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Elastic properties of civil engineering structures change when subjected to a dynamic excitation. The modal frequencies show a rapid decrease followed by a relaxation, or slow recovery, that is dependent on the level of damage. In this article, we analyze the slow recovery process applying three relaxation models to fit real earthquake data recorded in a Japanese building that shows variant structural state over 20 years. Despite the differences in conditions, the different scales and the complexity of a real-scale problem, the models originally developed for laboratory experiments are well adapted to real building data. The relaxation parameters (i.e. frequency variation, recovery slope, characteristic times and their amplitudes, and range of relaxation times) are able to characterize the structural state, given their clear connection to the degree of fracturing and mechanical damage to the building. The recovery process following strong seismic deformation, could, therefore, be a suitable proxy to monitor structural health.

A sparsely instrumented 55-story building in Osaka, Japan, had recorded unprecedented, severe, and long-duration, long-period resonating responses during the 11 March 2011 M9.0 Tohoku earthquake that occurred at 767 km distance. Thereafter, studies of the records resulted in the implementation of a significant retrofit design, comprising dampers and buckling restrained braces (BRBs). The responses of the retrofitted building were also recorded during the 24 April 2016 M7.3 Kumamoto earthquake that occurred at 478 km. The earthquake and on-demand acquired ambient response data are analyzed in this study to assess the impact of this rare, retrofitted and instrumented tall building subjected to long period strong shaking from events originating at far distances. As expected, the fundamental frequency and critical damping ratio of the building increased, albeit small, after the retrofit as compared to before the retrofit. Increase of damping percentage is a positive finding and indicates that even larger percentages may be attained under shaking stronger than the 2016 event. The records indicate that the building still experiences significant resonance, torsion, as well as a beating effect.

The Building Research Institute and the Japan 2x4 Home Builders Association jointly built a full-scale six-story wooden experimental building. The building was densely equipped with two types of accelerometers to investigate its dynamic characteristics in detail. The fundamental natural frequencies and damping ratios were identified using the strong motion data from more than 100 earthquakes. Identified results widely varied affected by several factors. As one of the factors, the seasonal fluctuation in the dynamic characteristics was recognized by the system identification using ambient vibration data. The fluctuation can be explained as the influence of temperature and relative humidity. The dependence of the dynamic characteristics on response amplitude could be observed in the analytical result using strong motion data as well. Finally, the soil-structure interaction effect was examined from the strong motion data using a simplified rocking-swaying model. As a result, the considerable soilstructure interaction effect was recognized.

The objective of this study is to investigate the effects of the three-dimensional (3-D) configuration of the soil-structure system from the viewpoint of dynamic building response to wave propagation. The authors have been motivated to make a study on this subject by the fact that strong ground motion records of the aftershocks of the 2011 off the Pacific coast of Tohoku earthquake were obtained at Iwaki City Hall located in Fukushima Prefecture, Japan. At the North station of the observation site, the shorter predominant periods ranging from 0.2 to 0.3 sec. were observed, while at the South station, the periods were longer ranging from around 0.8 to 1.2 sec. at a distance of about 150 meters from the North station. Therefore, it is interesting to investigate how the underground irregularities affect the ground motions and the seismic responses of the soil-structure. The analysis method is a combination of the 3-D finite element and the 2.5-D thin layered and 2.5-D finite element methods in conjunction with a substructure technique. It was verified that the wave field shows very complex patterns when there exist inclined boundary layers and an embedded structure, causing a big difference in the result between 2- and 3-D methods at a station close to the structure.

Nonlinear elasticity has been observed in different materials and at different scales (from laboratory tests to the crust of the Earth). This behavior is evidenced by nonlinear response accompanied by slow dynamics, a process in which elastic properties recover (fully or partially) values previous to the excitation. In this study we show that nonlinear elasticity also occurs at buildings scale, where nonlinear signatures can be observed even for low levels of strain, and slow dynamics is seen at both short and long-term monitoring. Although the physics behind this response remains unknown, it is believed that this fascinating behavior is controlled by the presence of cracks, heterogeneities and dislocations; and therefore, it might be linked to the level of structural damage. By using data from Japan, we analyze the dynamic response of 24 buildings facing strong motions. The objective is to monitor variations of fundamental frequency over time and correlate them with different levels of structural drift and loading. Analyzing nonlinear elastic behavior might become a very useful and valuable way to monitor the time evolution of structural health, and hopefully, a predictive method to estimate deterioration and likely structural damage over time.

The Building Research Institute (BRI) of Japan operates a strong motion network that covers buildings in major cities across Japan. A great number of strong motion data has been accumulated in the observing stations integrated in the strong motion network over many years. This paper selects four high-rise steel-framed buildings from the BRI strong motion network. These buildings suffered severe shakings in the 2011 Tohoku Earthquake. To discuss the dynamic characteristics of the buildings, the fundamental natural periods and damping ratios of the target buildings are identified using strong motion data. First, changes in natural periods and damping ratios with time are discussed. In general, the natural periods gradually increase with time. Caused by the severe motions in the Tohoku Earthquake, significantly changes in natural periods are observed in all the buildings. Second, the amplitude dependence of natural periods and damping ratios are discussed minutely. The obvious amplitude dependence of natural periods can be recognized in all cases. The types of the relations established between the response amplitudes and damping ratios are dependent on the buildings. Finally, the relations between the response amplitudes and natural periods were formulated based on the regression analysis. The result makes it easy to quantify the changes in the natural periods.

The 11 March 2011 M 9.0 Tohoku earthquake generated long-duration shaking that propagated hundreds of kilometers from the epicenter and affected tall buildings in urban areas several hundred kilometers from the epicenter of the main shock. Recorded responses show that tall buildings were affected by long-period motions. This study presents the behavior and performance of a 37-story building in the Tsukuda area of Tokyo, Japan, as inferred from modal analyses of records retrieved for a time interval covering a few days before, during, and for several months after the main shock. The U.S. “code-type” array comprises three triaxial accelerometers deployed at three levels in the superstructure. Such a sparse array in a tall structure limits a reliable assessment, because its performance must be based on only the average drift ratios. Based on the inferred values of this parameter, the subject building was not structurally damaged.

During the 2011 Tohoku earthquake, a vast number of buildings suffered violent shaking that has never been experienced in the past. Strong motion data of the 2011 Tohoku Earthquake were obtained in many buildings, and many meaningful analytical results have been reported. We introduce several findings obtained through the analysis of the strong motion data recorded in buildings such as; change in dynamic characteristics of damaged buildings, amplitude-dependence of dynamic characteristics, and long period long duration strong motion and response of tall buildings. In addition, we report the application of the real-time S2HM (real-time building damage assessment system) to emergency management in high-rise buildings during the Tohoku earthquake.

Monitoring of seismic response of structures has been widely employed for decades in Japan. From the structural point of view, the objectives of seismic monitoring are mainly to verify designed load assumption, evaluate seismic response and structure performance during large earthquake, detecting possible damages due to earthquake and monitoring for verification of structural retrofit. Seismic monitoring is very important for large, important structures and structures with specific features and innovative technologies such as seismic isolation or response control systems. This paper describes several case-studies and lessons learned of seismic monitoring on seismically isolated bridges and buildings in Japan.

A sparsely instrumented 55-story building in Osaka, Japan, recorded unprecedented severe and long-duration, long-period resonant responses during the March 11, 2011, M9.0 Tohoku earthquake that occurred at 767 km distance. Thereafter, studies of the records resulted in implementation of a significant retrofit design, comprising dampers and buckling restrained braces (BRBs). The responses of the retrofit building were subsequently recorded during the April 24, 2016, M7.3 Kumamoto earthquake that occurred 478 km away, but at an azimuth nearly opposite to that for the Tohoku event. The earthquake records and ambient response data analyzed in this study represent a rare opportunity to assess the impact of retrofitting on an instrumented tall building subjected to strong long-period shaking from distant earthquakes. As expected, the fundamental frequency and critical damping ratio of the building increased, albeit by small amounts, after the retrofit. Increased damping percentage is a positive finding, and suggests that even larger percentages might be attained for shaking stronger than occurred following the 2016 event. The records indicate that the building still experiences significant resonance and torsion, as well as a beating effect.

A method for an evaluation of the building structure response reduction by soil‐structure interaction (SSI) based on the seismographs of the soil‐structure system is presented. In the proposed method, a simplified SSI model is employed, and the parameters of model are determined by system identification. For system identification, both the inertial interaction (II) and kinematic interaction (KI) are considered. The method that indicates a reduction in the structure response by II and KI separately is also presented. The proposed procedures are applied to the observed records of the 2011 off the Pacific coast of Tohoku Earthquake. The findings demonstrate that the reduction effect of II is reduced when the response of building structure enters the inelastic range, and vice versa. In the superstructures with pile foundations, the reduction effects of KI are relatively smaller than those of II.

The generalized inversion of S-wave amplitude spectra from the free-field strong motion recordings of the China National Strong Motion Observation Network System (NSMONS) are used to evaluate the site effects in the Wenchuan area. In this regard, a total of 602 recordings from 96 aftershocks of the Wenchuan earthquake with magnitudes of M3.7-M6.5 were selected as a dataset. These recordings were obtained from 28 stations at a hypocenter distance ranging from 30 km to 150 km. The inversion results have been verified as reliable by comparing the site response at station 62WUD using the Generalized Inversion Technique (GIT) and the Standard Spectral Ratio method (SSR). For all 28 stations, the site predominant frequency Fp and the average site amplification in different frequency bands of 1.0–5.0 Hz, 5.0–10.0 Hz and 1.0–10.0 Hz have been calculated based on the inversion results. Compared with the results from the horizontal-to-vertical spectral ratio (HVSR) method, it shows that the HVSR method can reasonably estimate the site predominant frequency but underestimates the site amplification. The linear fitting between the average site amplification for each frequency band and the Vs20 (the average uppermost-20 m shear wave velocity) shows good correlation. A distance measurement called the asperity distance DAspt is proposed to reasonably characterize the source-to-site distance for large earthquakes. Finally, the inversed site response is used to identify the soil nonlinearity in the main shock and aftershocks of Wenchuan earthquake. In ten of the 28 stations analyzed in the main shock, the soil behaved nonlinearly, where the ground motion level is apparently beyond a threshold of PGA > 300 cm/s2 or PGV > 20 cm/s, and only one station coded 51SFB has evidence of soil nonlinear behavior in the aftershocks.

During the 2011 Tohoku-oki earthquake, a large number of response records from various types of buildings were obtained. The records give us the actual motions of buildings during the exceptionally large earthquakes that are considered comparable with design earthquakes. Various types of earthquake response records have been collected, and their properties with several buildings that will exhibit implications for structural design are discussed.

To make sure the relationship between strong ground motions and building damages, dynamic earthquake response analyses of a building, which had a slight damage during the 2007 Niigata-ken Chuetsu-oki earthquake, are performed. Earthquake records at ground surface have been observed by the K-NET system near the building. Also the building responses were observed during after-shock. The analysis model of the building is estimated based on results of a push-over analysis. Sway and rocking impedances are also installed in this model. The predominant frequency of the building in elastic condition is corresponding to that in after-shock observation. It is demonstrated that the response of the building, which is calculated considering the nonlinear characteristics of soil and building during the main-shock, is larger than the estimated response based on the damage situation of the building.

Building Research Institute (BRI) is operating a nationwide strong motion network. Total 75 stations are disposed in major cities throughout Japan. The observation project aims to contribute to advance in seismic design technology for buildings. Therefore seismic response of buildings is the main target of the observation. A great number of strong motion records have been obtained and utilized to study local site and geological effects, soil-structure interaction and dynamic behavior of buildings. The paper introduces outline of the BRI strong motion network and its recent topics.

One of the most significant effects of the M9.0 Tohoku, Japan, earthquake of March 11, 2011, is the now well-known long-duration (>10 min) shaking of buildings in Japan – particularly those in Tokyo ( 350–375 km from the epicenter) and in places as far as Osaka ( 770 km from the epicenter). Although none collapsed, the strong shaking caused many tall buildings not to be functional for days and weeks. The purpose of this entry is to discuss the drift ratio issues related to tall buildings in Japan affected by long-period ground motions at far distances from the epicenter of the Tohoku earthquake ofMarch 11, 2011 (M1⁄4 9.0). We review the inferred long-period long-distance effects of the ground motions from recorded response data from four tall buildings in Osaka and Tokyo and the spectral properties of ground motions recorded from stations nearby these buildings. Therefore, three other relevant topics are also discussed in this entry

To detect and evaluate the earthquake damage of a 9-story reinforcement concrete building in the main shock of Miyagi-ken Earthquake, Sep. 15, 1998, the parameters of the building are identified and analyzed by using off-line system identification method from the structural strong motion records in the foreshock and aftershock before and after the main shock event. The results show that the structural parameters can be identified accurately from the seismic response data in small earthquakes during which the structure can be assumed as the time-invariant system. By comparing the parameters identified from the small earthquakes before and after the main shock, the structural damage in the main shock can be detected and evaluated primarily. It is found that the damage occurrence can be detected from the decreasing of the natural frequency and equivalent stiffness of the building, whereas the damping ratios are not suitable to detect the damage because the variations of damping ratios of the building are irregular after damaged in the main shock. The results can be referenced by the structural earthquake damage detection, diagnosis, and related work.

To track and detect the time-varying characteristics of building during small and great earthquakes, the time-varying parameters of a 5-story building are identified from its seismic response data in three earthquakes by using recursive system identification method. The results show that the time-varying modal parameters of structure can be predicted accurately from the seismic response records, and the structural parameters of buildings before and after damage can be determined from the time-varying parameters by only using records in one earthquake. The natural frequency does not change in small earthquake, but decreases gradually until it reaches the lowest point value, and then increases until the end of record in great earthquake, but it can not be recovered to the initial value when the building is damaged in earthquake. It can be concluded that the performance of structure could be recovered to a certain extent after earthquake compared with the most dangerous state when the response reaches the maximum value in earthquake. Furthermore, the damping ratio changes irregularly during the great earthquake. The results provide means for structural seismic damage detection, diagnosis and evaluation.

The time-invariant modal parameters of a 5-story building are identified from the seismic response time histories in 3 earthquakes by using off-line system identification method ARX model, and all the parameters are compared to find out the parameter changes and the seismic damage information primarily. The results show that the modal parameters of the structure can be identified accurately and easily by using the ARX method, and the structural parameters of the building before and after damage can be obtained from the two small earthquake response data before and after the main shock. The parameters identified from the data in the main shock are similar to the results from the data in the aftershock, but different with the results from the data in the small earthquake just before the main shock. It is found that the modal frequencies and equivalent stiffness decrease but the damping ratios increase after the main shock. From the results, it can be concluded that the building are damaged in the main shock. The results can be referenced by the seismic damage detection, diagnosis, and related tasks.

To diagnose the seismic damage of one 5-story reinforced concrete building under the excitation of great earthquake, the off-line ARX method and recursive on-line RARX method are applied to identify the structural time-invariant and time-varying parameters from its seismic response records. The results show that the time-invariant and time-varying parameters can be determined from the seismic response records by two methods, especially the structural time-varying characteristics can be identified accurately by using on-line RARX method through forgetting factor. Moreover, the structural parameters of the building before and after damage can be obtained from the time-varying parameters, and the seismic damage can be detected from the comparison among the parameters of structural different state. It is also found that the natural frequency decreases gradually until it reaches the lowest point value, and then increases until the end of the earthquake, but can not be recovered to the initial value. The performance of structures could be recovered to a certain extent after the earthquake compared with the most dangerous state when the natural frequency is at the lowest point. The results can be referenced by structural damage diagnosis and related tasks.

The final purpose of our study is development of the structural health-monitoring technique for the real existing buildings. In this paper, the results of the analytical studies are shown as the first step of our health-monitoring scenario. One of the purposes of the first step is to make the analytical models for evaluating the structural performance. The target building is the annex of Building Research Institute, of SRC building with 8 stories. Three-dimensional model was made and comparisons with the records and analysis during earthquake were carried out. The simple structural model tended to show longer period than the real building records due to the effects of walls, slab stiffness, rigid zone. The final model was modified including the effect of walls, finishing elements. The static elasto-plastic analysis is also carried out to predict the collapse mechanism under severe earthquakes. Base on the results of the analytical study, the peak strain sensors will be installed at the member in which damages are expected to occur. The examination of the durability and reliability of sensors, and of data collection and power-supplying system will be required in near future.

Building Research Institute (BRI), Ministry of Construction, Japan, started the strong motion observation project in 1957. The observation network has been enriched and enlarged in the past forty years. BRI is now operating several networks with a number of strong motion instruments. In this paper, we are introducing a new system that is installed to buildings and underground at BRI, and some observation results.