Chapter 11!
Smart Structures
Field of Expertise: Structural and Earthquake Engineering

Ryuta Enokida & Kohju Ikago
Summary
A smart structure is a structural system equipped with devices to measure and control its responsewhen subjected to external excitations, such as earthquakes and strong winds. This structuralsystem is a promising measure to mitigate seismic damage from severe earthquakes, such as theNankai Megathrust Earthquake expected to occur within the next 30 years. This chapter discusses
the realization of smart structures and state-of-the-art structural health monitoring techniques.
Keywords: Smart structure, long-period ground motion, high-rise building, structural healthmonitoring, damage detection

Introduction
Thousands of high-rise and base-isolated buildings, classified as long-period structures,
have been built in Japan. During the 2011 Great East Japan Earthquake, these structures weresubjected to long-duration shaking induced by long-period ground motions. Since then, the issue oflong-period structures subjected to long-period, long-duration ground motions has been taken
seriously by the Japanese structural engineering community. To address this issue, we briefly
describe our activities.

1: Problems Revealed by the Great East Japan Earthquake
What happened?
During the Great East Japan Earthquake (magnitude 9.0) on March 11, 2011, many
seismometers located in the Tohoku region recorded very high acceleration values exceeding theacceleration due to gravity (= 9.8 m/s2). This high acceleration induced the falling of ceilings in many
gymnasiums and other large-span structures that were expected to serve as evacuation stations in
the event of a disaster. Consequently, many of these structures were not functional for theanticipated purpose. However, the recorded data did not significantly contain the “killer pulse”
component within the range of 0.5 to 1 Hz, which is known to affect low-rise structures critically.
Because of their characteristics, low-rise buildings (for example, detached houses) were not assignificantly damaged during this earthquake as during the 1995 Kobe Earthquake, in which the killer
pulse component predominated1).
In contrast, many high-rise buildings in Tokyo, located more than 300 km away from the1

epicenter, were significantly shaken by long-period ground motions for up to 10 min, resulting in
malfunctions and failures of non-structural components. Furthermore, a 55-story building
constructed on reclaimed land near Osaka Bay, approximately 770 km from the epicenter, underwent
prolonged shaking for approximately 10 min; its top floor shook up to 2.7m horizontally. A subsequent
survey revealed that the number of damages amounted to approximately 360, including the fallingof ceiling components and cracks on the floors. The Great East Japan Earthquake indicated that
attention should be paid to high-rise buildings subjected to near-fault earthquakes and far-field megaearthquakes.
Many high-rise buildings in the vast area of eastern Japan were influenced by this strong
earthquake. There was an increasing demand to assess the post-earthquake structural conditions
because the safety of buildings needed to be guaranteed against subsequent aftershocks. However,visual inspection relying on human effort is not the best method in such a disastrous situation
because it requires inspectors to work under high risks of aftershocks in potentially unsafe buildings.
The importance of structural health monitoring (SHM) techniques that do not significantly rely onhuman effort has been widely recognized by the Japanese structural engineering community.
The Great East Japan Earthquake identified the importance of the following challenges.
1) Enhancement of the seismic performance of high-rise buildings
2) Postearthquake structural condition assessment based on measurement data
In this regard, we summarize our efforts on the above and highlight the future challenges in thefollowing sections.
1. Smart structural systems to enhance seismic capability of high-rise buildings
The fundamental concept of a smart structure can beunderstood from the analogy of a person standing on avehicle, such as a train or a bus. When the vehicle moves
suddenly, the person steps on his or her feet or holds a
strap to maintain stability for safety. This behavior issomewhat natural when we are on a board, and smart
structures also have functionalities similar to earthquakes.
In a smart structure (human), measured data in sensing
devices (senses) are transferred to a computer (the brain), Fig. 1 Rotary inerter damperand a dynamic jack (muscles) moves for safety.
The unprecedently high economic growth of Japan fromthe late 1980s to the early 1990s facilitated theconstruction of smart structures. However, this prosperous
period did not last long and declined, partly because of theend of the high-growth period and concerns about itsperformance. For example, a smart structure stops
functioning if a power failure occurs during a disaster (forexample, an earthquake or typhoon), or it might accelerate
the structural damage, owing to the unexpected behavior
or malfunction of dynamic actuation systems.
Although the same situation for this structural
system prevailed even after 2000, the classical passive Fig. 2 Variable orifice damper
vibration control approach was recently revitalized andrecognized as another form of smart structure. The
dynamic vibration absorber and tuned mass damper (TMD) are examples. Currently, a high-rise2

building in Shinjuku, Tokyo, is equipped with TMDs (with a total weight of 1,800 tons) as acountermeasure against long-period motions2). When this building is shaken by seismic motions orstrong winds, its pendulums suppress the vibration. As shown above, the passive approach does
not require sensors or dynamic actuation systems driven by electricity, which are vital elements ofthe conventional smart structure. However, in this approach, the suspended weights passively sense
their structural vibrations, and an energy-absorbing device attached to the mass controls thevibration without using electricity. Because of this feature, the passive approach is generally
regarded as more cost-effective and reliable than the active approach, which relies on electricity.
This cost-effective aspect promotes its application in practice, which will become mainstream inenhancing the seismic capability of high-rise buildings.
Our research group at Tohoku University and private companies jointly developed passive
devices to construct passive smart structures. One of the devices was an inerter-based device (Fig.
1), and the other was a variable orifice damper (Fig. 2). The rotary inerter damper records anapparent mass several thousand times larger than the physical mass of the cylindrical flywheel via
a ball-screw mechanism, which converts linear motion into rotational motion. This device isinnovative in decreasing the weight of the device, although conventional TMDs typically require
hundreds or thousands of weight larger than this innovative device. This device has already beenused to retrofit three actual high-rise buildings. A variable orifice damper was developed to suppress
excessive displacement in a base-isolated structure, particularly under long-period ground motion.
This device increases the control force when the displacement of the structure exceeds theprescribed limit. The research and development of this device are actively in progress forapplications in actual structures.
2. SHM techniques for post-earthquake assessment
Visual inspection is the most straightforward approach for detecting structural damage andits severity in a building. However, inspection accuracy depends on the experience and skill of theinspectors and the visibility of damage. Different SHM techniques have been developed based on
the data acquired using measurement
equipment placed on the structure tominimize
reliance on visual inspection. The frequencydomain
approach, one of the most widely used SHM
techniques, is typically effective only for
globalstructural condition assessment. However,thisapproachisunsuitable
for
local
assessments, which require more detailed
information, such as the location and severity
ofstructural damage. For local assessments inSHM, atechnique for directly evaluating the physicalparameters of damaged structures isrequired.
Fig. 3 E-Defense test with three-story
To address this requirement, westructure.
developed a method called simple piecewise
linearisation in time series (SPLiTS)3), which identifies the physical parameters of a structure shaken
by an earthquake. Its efficiency was verified through shake table tests on a full-scale three-storysteel structure (width: depth: height = 18 m: 12 m: 13.5 m; total weight = 140 t), as shown in Fig. 3.
The shake table tests were performed at E-Defense, the world’s largest 3D shake table testing facility.
SPLiTS detected changes in the stiffness and damping of the structure in the time history andstructural energy absorption, as shown in Fig. 4. It was also found that SPLiTS could provide
3information on the time of occurrence and severity of structural damage.
Current SPLiTS requires acquiring acceleration and displacement data using measurementdevices on each floor. In terms of SPLiTS applications,acquiring displacement data is challenging, althoughobtaining acceleration data is significantly easier because ofrecent advancements in wireless sensing technology. For its
application to structures in cities, the current SPLiTS should
be investigated to achieve similar estimation accuracy using
only acceleration data without relying on displacement data.
Epilogue
From past severe earthquakes, Japan has learned
that various factors associated with earthquakes lead tosignificant casualties and loss of property: the tsunamiFig. 4 Results of SPLiTS
during the 2011 Great East Japan Earthquake, the collapseof structures during the 1995 Kobe Earthquake, and the fire of the 1923 Great Kanto Earthquake.
Earthquakes are unpredictable in terms of their occurrence dates, locations, and degree of severity.
Several factors that increase the risk of loss and casualty need to be considered during preparation
to mitigate losses caused by such unpredictable disasters. From a long-term perspective, we areapplying various advanced techniques, such as artificial intelligence, control theory, and wirelesstechnology, to structural control and SHM to prompt preparation against future strong earthquakes.
References
Cabinet Office, Government of Japan,
http://www.bousai.go.jp/kaigirep/chousakai/tohokukyokun/8/pdf/sub2.pdf [accessed on 12/13/2021]
https://www.mitsuifudosan.co.jp/corporate/news/2015/0514/ [accessed on 12/13/2021]
Ryuta Enokida, Koichi Kajiwara, Simple piecewise linearisation in time series for time-domain
inversion to estimate physical parameters of nonlinear structures, Struct. Control Health Monit.,
Volume 27, 2020.

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