FACULTY OF CIVIL ENGINEERING. Eng. Horaţiu-Alin Mociran. Ph.D. THESIS - ABSTRACT -

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1 FACULTY OF CIVIL ENGINEERING Eng. Horaţiu-Alin Mociran Ph.D. THESIS - ABSTRACT - CONTRIBUTIONS TO THE SEISMIC PERFORMANCES EVALUATION OF STRUCTURES EQUIPPED WITH FLUID VISCOUS DAMPERS Ph.D. Supervisor, Prof. Eugen PANŢEL, Ph.D. Eng

2 CONTENTS Chapter 1. Introduction Chapter 2. Consequences of earthquakes Chapter 3. Structural control systems Chapter 4. Computational elements for structures equipped with linear fluid viscous dampers Chapter 5. Methods of analysis for structures equipped with linear fluid viscous dampers Chapter 6. Numerical studies Chapter 7. Performance indices Chapter 8. General conclusions, contributions and future work References Appendices Appendix I. Characteristics of used ground motions Appendix II. Diagrams use in the numerical studies Appendix III. Graphical representations of performance indices Appendix IV. Computer program SEISMPROT

3 1. Introduction Earthquakes are one of the nature s greatest disasters. The main concern of earthquake engineering is to enhance the protection of structures and its occupants. The philosophy in the conventional seismic design aims that structures prevent collapse by allowing structural members to dissipate the input energy by inelastic deformations. Energy dissipation occurs in specially detailed regions (plastic hinges). However, such design permits a certain degree of structural damage which often requires expensive repairs. An alternative to the conventional seismic design is the use of passive energy dissipation devices. The goal of using these devices is to dissipate the major portion of earthquake input energy, so that less is available to cause deformation of primary structural members. The main objectives of the PhD thesis are: - to study the devices used in structural control systems; - to derive the mathematical models for structures equipped with linear fluid viscous dampers - to assess the seismic performances of structures equipped with fluid viscous dampers - to study the influence of different parameters upon the iciency of seismic protection systems with fluid viscous dampers - to propose adaptations of the methods of analysis provided by romanian seismic design code to make them suitable for the design of new strucutres equipped with fluid viscous dampers. 2. Consequences of earthquakes The consequences of earthquakes are classified as follows: - ects on human beings (deaths and injuries); - ects on built environment (ects on buildings, social ects and ects on cultural heritage); - ects on natural environment (faults, landslides, liquefactions, tsunamis). In this chapter are presented in tabular form the major earthquakes that occured worldwide in the 20th century and the number of deaths associated with them. The deadliest earthquakes around the world in the 20th century are also listed. Examples of traditional construction techniques adapted to the seismicity of territory are shown.

4 3. Structural control systems Chapter 3 presents an updated state-of art review on structural control systems: passive, active, semiactive and hybrid. Passive control systems, the focus of this study, reduce the structural response by passive devices that do not require power to operate and develop control forces opposite to the motion of building. Passive devices are divided in two: seismic isolators and passive energy dissipation devices. Examples of passive devices include velocity dependent systems (viscoelastic and fluid viscous dampers), displacement dependent systems (metallic yielding and friction dampers) and vibration absorbers (tuned mass dampers and tuned liquid dampers). The construction of the main devices use in structural control systems, their principle of operation, their advantages and disadvantages and significant examples of their implementation are presented. 4. Computational elements for structures equipped with linear fluid viscous dampers Chapter 4 starts with a description of three mathematical models of fluid viscous dampers: linear viscous dampers, Kelvin model and Maxwell model. In general, frame structures are equipped with fluid viscous dampers install in diagonal or chevron brace configuration. These configurations produce damper displacements that are less than or equal to interstory drift. Rigid frame structures subjected to earthquake loadings have small interstories drifts and velocities. Small device dispalcements necessitate large damper force and expensive damper. The diagonal and chevron configuration are suitable for flexible structures. Two new configurations were proposed in this PhD thesis that produce damper displacement (velocity) greater than interstory drift (velocity) and therefore reduce the damper force, damper size and cost. The equations of motion for a single and multiple degree of freedom frame structures equipped with fluid viscous dampers in the four aforementioned configurations and subjected to ground acceleration are derived. The energy balance equations for structures equipped with fluid viscous dampers are given and discussed. 5. Structural control systems Chapter 5 presents the methods of analysis for structures described in the romanian seismic design code P100-1/2006: lateral force method, modal response spectrum analysis, pushover analysis, linear time history analysis and non-linear time history analysis. Recommendations were made to adapt these methods in order to be

5 use for the design of new structures equipped with fluid viscous dampers. The formulas of supplemental damping ratio introduced by dampers ξ s for the two new configurations proposed in this thesis were derived. Two comparative studies on the ectiveness of four damper configurations were performed. The results in both variants indicated superior performance in the cases of the proposed configurations. In this chapter were also presented the main provisions regarding the performance based design of american, european and romanian seismic design codes. At the end of this chapter, two flow charts were proposed for the design and retrofit of new or existing structures equipped with fluid viscous dampers. 6. Numerical studies In order to study the iciency of linear fluid viscous dampers in reducing the seismic response, three steel frame structures with 1, 5 and 9 stories and three bays were considered. All structures were analyzed in two cases: unequipped and equipped. The assessment of the ectiveness of fluid viscous dampers was made by comparing the seismic response of unequipped and equipped frame structures. The unequipped structures were design to remain in the elastic range under the considered earthquakes. The structures were equipped with fluid viscous dampers installed in a proposed configuration in all stories of the central bay of frames. The inherent damping ratio of the unequipped structures in the fundamental mode was assumed 5% and the total damping ratios of the equipped structures were designed as 15%, 25% and 35%. Time history analyses were conducted for five historic earthquakes: Vrancea, Kobe, Northridge, Imperial Valley and Kocaeli. The structural response parameters taken into account were: - story drift/ height; - story acceleration; - story shear and base shear; - damper force displacement loop; - energy time history. The numerical results proved the feasibility of these fluid viscous dampers for the seismic protection of frame structures and validated some of the theoretical formulas proposed in the thesis. The chapter ends with a description of the computer program SEISMPROT, written by the author in Java, for the time history analysis of single degree of freedom frame structure equipped with fluid viscous dampers in three different configurations.

6 7. Performance indices In the 7th chapter, four seismic performance indices were proposed for the evaluation of the iciency of protection systems with fluid viscous dampers. The iciency of these systems is investigated by comparison the values of the performance indices for the structures with 1, 5 and 9 stories, analyzed in chapter 6, for different damping ratios. 8. General conclusions, contributions and future work The main general conclusions of the thesis are: 1. The equipping of structure with linear fluid viscous dampers produced significant reductions of seismic response in comparison with the unequipped structure: - the peak interstory drifts were reduced by 17% to 62.2%; - the peak story accelerations were reduced by 23.3% to 58.5%; - the peak base shears were reduced by 6.1% to 55.4%. 2. The reduction of seismic response was in proportion with the ective damping ratio of the structure: - in the case of maximum interstory drifts, the reduction were ( )% for the ective damping ratio ξ = 0.15 and ( )% for the ective damping ratio ξ = 0.35; - in the case of maximum story accelerations, the reduction were ( )% for the ective damping ratio ξ = 0.15 and ( )% for the ective damping ratio ξ = 0.35; - in the case of maximum base shears, the reduction were ( )% for the ective damping ratio ξ = 0.15 and ( )% for the ective damping ratio ξ = The optimum values for the ective damping ratios have been found ξ = 0.25 and ξ = The author s personal contributions are: 1. A comprehensive review of the literature on structural control systems; 2. The proposal of two new damper configurations for frame structures; 3. The derivation of equations of motion for a single and multiple degree of freedom structures equipped with fluid viscous dampers installed in the proposed configurations; 4. The elaboration of SEISMPROT computer program for the time history analysis of single degree of freedom structure equipped with fluid viscous dampers;

7 5. Recommendations to adapt the methods of analyses presented in the romanian seismic design code P100-1 in order to be use for the design of new structure equipped with fluid viscous dampers; 6. The proposal of two charts for the performance based design of existing and new structures equipped with fluid viscous dampers; 7. The comparison of the iciency of different damper configurations in frame strucure; 8. The assessment of seismic performance of three steel frame structures with 1, 5 and 9 stories equipped with fluid viscous dampers installed in a proposed configuration and subjected to five well known ground motions (including NS component of Vrancea 1977); 9. The proposal of four seismic performance indices. SELECTIVE REFERENCES [1] Abbas, H., Kelly, J. M., A Methodology for Design of Viscoelastic Dampers in Earthquake-Resistant Structures, Raport nr. UCB/EERC-93/09, Earthquake Engineering Research Center, Berkeley, [6] Aiken, I. D., Energy Dissipation Devices. În: 100 th Anniversary Earthquake Conference Commemorating the 1906 San Francisco Earthquake, San Francisco, S.U.A., aprilie, [15] Alexa, P., Cătărig, A., Mociran, H., Mathe, A., Numerical Studies on Base Isolated Steel Structures. În: Proceedings of the International Symposium on Shell and Spatial Structures, vol. I, Bucureşti, 6-10 septembrie 2005, pg [17] Alexa, P., Mociran, H., Lădar, I., Availble Ductility of RBS Steel Structures. Comparative Studies. În: Proceedings of 6 th International Conference on Behaviour of Steel Structures in Seismic Areas, Philadelphia, august [22] Alexa, P., Petrina, M., Mociran, H., Socaciu, N., Ductility of RBS versus base isolated steel structures. În: International Symposium New Olympics New Shell and Spatial Structures, Beijing, octombrie 2006, pg [32] Blue book, Appendix A: Guidelines for Buildings Using Passive Energy Dissipation Systems, Energy Dissipation Committee, Structural Engineers Association of California, Sacramento, S.U.A., [33] Bozorgonia, Y., Bertero, V. V., Earthquake Engineering From Seismology to Performance Based Engineering, CRC Press, Boca Raton, [37] Castellano, M. G., Colato, G. P., Infanti, S., The Experience of FIP INDUSTRIALE in the Retrofit of Bridges through Seismic Isolation and Energy Dissipation. În: Buletinul AGIR, nr. 2/3, aprilie-septembrie 2009, pg [50] Chopra, A. K., Dynamics of structures. Theory and Applications to Earthquake Engineering (ed. 3), Pearson Prentice Hall, New Jersey, [53] Clough, R. W., Penzien, J., Dynamics of Structures (ed. 3), Computer & Structures, Inc, Berkeley, [56] Constantinou, M. C., Soong, T.T., Dargush, G.F., Passive Energy Dissipation Systems for Structural Design and Retrofit, Multidisciplinary Center for Earthquake Engineering Research, New York, [59] Constantinou, M. C., Tsopelas P., Hammel W., Testing and Modeling of an Improved Damper Configuration for Stiff Structural Systems, Raport Tehnic, University of Buffalo, New York, [71] Dubină, D., Lungu D., Văcăreanu R., ş. a., Construcţii amplasate în zone cu mişcări seismice puternice, Editura Orizonturi Universitare, Timişoara, [77] Eurocode 8, Design of Structures for Earthquake Resistance/ Part 1: General Rules, Seismic Actions and Rules for Buildings, European Committee for Standardization, Brussels, [89] Gawronski, W., Advanced Structural Dynamics and Active Control of Structures, Springer-Verlag, New- York, 2004.

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