EARTHQUAKE RESPONSE ANALYSIS OF A MID-STORY SEISMIC ISOLATED BUILDING

Transcription

1 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August -, Paper No. EARTHQUAKE RESPONSE ANALYSIS OF A MID-STORY SEISMIC ISOLATED BUILDING Ming HU, Hongjun SI, Kangning LI, Jiandong ZHOU, Jiqi HUANG and Masayasu UCHIDA SUMMARY In this paper, the effectiveness of the mid-story isolation technique is evaluated by conducting nonlinear time history analysis of a building that implements the mid-story isolation. The building is a -story SRC/RC building combining office/residential uses. Seismic isolation was installed at the th story. The whole structure was idealized as a stick model with its mass concentrated at points in the analysis. Each story of the building was simplified as a mass point with a lateral spring. The nonlinear time history analysis was carried out by using several seismic waves with different types. The analytical results indicate that the mid-story isolation technique is effective for the earthquake-resistant of the building with its responses of both the upper stories and the lower ones reduced greatly. INTRODUCTION After the Hyogo-ken Nanbu earthquake, the seismic isolation technologies have been implemented to more and more new building structures. This is because the earthquake- resistant design of building structures must be able to save their occupants life and minimums properties loss if they suffered from an earthquake. As a result, seismic isolation technologies have been increasingly applied to seismic strengthening and seismic retrofitting of existing buildings. In earlier days, the isolation layer was installed under foundations, which is named the foundation isolation. Due to its limitation of functions and configurations, many buildings were not suitable for use of the foundation isolation. This is especially true in the case of strengthening of existing buildings, since it is often very difficult, sometimes impossible, to install the isolation layers under the foundations of the existing buildings. Hence, for existing building, the isolation layers are required to be installed in mid-stories of the building. This paper describes mid-story isolation technique and presents nonlinear dynamic analytical results of a public building with the techniques adopted. Engineer, Lanton Design Consultants Co. Ltd, China. huming@hotmail.com Ph.D, Kozo Keikaku Engineering Inc., Japan, shj@kke.co.jp Ph.D, Canny Structural Analysis, Canada. Canny@hotmail.com Ph.D, P.E., Tokyo Techno-Consultant Ltd., Japan. zhoujiandong@yahoo.co.jp Engineer, Lanton Design Consultants Co. Ltd, China Engineer, Uchida Architect, Japan. Masa@aol.com

2 DESCRIPTION OF THE BUILDING AND ITS STRUCTURRE Description of the Building The building with total area of m is located near Tokyo, Japan. It was constructed in, and consists of two blocks A and B, both of which have nine stories. Its st to th stories are of SRC structures and its th to th stories are of RC one. The normal concrete was used for st to th stories with its design strength of N / mm. The lightweight aggregate concrete was used for th to th story with design strength N / mm. Its st to rd stories are used as government offices, and th to th stories as normal residential apartments. Its th story acts as a transitional story and is used to connect corridor and shared space for the upper story residents. This story is a pure frame structure without any shear wall, as shown in Fig.. The foundation of the building adopts an independent footing foundation, which in turn sits on a clay rock with bearing strength kn / cm. The natural period of the building structures is.~. seconds. Since the residential floors and office floors are already in use, it is taken into account that the seismic strengthening works of the building structures should not affect its normal uses. In this paper, as a typical one, only block A is adopted for seismic analysis and discussion. (a) Lateral View (X) (b) Longitudinal View (Y) Fig. Block A Seismic Diagnosis The seismic diagnosis was carried out according to the third examination method prescribed in the Standard for Seismic Diagnosis of Existing Buildings [] and in the Manual for Seismic Diagnosis of Existing Buildings []. The results of seismic diagnosis are shown in Fig.. After considering the regional effect factor, the seismic performance of the building structures shall meet the following requirements: Is., CTSD.. Fig. indicates that all the stories in longitudinal direction and rd & th story in lateral direction do not meet the requirements. These stories may suffer from damages during a significant earthquake; thus, seismic strengthening measures, namely mid-story isolation technique, should be taken to improve the earthquake-resistant of the building structures.

3 Floor X Y.... Is.... C TS D Fig. Results of Seismic Diagnosis X Y Selection of Story for Isolation As described previously, the st to rd stories of the building are used as office department; the th to th stories as normal residential and its th story as a transitional story; the vertical structure is formed of independent columns only. The th story is structurally weak floor because it has pure frame structure with a large open space without any shear wall. After careful analysis and comparison, the mid-story isolation technique is adopted for the seismic strengthening of the building structures by installing a laminated natural rubber bearing and viscous dampers on the th floor of the building to form a mid-story isolation structure, which separates the whole structures into two, one above the isolation story and another below the isolation story, and eventually improve the earthquake-resistant of the building structures. Description of the Isolation System Generally there are types of rubber bearings used in seismic strengthening of structures, that is, laminated natural rubber bearing, laminated high damping rubber bearing, and laminated lead rubber bearing. After analysis and comparison, the laminated natural rubber bearing is selected for the use in this building. This rubber bearing has the following characteristics: large non-linear scope and high reliability for modeling; low dependency on thermal; small creeping; high tensile strength and deformation capacity bearing a tension force. Its restoring force curve under a compressive stress of. N / mm is shown in Fig.. The isolation system was installed into all Horizontal load (kn) Horizontal displacement Fig. Restoring force curve of rubber isolation matrices (G. φ)

4 SRC columns on the th story of the building one by one. each column was cut off at its middle level before the rubber bearings was set up instantly between its two cuts. Totally sets of laminated rubber bearings (G.) and sets of dampers (KN) were installed into the building structure. The laminated natural rubber bearings used in the building has mm diameter. Its elastic shear modulus is. N / mm. Its first and second shape factors are.,., respectively. Its horizontal and vertical rigidity is., kn / cm, respectively. Its maximum allowable long-term and short-term compressive stresses is. N / mm,. N / mm, respectively. Its maximum allowable horizontal displacement is cm. Its restoring force curve is shown in Fig.. QD(kN) Q α C * Q C * -V -V V V ΔV(cm/sec) -Q α C * -Q Fig. Restoring force curve of damper(kn) C * =. KN sec/cm/sets α =. Q = KN Q = KN V = cm/sec V = cm/sec EARTHQUAKE RESPONSE ANALYSIS Modeling of the Building Structure The building structure with an isolation system is idealized an equivalent shear model with mass points shown in Fig.. The mass of each story of the building is concentrated to its floor level. The horizontal shear spring model is adopted to simulate the shear RF viscous loop of natural rubber. The damping ratio of the building is taken as % (initial rigidity proportionally descending). Damping ratio of the isolation story is taken as zero. F F F 隔震层上部结构 Upper structure Scenario Waves Four types of eight in total seismic waves were used in the analysis. They are the conventionally used waves such as EL CENTRO NS and TAFT EW, the long period waves, HACHINOHE NS and YOKOROCK, the synthetic wave from the scenario Kanto earthquake as well as artificial waves based on the Notification No. of the Ministry of Construction of Japan (present the Ministry of Land, Infrastructure and Transport Government of Japan, refer to as Notification wave hereafter). The phase characteristics of the Notification waves were taken from the records of HACHINOHE NS, JMA Kobe, and a random wave, respectively. The parameters of each wave are listed in Table. F F F F F F BF 隔震层 Fig. Analysis Model Isolation story 隔震层下部结构 Lower structure

5 Table Parameters of the seismic waves used in the analysis PGA (cm/s ) PGA(cm/s ) PGV(cm/s ) Wave Name Level Level Duration (sec) No. Site(Kanto wave). No. Site (YOKOROCK). No. Kobe (Notification wave). No. Hachi (Notification wave). No. Random (Notification wave). No. EL CENTRO NS.... No. TAFT EW.... No. HACHINOHE NS.... Veloocity (cm/s) Frequency Acceleration (gal) Period Fig. Velocity response spectra (Hz).. The synthetic wave from the scenario Kanto earthquake is made up in the following steps. Firstly, the response spectra are calculated by using the method proposed by Kobayashi and Midorikawa [] to consider the fault rupture process and the geology condition at the site. Secondly the time history is sorted out by fitting the spectra derived above and by using a random phase. The artificial seismic wave on the stiff ground, YOKOROCK, is developed by Yokohama city based on scenario earthquakes including the Kanto earthquake. No. No. No. No... Displacement (cm).. No. No. No. No (sec)

6 The pseudo velocity response spectra of the above seismic waves are shown in Fig.. It can be seen that the predominant period of EL CENTRO NS waves is relatively short, but those of the waves Kanto and HACHINOHE NS are longer than. second, sometimes exceed the amplitude of the published waves. Seismic Design Standard Seismic Design Standard : the principle structural members shall be in elastic state in the earthquake that the building may experience once or more times within its lifetime (refer to as level ); Seismic Design Standard : the stress in each structural member shall be less than the member s ultimate strength due to the strongest earthquake that the building may experience within its lifetime in the region (refer to as level ). The seismic waves used in time history analysis are modified based on the peak velocity of the waves. Namely, for the standards and, the peak velocity of the wave were taken as and cm / sec, respectively. Analysis Method The time history analysis was carried out by using a computer program CANNY. The restoring force model of upper structure used equivalent shear model and the isolation model adopted horizontal shear model. The durations of the seismic waves were to sec. The ambient temperature of the isolation devices is assumed to be. The stiffness variation in analysis is between +%~-% to consider the aging effects of natural rubber laminates DISCUSSION OF ANALYTICAL RESULTS Characteristic Parameters The characteristic parameters of the building structure before and after the installation of isolation are shown in Fig.. It is clear that the large displacement of upper structure above the isolation layer is caused by the first vibration mode. The natural period of the structure increases from about.~. seconds to about. seconds due to the installation of isolation. Floor Fig. Vibration modes and parameters of the structure before and after installing isolation

7 Earthquake Response The analytical results were obtained by using the seismic waves with peak velocity of cm/sec are shown in Fig. to. Maximum Displacement Response The maximum response displacements of the building structure in its lateral X and longitudinal Y directions are shown in Fig.. The maximum inter-story displacements of the isolation story are listed in Table. It is clear that displacements of the structure under the isolation layer are very small. A sharp increase of about.mm of the inter-story displacement occurred at the isolation story, but the structure above the isolation story performs almost like a rigid body with an identical horizontal displacements. The seismic displacement of the building structure have been concentrated on the isolation story with a maximum displacement of. cm well below the allowable value cm. Hence, the installation of the isolation layer has altered the displacement pattern of the building structure and consequently improved its earthquake resistant. Maximum Acceleration Response The maximum acceleration responses are shown in Fig.. As expected, the acceleration response of the structure under the isolation story is much greater than those of the upper structure. Hence, the mid-story isolation has greatly reduced the response acceleration of the upper structure. Floor No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. Fig. Results of time history analysis (maximum response displacement) Table The maximum displacement of the stories (Level ) Direction Horizontal displacement Limit Response Waves Long edge cm.cm No. Short edge cm.cm No.

8 Maximum Story Shear Force The maximum response shear forces of the building structure with an isolation layer installed are shown in Fig.. The thick solid line in the figure represents the structural bearing capacity. The figure shows that the mid-story isolation has greatly reduced the earthquake forces. The earthquake shear forces of the upper structure are well below their earthquake resistant and therefore they don t need to be strengthened. Only the lower story of the building structure under the isolation layer need to be strengthened in its longitudinal direction since its earthquake shear force exceeds its shear capacity. Floor No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. No. Fig. Results of time history Analysis(maximum response acceleration) Floor No. No. No. No. No. No. No. No. C apacity No. No. No. No. No. No. No. No. C apacity Fig. Results of time history Analysis (maximum story shear force)

9 CONCLUSIONS For the building with a structural weak floor at the mid-story and with different structural types for the upper and lower stories, the seismic performance can be greatly improved by using the mid-story isolation technique. For the case analyzed in this study, the building are strengthened to meet the earthquake resistant requirements and service requirements strengthening only the isolation story and the lower structure by using the mid-story isolation technique. The occupants can keep using the building when the strengthening works was going on. The mechanical properties of the upper structure are similar to that of a foundation-isolated structure, and its seismic isolation is achieved in a similar way. For the lower structure, the seismic reaction is analyzed, showing that the seismic forces acting on the structure have been significantly reduced and the expected strengthening effects have been achieved. The method used in this study is particularly suitable for middle to high rise buildings located in intensive seismic area. It can be predicted that as the demand of seismic isolation and strengthening will increase, the mid-story isolation technique will be applied into to more and more building structures. REFERENCES. The Japan Building Disaster Prevention Association. Standard for Evaluation of Seismic Capacity of Existing Reinforced Concrete Buildings (Revised Edition),.. Japan Association for Building Research Promotion and Structural Research Consulting Association. Technical Manual for Seismic Diagnosis and Seismic Retrofit Design of Existing Buildings ( edition),.. Kobayashi H, Midorikawa S. Semi-empirical Method for Estimating Response Spectra of Near-field Ground Motions with regard to Fault Rapture. Proceedings of the th European Conference on Earthquake Engineering. : -.