses. Values obtained from the tables are based on 1.2, 0.6 and 1.3 partial factors of safety for the dead load, imposed load and seismic load respectively. In the tables, three conditions were considered. Condition 1 is without any damping device. Conditions 2 and 3 are in the presence of the hydraulic displacement amplification systems, and with the displacement amplification equals to 10 and 30 respectively.

8 Table 1. Summary of the numerical analyses for the 400mm by 550mm beam. Case Condition Shear Moment (kn) (knm) Member Capacity Strong Earthquake Frequent Earthquake Table 2. Summary of the numerical analyses for the 400mm by 400mm column. Case Condition Shear (kn) Axial (kn) Moment (knm) Reinforcement (mm 2 ) Member Capacity Steel Area (8T20) = 2513 mm Not Achievable Strong Earthquake Not Achievable Frequent Earthquake Significant reduction in the design forces is achieved when the displacement amplification factor is equal to 30, and acceptable design forces are obtained when the factor is equal to 10. To provide a 30 times displacement amplification, it is estimated that the bore size ratio between the large and small bore size hydraulic cylinders is approximately Conclusions An energy dissipation system with displacement amplification is proposed. A prototype in 1/3 scale was constructed and tested by subjecting to cycles of sinusoidal displacements. Experimental studies have indicated that the hydraulic displacement amplification system can provide significant contribution to dissipate energy and can improve the performance of the viscous fluid damper. However, the displacement amplification achieved by the system is far from satisfactory. Further studies are in progress to improve the efficiency of the amplification system. Time-history analyses were carried out on a two-story reinforced concrete frame design to non-seismic requirements when subjected to strong and frequent earthquake attacks. Significant reduction in the design forces is achieved when the displacement amplification factor is equal to 30. By optimizing the displacement amplification factor, it is possible to reduce the member forces to acceptable level.

9 7. Acknowledgement The authors are grateful to the financial supports from the Research Grants Council of Hong Kong (RGC No: PolyU BQ-576). 8. References BS8110 (1985), Part 1. Code of practice for design and construction, BS 8110: Part 1 Part 3, Structural Use of Concrete, National Standard, British Standard Institution. Berton, S., 2004, Displacement Amplification Method and Apparatus for Passive Energy Dissipation in Seismic Applications, U.S. Pat. No. 6,672,573 B2, 6-Jan Constantinou, M., 1994, Application of Fluid Viscous Dampers to Earthquake Resistant Design, presented at Research Accomplishments, , September 1994, pp Buffalo: National Center for Earthquake Engineering Research. GN , Code for Seismic Design of Buildings, National Standard, Beijing, China. Hanson, R.D. and T.T. Soong, 2001, Seismic Design with Supplemental Energy Dissipation Devices, CRC Press. Lee, C.F. et al, 1996, Seismic hazard Analysis of the Hong Kong Region, presented at GEO Report No. 65, Civil Engineering Services Department, Hong Kong Government. Ribakov, Y, J. Gluck and N. Gluck, 2000, Practical Design of MDOF Structures with Supplemental Viscous Dampers Using Mechanical Levers, 8 th ASCE Specialty Conference on Probabilistic Mechanics and Structural Reliability.