PLASTIC DEFORMATION CAPACITY AND HYSTERETIC BEHAVIOR OF U-SHAPED STEEL DAMPERS FOR SEISMIC ISOLATED-BUILDINGS UNDER DYNAMIC CYCLIC LOADINGS

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1 NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 2-25, 24 Anchorage, Alaska PLASTIC DEFORMATION CAPACITY AND HYSTERETIC BEHAVIOR OF U-SHAPED STEEL DAMPERS FOR SEISMIC ISOLATED-BUILDINGS UNDER DYNAMIC CYCLIC LOADINGS Y. Jiao S. Kishiki 2 D. Ene 3 S. Yamada 4 N. Kawamura 5 and Y. Konishi 6 ABSTRACT U-shaped dampers have been widely used for different types of isolated structures since the 995 Kobe earthquake. Previous research work provides static tests to estimate the performance of U- shaped dampers. However, the ultimate plastic deformation capacities and hysteretic behaviors of full-scale U-shaped dampers under dynamic excitations still remain unclear. There is another problem whether or not the initial temperature has any effect on the hysteretic behavior of U- shaped dampers. In the present paper, two sets of dynamic loading tests of U-shaped steel dampers were conducted to evaluate the above mentioned issues. The major findings are: ) Ultimate plastic deformation capacities of U-shaped steel dampers with various sizes were evaluated through a Manson-Coffin relation-based equation based on the horizontal shear angle which is defined as the lateral deformation divided by the height of the dampers. 2) Loading speed had little effect on the plastic deformation capacity of U-shaped dampers. 3) Initial temperature hardly affected the hysteretic behavior of U-shaped dampers. Assistant Prof., Dept. of Architecture, Tokyo University of Science, Tokyo, Japan, 25-5, yujiao@rs.tus.ac.jp 2 Lecturer, Dept. of Engineering, Osaka Institute of Technology, Osaka, Japan, , kishiki@archi.oit.ac.jp 3 Graduate Student, Dept. of Env. Sc.&Tech, Tokyo Institute of Technology, Yokohama, Japan, , ene.d.ab@m.titech.ac.jp 4 Associate Prof., Structural Engineering Research Center, Tokyo Institute of Technology, Yokohama, Japan, , yamada.s.ad@m.titech.ac.jp 5 Engineer, Nippon Steel&Sumikin Eng. Co., Ltd., Tokyo, Japan, 4-32, kawamura.norihisa@eng.nssmc.com 6 Engineer, Nippon Steel&Sumikin Eng. Co., Ltd., Tokyo, Japan, 4-32, konishi.yoshinao@eng.nssmc.com Jiao Y, Kishiki S, Ene D, Yamada S, Kawamura N, Konishi Y. Plastic deformation capacity and hysteretic behavior of U-shaped steel dampers for seismic isolated-buildings under dynamic cyclic loadings. Proceedings of the th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 24.

2 NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 2-25, 24 Anchorage, Alaska Plastic Deformation Capacity And Hysteretic Behavior Of U-Shaped Steel Dampers For Seismic Isolated-buildings Under Dynamic Cyclic Loadings Y. Jiao 2 S. Kishiki 2 D. Ene 3 S. Yamada 4 N. Kawamura 5 and Y. Konishi 6 ABSTRACT U-shaped dampers have been widely used for different isolated structures since the 995 Kobe earthquake. Previous research work provides static tests to estimate the performance of U-shaped dampers. However, the ultimate plastic deformation capacities and hysteretic behaviors of fullscale U-shaped dampers under dynamic excitations still remain unclear. There is another problem whether or not the initial temperature has any effect on the hysteretic behavior of U-shaped dampers. In the present paper, two sets of dynamic loading tests of U-shaped steel dampers were conducted to evaluate the above mentioned issues. The major findings are: ) Ultimate plastic deformation capacities of U-shaped steel dampers with various sizes were evaluated through a Manson-Coffin relation-based equation based on the horizontal shear angle, which is defined as the lateral deformation divided by the height of the dampers. 2) Loading speeds had little effect on the plastic deformation capacity of U-shaped dampers. 3) Initial temperature hardly affected the hysteretic behavior of U-shaped dampers. Introduction Seismic isolation is a simple structural design approach to reduce earthquake damage by decoupling the structure from the horizontal components of the ground motion by interposing base-isolation system with low horizontal stiffness between the structure and the foundation []. Various seismic isolation systems such as high damping rubber bearings, sliding elements, etc., have been adopted all over the world in earthquake prone areas [2][3][4]. Among them, one of the commonly used isolation systems is comprised of low-damping natural rubber bearings and some forms of mechanical dampers including hydraulic dampers, metallic dampers [5][6] etc.. The former is Assistant Prof., Dept. of Architecture, Tokyo University of Science, Tokyo, Japan, 25-5, yujiao@rs.tus.ac.jp 2 Lecturer, Dept. of Engineering, Osaka Institute of Technology, Osaka, Japan, , kishiki@archi.oit.ac.jp 3 Graduate Student, Dept. of Env. Sc.&Tech, Tokyo Institute of Technology, Yokohama, Japan, , ene.d.ab@m.titech.ac.jp 4 Associate Prof., Structural Engineering Research Center, Tokyo Institute of Technology, Yokohama, Japan, , yamada.s.ad@m.titech.ac.jp 5 Engineer, Nippon Steel&Sumikin Eng. Co., Ltd., Tokyo, Japan, 4-32, kawamura.norihisa@eng.nssmc.com 6 Engineer, Nippon Steel&Sumikin Eng. Co., Ltd., Tokyo, Japan, 4-32, konishi.yoshinao@eng.nssmc.com Jiao Y, Kishiki S, Ene D, Yamada S, Kawamura N, Konishi Y. Plastic deformation capacity and hysteretic behavior of U-shaped steel dampers for seismic isolated-buildings under dynamic cyclic loadings. Proceedings of the th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 24.

3 Figure U-shaped steel dampers employed to separate the upper structure from the foundation, while the latter is necessary in order to control the deformation in the isolation system as well as dissipate the earthquakeinduced energy. U-shaped steel dampers are often used as energy dissipation devices for seismically isolated structures [7]. This kind of device can be installed around the rubber bearings or be set up separately at various locations in the isolation system (Fig. ). After the 2 Great East Japan earthquake, none of the U-shaped steel dampers installed in the based-isolated buildings has been reported fracture or seriously damaged, not even in the severely afflicted areas, which proved their outstanding plastic deformation capacities [8]. Steel dampers for seismic isolation systems are required to have adequate plastic deformation capacities so that they would not reach failure during strong ground motions and the aftershocks. Previous research work during or soon after the developing procedure of the U- shaped damper provide static tests to estimate the damper performance [7]. Nevertheless, due to the lack of systematic experiments, the ultimate plastic deformation capacities of full-scale U- shaped dampers under random dynamic excitations, which are important in the design procedure of base-isolated structures, still remain unclear. Moreover, the influence of dynamic cyclic loadings on the dampers hysteretic characteristics is also an essential topic. Another issue is the influence of temperature, on which very little research has been conducted. It is unclear whether the initial temperature has any effect on the hysteretic behavior and plastic deformation capacity of U-shaped dampers. Additionally, the inner temperature of the damper increases when it deforms during earthquakes. The effect of the temperature rise on the performance of the dampers is indistinct at the moment. In the present paper, a series of dynamic loading tests toward U-shaped steel dampers were conducted to seek the answers to the above mentioned problems. The experiment programs contain single damper tests and damper unit tests. The single damper dynamic loading tests were carried out in Tokyo Institute of Technology (Tokyo Tech.) and University of California San Diego (UCSD). A damper unit consists of two dampers of same sizes that are set 9 o against each other. The damper unit tests were conducted in Tokyo Tech. The parameters of these tests were the loading direction, loading speed, initial temperature, and the size of the damper. Furthermore, some experimental data from [7] and [9] were collected to enlarge the size of the database. The effects of loading speed and temperature on the hysteretic behavior and plastic deformation capacity of U-shaped dampers were discussed through the experimental data. The plastic deformation capacity of U-dampers under one-directional cyclic loadings was evaluated, with the previously mentioned effects being taken into consideration.

4 Dynamic Loading Tests of Single U-shaped Damper Specimen The dynamic cyclic loading tests of single U-shaped dampers were mainly conducted in Tokyo Institute of Technology. Due to the limitation of the stroke of the loading equipment in Tokyo Tech, one group of single U-shaped damper specimens was tested under large deformation amplitudes at the University of California San Diego. The objective of these tests is to fully gain a clear idea of how the loading direction and loading speed affect the plastic deformation capacity of the damper. All specimens were loaded till ultimate states in these tests. Specimens U-shaped dampers are made of structural steel SN49B, which is known for its adequate plastic deformation capacity and stable yield strength. The special U shape (Fig. 2) is formed through cold bending and the post-forming U-shaped dampers are subjected to heat treatment. Full scaled U-shaped dampers of three most commonly used sizes (UD4, UD5, UD6) were tested in the Tokyo Tech experiments. The measurements of UD4~UD6 are listed in Figure 2. Coupon tests were conducted with the material from the same lot as the damper specimens. Table shows the coupon tests results. Test Setup Fig. 3 shows the test setup for the experiments in the Tokyo Tech experiments. Single damper specimens were tested under horizontal loading along degrees or 9 degrees with respect to the dampers symmetry axis. The specimens are connected to the reaction beam through some base plates and the reaction jig at their bottom part to get reaction force, while the upper part of the damper specimens are connected to the loading unit through the loading jig and base plates. The actuator (2kN) is mounted to the loading unit which is placed on the parallel rails so that the whole system is subjected to horizontally parallel moving during loading. The force afford by the damper unit was recorded by the built-in load cell in the actuator. Wire displacement transducers were used in this experiment to measure the horizontal deformation of the damper unit. By rotating the specimen, the loading direction was set to be degrees or 9 degrees. Test Program The variables of these tests are the size of the specimen, loading direction, loading speed, and the deformation amplitude of the loading histories. Table 2 lists the loading information of all 23 single damper specimens. UD4, UD5 and UD6 damper specimens were tested at Tokyo Tech. Among them one group of UD4 dampers were loaded under static tests with deformation amplitude of 5mm, while the other specimens were subjected to dynamic cyclic loadings (sine-wave). The loading period T varies from 3 seconds to seconds (quasi static tests). The maximum loading speed was up to 524mm/sec. Due to the limitation of the actuator, the total deformation of each excitation is about 2mm, with an excitation interval of minutes. A certain amount of the heat generated in the specimens during each excitation can escape during this minutes. Only one group of UD5 dampers were tested dynamically (T=4sec) at UCSD with deformation amplitude of 75mm. Three cycles were completed in each excitation.

5 t H w w 2 Measurements (mm) t W W 2 l H UD UD UD Figure 2 U-shaped steel dampers Actuator Loading Unit Specimen B Loading Jig Loading Jig Specimen A Specimen B Specimen A Parallel Rails Reaction Jig Reaction Jig Reaction Beam ELEVETION FRONT VIEW Figure 3 Table Setup of the U-shaped steel damper tests Results of the material coupon tests Thickness Yield strength Tensile strength Yield ratio Elongation (mm) (N/mm 2 ) (N/mm 2 ) (%) (%) UD UD UD Table 2 List of single U-shaped damper tests specimens Tokyo Tech. Damper Size UD4 UD5 Loading Direction Loading History (Amplitude) (mm) Loading Period (sec) Maximum Loading Speed (mm/sec) o 9 o +5 (Monotonic) ± ± ± ± ± ± ± UD6 ± UCSD UD5 ±

6 Measuring System Force-deformation relationships of the specimens were recorded in these experiments. Horizontal force afforded by the damper specimen was obtained by summing the measured data from the load cells between loading jig and the loading unit. The deformation of the specimen was obtained through the real-time displacement recorded by a built-in displacement sensor on the actuator. Moreover, in order to track the real-time deformation of the specimens under monotonic loading, tracers were attached to the specimens. The bending behavior of the U-shaped damper during loading was recorded by digital cameras set at fixed locations. The pictures were taken at constant time intervals. Single U-Shaped Damper Behavior under Monotonic Loading Monotonic loading tests of single damper specimens were conducted to find out the damper s basic mechanical behavior. Three UD4 dampers were loaded along degrees or 9 degrees with respect to the dampers symmetry axis respectively. Fig.4 shows the force-deformation relationships and the deformation during the monotonic loading. Due to the stroke limitation of the actuator, the maximum deformation in the monotonic loading tests was 5mm. No cracks were observed on any of the specimens during loading. Under o loading, the force afforded by the specimen reached maximum value (38kN) at the point when the deformation was around mm. After that, the force slightly decreased for a while with the increase of the deformation, when the deformation reached about 4mm, the force raised again till the end of the loading procedure. About the 9 o loading, the 2 nd order stiffness after yielding was relatively high, the force kept increasing during the whole time. The shapes of the damper under loading from two directions were different (Fig.4). Vertical deformation was observed during the loadings from o. The maximum value of the vertical deformation reached about 25mm when the horizontal deformation was 4mm during o loading. Therefore, attentions should be paid during the design of base isolated building with U-shaped dampers to allow enough space for the potential vertical deformation. The experimental yield strength P y, yield deformation y, and initial stiffness k are listed in Table 3. Here, the yield point (P y, y ) was defined as the point when the tangential stiffness dropped to /5 of the initial stiffness. Fatigue Characteristics of Single U-shaped Dampers Fig. 5 shows a group of examples of the force-deformation relationships of UD4 loaded under three directions (T=sec). Specimen under each loading direction has full and stable hysteresis loops, which indicates good energy dissipation. The maximum force at each cycle slowly decreased during loading, and ductile fracture was confirmed at the end of the test when the force of the specimens dropped rapidly and showed a negative high-order stiffness. Fatigue behavior is one of the most important characteristics of dampers. In order to evaluate the fatigue behavior of dampers of various sizes using the same index, the total deformation amplitude t was converted to horizontal shear angle t. The definition of t. is shown in Eq. and Fig. 6, where t is the total deformation amplitude, and h is the height of the U-shaped damper. The horizontal shear angle t shares the same value with the shear angle of the

7 isolator, which corresponds with the drift of the isolated story. t t h () The relationships between the horizontal shear angle t and the number of loading cycles till fracture Nf of single U-shaped damper under three loading directions were plotted in a Manson-Coffin plot (Fig. 7). For the purpose of receiving better accuracy, the experimental data published in [7] and [9], together with some unpublished results from the tests conducted by the Research Center of Nippon Steel & Sumikin Engineering were also processed in the same way and plotted in Fig.7. The specimens tested in these experiments also include UD45 and UD55, which have similar shape as the specimens used in the Tokyo Tech. tests with each measurement meets the similitude criterions. Degree 9 Degree P [kn] [mm] x 5 P [kn] 6 [mm] x y y 7 7 [mm] [mm] Force-deformation relationships & deformation during loading of UD4 under monotonic loading 5 Force (kn) Degree Figure Degree Force (kn) Figure Def (mm) Def (mm) Force-deformation relationships of UD4 (T=sec) 2 3

8 Table 3 Yield strength P y, yield deformation y, and initial stiffness k of UD 4 Yield Strength P y (kn) Yield Deformation y (mm) Initial Stiffness k (kn/m) o o o δ Isolator γ= δ/ h U-shaped U ダンパー Damper h Figure 6 Definition of the horizontal shear angle Influence of the Loading Direction Compared with the loading direction of 9 o, when t is relatively large (more than 4%), the specimens loaded along o survived less loading cycles. It is because that when the damper is loaded towards its symmetry axis, the strain along the damper is more likely to concentrate, which causes cumulative damage. When t is between 2% to 4%, the specimens loading on three directions show similar fatigue behavior. However, when t is small (less than 2%), the specimens on 9 o reached fracture slightly earlier than those loaded along o because of the strain due to the torsion occurred at both ends of the U-shaped dampers is more critical than the axial strain. Influence of Loading Speed The experimental results of UD4 specimens tested under dynamic loading at Tokyo Tech were discussed as an example to illustrate the influence of loading speed on the damper s fatigue behavior. The relationship between the maximum loading speed and the number of loading cycles before fracture total energy dissipation ( W) is shown in Fig. 8. The Y-axis indicates the values of the considered variable under dynamic loading normalized by the value of the same variable when the specimens was loaded at the period of sec (quasi static tests). The X-axis indicates the peak loading speed during dynamic loading normalized by the peak loading speed when the specimens was loaded at the period of sec(quasi static tests). When the dampers were loaded under dynamic loading for both directions, N f and W were slightly larger than those obtained from quasi static tests. The normalized N f under dynamic loading is between.6 and.6, while the normalized W under dynamic loading ranging from. to.2. The fatigue behavior of U-shaped dampers under dynamic loading is slightly better than that under static loading, therefore, during structural design, it is considered safe to neglect the effect of loading speed on the fatigue behavior.

9 Fatigue Life Evaluation of U-shaped Dampers Based on the above discussion, the fatigue life of U-shaped dampers can be evaluated using Eq. (2) and (3). These Manson-Coffin type empirical equations were obtained from the experimental results shown in Fig. 7. o Direction o t o N f ( 2% 5%) o t 9 o Direction.55 o o N ( 2% %) o t t 9 f o o N ( 253.5% o 5%).23 f t t (2) (3a) (3b) t [%] Degree t [%] 9 Degree (2) (3b) (3a) N f N f Figure 7 Fatigue behavior of U-shaped dampers N f / N f (T=sec) Degree 9 Degree Max Speed/Max Speed (T=sec) W/ W (T=sec) Degree 9 Degree Max Speed/Max Speed (T=sec) Figure 8 Maximum loading speed v.s. N f Maximum loading speed v.s. W

10 Details of the Experiments Dynamic loading tests of Damper Unit Specimens In isolated buildings the U-shaped dampers are set as units, of which two dampers of same sizes are set 9 o against each other. In order to clarify the effect of loading speed and initial temperature on the hysteretic behavior of U-shaped dampers during the life cycle of the isolated building, damper unit with two dampers perpendicular to each other were tested. Four cycles of loading were applied to the specimens in these experiments. Setup for the tests under various loading speed and initial temperatures is shown in Figure 3. Damper unit is set between the loading jig and the reaction jig. The variables of this test are loading speed and initial temperature (Table 4). Specimens U~U4 were tested under dynamic loading with different speed at room temperature. U5~U9 were loaded dynamically at various initial temperature from -o to 4o. Table 4 Information of the cyclic loading tests of U-shaped damper units Specimen Amplitude (mm) Maximum Loading Speed (mm/sec) Loading cycles U ±2 2 Cycles 2 Sets U2 ± Cycles 2 Sets U3 ± Cycles 2 Sets U4 ± Cycles 2 Sets Initial Temperature (ºC) Room temp. U5 ± Cycles 2 Sets - U6 ± Cycles 2 Sets U7 ± Cycles 2 Sets U8 ± Cycles 2 Sets 4 U9 ± Cycles 2 Sets 4 Force (kn) Unit_mm/s Unit2_57mm/s Unit3_34mm/s Unit4_49mm/s Force (kn) Unit6_34mm/s@- Unit7_34mm/s@ Unit3_34mm/s 8 Unit8_34mm/s@ (A) - Deformation (mm) (B) - Deformation (mm) Figure 9 Comparison of hysteresis loops subjected to different loading speed and temperature

11 Effect of Loading Speed and Initial Temperature on the Hysteretic Behavior of U-shaped Dampers The comparison of hysteresis loops (second loading cycle) of the specimens testes under different loading speed and initial temperature, based on the tests of U-shaped damper sets, is shown in Fig. 9. There is no significant change of stiffness due to higher loading speed or varying initial temperature. Compared with that from static loading, force measured during dynamic loading is slightly larger (less than 3%). Therefore, the effect of loading speed can be neglected. On the other hand, when the initial temperature was low, the area that the hysteresis loop covered was larger. The increase of force is within % at - o. However, the specimens were heated-up very fast due to plasticification of steel, therefore, although the effect of initial temperature is larger than that due to high loading speed, it is reasonable to ignore the tentative effect of initial temperature. Conclusions U-shaped dampers have been widely used for different isolated structures such as hospitals, plants and residential buildings since the 995 Kobe Earthquake. Two sets of full scale experiments were carried out in this study. The plastic deformation capacities of U-shaped steel dampers with various sizes were evaluated through Eq.(2) and (3). Loading speed and initial temperature hardly affected the hysteretic behavior of U-shaped dampers. U-shaped steel dampers are subjected to 2 directional random loadings due to the relative motion of isolated upper structures against the fixed base. Future study will focus on the fatigue behavior of U- shaped dampers under 2 directional random horizontal loadings. References. Naeim F, and Kelly J M. Design of seismic isolated structures-from theory to practice. John wiley & sons Derham CJ, Thomas AG, Eidinger JM, and Kelly JM. (98) Natural Rubber Foundation Bearings for Earthquake Protection Experimental Results. Rubber Chemistry and Technology, Vol. 53, No., pp Mokha, A, Constantinou, M, Reinhorn, A, and Zayas, V. Experimental Study of Friction Pendulum Isolation System. J. Struct. Eng., 7(4), Robison WH, and Cousins WJ. Lead dampers for base-isolation. 9th World Conference on Earthquake Engineering. Kyoto Suzuki K, Watanabe A, and Saeki E. Development of U-shaped Steel Damper for Seismic Isolation System. Nippon Steel Technical Report No. 92 July Suzuki K, Saeki E, and Watanabe A. Experimental Study of U-shaped Steel Damper. Part: Test of single U- shaped Damper. Part 2: Test of U-shaped Dampers with Rubber bearings (In Japanese). Proceedings of Architectural Institute of Japan (AIJ) Annual Conference, B-2, PP Konishi Y, Kawamura N, Terashima M, Kishiki S, Yamada S, Aiken I, Black C, Murakami K, and Someya T. Evaluation of the fatigue life and behavior characteristics of U-shaped steel dampers after extreme earthquake loading. 5th World Conference on Earthquake Engineering. Lisboa Suzuki K, Watanabe A, and Takayama M. Experimental Study of U-shaped Steel Damper. Part 3: The effect of velocity and temperature. Part4: Two-Dimensional Loading Test. Part5: Investigation of proportional relations. Part6: Test for products of U-shaped steel damper. (In Japanese). Proceedings of Architectural Institute of Japan (AIJ) Annual Conference, B-2, PP