Earthquake early warning-enabled smart base isolation system

Automation in Construction 115 (2020) 103203

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Automation in Construction journal homepage: www.elsevier.com/locate/autcon

Earthquake early warning-enabled smart base isolation system a

a,⁎

Yan-Shing Lin , Ricky W.K. Chan , Hiroshi Tagawa a b

T

b

School of Engineering, RMIT University, Australia Graduate School of Engineering, Hiroshima University, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Earthquake early warning Base isolation Smart structures Internet of things

Recent deployments of Earthquake Early Warning (EEW) system in Japan and some other earthquake-prone regions provide the vital warning signals prior to the arrival of destructive ground motions. The EEW system uses the different traveling speed of seismic P- and S-waves to achieve the goal of earthquake warning. This technology is primarily used to produce warning signals to alert the public to avoid potential risks, such as to evacuate from buildings. It is also used to mitigate other risks such as reducing train speed of Shinkansen trains. The present study suggests a new seismic-risk mitigation technique by connecting a base isolation system with the EEW. Base isolation is a mature technology which decouples structure from its base and lengthens its natural period of vibration. However, existing base isolation devices must possess certain lateral resistance to withstand service lateral forces such as wind, and such stiffness hinders the effectiveness of vibration isolation. In addition, supplementary damping devices are sometimes added to control excessive displacements in isolation level. This paper proposes a smart system which changes the property of a base isolation system upon EEW signal. In normal times when earthquake risk is not present, the base isolation system is locked by shear keys. When an earthquake is signalled by the EEW system, a mechanical system releases the base isolation system. When the earthquake ceases, the system resets and base isolation is locked again. On-board vibration sensors are added to activate the system in case of EEW fails to detect incoming waves. A crucial benefit of the system is that supplementary damper is no longer required to control excessive isolator displacements, and elastic stiffness of base isolation is no longer required to re-enter the base isolation system. The result is that it maximizes the vibration isolation effectiveness. A conceptual framework of proposed system is described and demonstrated by laboratory-scaled experiments. A 6-storey test frame is excited on a shake table subjected to historical earthquakes. Results indicates that the proposed system is effective in reducing earthquake responses on the building. It is an IoT-enabled earthquake-risk mitigation system.

1. Introduction Earthquakes are constant threats to human civilization. In the last few decades, with the deployment of seismometers in many parts of the world, we are now able to detect tremors around the globe, including those in remote areas and seafloors. Government agencies such as the United State Geological Survey (USGS) and Japan Meteorological Agency (JMA) publish real-time or near real-time earthquake information. However, prediction of earthquakes is still difficult, and large earthquakes in areas near populous urban areas continue to cause enormous damages and casualties. Recent earthquakes such as the 2008 Sichuan Earthquake, the 2011 Tohoku Earthquake and 2015 Nepal Earthquake caused a large number of lives and widespread destructions. Continual efforts have been made to mitigate seismic risks. Recently, the Earthquake Early Warning (EEW) System has been

⁎

developed and deployed. The principle of EEW is based on the characteristics that the seismic P-Wave travels 1.73 to 1.85 times faster than the more destructive S-wave [1]. When P-wave is detected, the approximate location of the epicentre is estimated, and a warning signal is broadcasted to the affected areas prior to the arrival of the S-wave. The principle of EEW is presented in Fig. 1. Japan is the first nation in the world to offer nation-wide EEW to the public in 2007. Estimated seismic intensity [2] are broadcasted. Earthquake warning is broadcast through television, radio, mobile networks, the Internet and specific EEW-enabled devices. Similar warning systems are now available in Taiwan, parts of China and many other earthquake-prone regions of the world. In Japan, EEW provides earthquake warnings in the order of seconds, providing reaction time to mitigate larger earthquake disasters such as shutting down certain industrial operations, reducing train speed and evacuating personnel [3]. For areas close to the epicentre,

Corresponding author at: School of Engineering, RMIT University, GPO2476, Melbourne, VIC 3001, Australia. E-mail address: [email protected] (R.W.K. Chan).

https://doi.org/10.1016/j.autcon.2020.103203 Received 4 December 2019; Received in revised form 17 March 2020; Accepted 23 March 2020 0926-5805/ © 2020 Published by Elsevier B.V.

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Fig. 1. Schematic diagram of Earthquake Early Warning system.

2. A brief review of base-isolation techniques

there will be insufficient time to send out warning signal due to operational delays and this area is called the blind zone. The current blind zone of EEW is about 50–70 km2 [4]. The possibilities offered by EEW system may not be limited to the issue of warning signals. In highly urbanized areas, the time required to evacuate an entire building such as a high-rise structure is much longer than the time provided by the EEW. In contrast, the activation of certain mechanisms appears to be feasible to mitigate seismic risks in some industrial processes and to enhance the protection of critical systems. This paper presents an investigation into an EEW-enabled smart base isolation system. The proposed system consisted of a base isolation that can be disabled or enabled by locking or releasing movable components. Compared to traditional passive base isolation systems, the proposed system has the following key features:

2.1. General descriptions In the last few decades, there has been a massive growth for development of earthquake-resistant structures and their associated technologies. Supplemental energy dissipating devices such as viscous fluid dampers, friction dampers, metallic yielding dampers; bucklingrestrained-braces, etc. have been commercially available and widely applied worldwide. On the other hand, base-isolation is regarded as one of the most mature technologies. Isolation devices are typically positioned at the base of a structure which isolate or to reduce the transmission of energy to the main structure. Elastomeric bearings and friction sliding bearings have been widely studied, and applications are common in earthquake-prone countries or regions. Elastomeric bearings usually compose of alternating rubber layers and steel plates. The rubber material provides elastic restoring and damping force, while the steel plates enhance vertical load capacity. A lead core is sometimes added to increase damping properties. A sample elastomeric bearing is shown in Fig. 2. This bearing is cut open to show the inner lead core and layers of steel and rubber and it is demonstrated to the public in the Nagoya City Science Museum.

(1) When the base isolation system is disabled by a locking mechanism, the main structure possesses high lateral-force resistance and prevents excessive wind-induced displacements; (2) The base isolation system is designed with minimal friction and no supplemental stiffness or damping. It maximizes performance when the structure is subjected to horizontal ground motions; and (3) After ground motion ceases, actuators re-centre the main structure to its initial position and re-engages the shear keys to lock the base isolation, and (4) The system is fully-automated and repeatable. In a previous study, the authors investigated into a one-dimensional base-isolation system using EEW [5]. The previous design used electromagnetic switches to facilitate rapid deployment of base isolation. This paper presents a design which is a successor to the previous design that it is a two-dimensional base isolation system and it uses linear actuators instead of electromagnetic switches. This paper first presents the current state-of-the practice of base isolation, particularly focuses in the use of cross-linear bearings (CLBs) in Japan. It is followed by a numerical study into the effect of using supplementary damping devices in conjunction with CLB systems. The paper then describes the conceptual design of the proposed EEW-enabled smart base isolation system. A laboratory-scale experimental verification using a shake table is presented.

Fig. 2. A typical elastomeric base isolator. 2

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different average wind speeds and discussed the habitability of base isolated building. To control excessive displacements in the isolation level, linear or nonlinear viscous dampers are added. Wolff et al. [17] conducted a comprehensive experimental investigation into the effects of these additional damping devices. Over 200 shake table tests were conducted with near-fault and far-fault excitations on two structurally distinct test frames. They concluded that in general, negative effects including increase in story drift and storey shear forces with added damping. The proposed smart system in this work does not require supplementary dampers at the isolation level to limit its displacement, thus maximising the effectiveness of the isolation.

On the other hand, sliding isolators decouples the horizontal stiffness of a structure through moveable contact surfaces. They allow two contacting surfaces to slide against each other upon horizontal loadings. Most sliding isolators use polytetrafluorethylene (also known as Teflon) material and stainless steel at the sliding interface. Horizontal restoring force is produced by added springs or gravity through spherical sliding surfaces. Notable examples are Friction Pendulum bearing and Triple Pendulum bearing that have been widely studied and applied. A thorough literature review on elastomeric and sliding base isolation has been presented by Warn et al. [6]. Sasaki et al. [7] reported on shake table tests on a full-scale, 15 m-high five-storey base isolated structure in the E-Defence test facility at the Hyogo Earthquake Engineering Research Centre in Japan. On the other hand, semi-active or active base isolation systems have attracted much attention by researchers. Yoshioka et al. [8] tested a semi-active base isolation using magnetorheological (MR) dampers. Chang and Spencer [9] tested a laboratory-scaled active base isolation powered by hydraulic actuators. Fujita et al. [10] proposed and tested an intelligent base isolation system using air bearings. The system is connected to Japan's EEW system and once activated, the compressed air floats the superstructure (e.g. a computer server). Recently, Venanzi et al. [11] discussed the concept and numerically simulate an active controlled base isolation for museum artefacts. A type of sliding isolators called Cross Linear Bearings (CLB) are becoming more popular in low to medium-rise building structures in Japan. A schematic diagram of this isolator is shown in Fig. 3. Their design resembles linear stages which are widely used in manufacturing. A load supporting surface is mounted on a sliding block which is supported on ball-bearings. The ball-bearings are fitting within “raceways” which recirculate as the block moves. Very low friction coefficient is achieved: coefficient of friction in the range of 0.0012–0.009 is listed in manufacturer's specification [12]. CLB may be supplied as single direction or two perpendicular directions. Fundamental period of structures isolated by CLB may be extended to 5 s. Commercially available CLB's have a wide range of axial capacity, suitable for light structures such as timber residential developments to heavy structures such as medium-rise reinforced concrete buildings. Axial compression rating of more than 30MN is available [12]. On the other hand, applications of CLB typically requires supplemental dampers such as viscous fluid (VF) dampers to dissipate input energy and also to control CLB displacements. Elastomeric base isolators are usually added to provide re-centering force. Fig. 4 shows a schematic diagram of the arrangement of CLBs, VF dampers and elastomeric base isolators in combination. Typically, all these devices are connected to a planar steel frame, and the main structure is supported on this frame. The seismic retrofit of the Osaka Prefecture Government Building (built in 1926) involved a combination of base isolation devices described herein was completed in 2017 [13]. In general, base isolation systems reduce the overall horizontal stiffness of a structure and avoid the dominant excitation frequency of earthquakes. However, wind loadings on base isolated structures may cause excessive displacements. Henderson et al. [14] discussed that the wind effects of base isolated structures and indicated that base isolation may amplify the response due to wind. Chen et al. [15] conducted a statistical analysis to different base isolated structures and investigated the displacement response due to wind effects. Liang et al. [16] analysed the floor acceleration response of a base isolated building to

2.2. Mathematical representation of base-isolated structures Fig. 5 shows a schematic diagram of a base-isolated structure on sliding isolators. A single-degree-of-freedom system with mass m, lateral stiffness k and viscous damping c is supported on base structure of mass mb. The base structure is supported by a combination of crosslinear bearings, rubber bearings and viscous dampers. The system is subjected to one-directional ground shaking. The equations of motion can be expressed as,

Mx¨ + Cx ̇ + Kx + τf f = −Mιx¨g

(1)

m 0⎤ k + kb − k ⎤ c + cb − c ⎤ K=⎡ C=⎡ where M = ⎡ b c ⎦ k ⎦ ⎣ −c ⎣ 0 m⎦ ⎣ −k x is the relative displacement vector. M, C, and K are n x n mass, damping and stiffness matrices respectively. ι is an nx1 influence vector and x¨ g is one-dimensional ground acceleration. τ is a location matrix indicating the location of base isolation, and ff is a nonlinear frictional force. The Bouc-Wen resilience model [18] has been used to model elastomeric types [19] and sliding type [20] base isolators. Here, we assume a friction-based sliding isolation system and the frictional force (ff) is assumed proportional to weight of the main structure, the BoucWen model can be written as, f f = μ (m + mb ) gZ

(2)

where μ is the coefficient of friction of the base isolation system, m is the mass of superstructure, mb is mass of base, g is acceleration due to gravity and Z is a dimensionless parameter which is defined below:

YZ ̇ = Ax ̇ − γ |x |̇ Z |Z|n − 1 − βx ̇ |Z|n

(3)

where x is the relative displacement of dynamic system, Y is the elastic deformation, while A, γ, β and n control the shape of hysteresis. Eqs. (2) and (3) may readily be coupled with equations of motion in a dynamic system. 2.3. Effects of viscous dampers on base-isolated buildings To study the effects of additional damping provided by viscous dampers in base-isolation systems, a numerical parametric study is carried out. The two-degree-of-freedom base isolated structures as depicted in Fig. 5 is used for this study. For the main structure above base isolation level, fundamental periods of 0.5, 0.75, 1.0, 1.5, and 2.0 s are considered. Seismic masses of m = 300kNs2/m and mb = 0.5 m are assumed. The damping ratio of main structure is 2%. The friction coefficient μ and stiffness kb of the cross-linear bearing is assumed to be 0.05 and 0.0 respectively. A range of damping force cb, provided by viscous dampers, is considered. Eqs. (1)–(3) are used to numerically Fig. 3. Schematic diagram of a Cross Linear Bearing.

Supporting plate Raceway with ball-bearings Sliding block

3

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Planar frame

Viscous Fluid Damper Cross Linear Bearing

Elastomeric Bearing

Fig. 4. Cross linear bearings used in combination with other base isolators.

Fig. 5. Base isolation system with rubber bearing and viscous dampers.

conducted by Wolff et al. [17]. From this sensitivity study, we may conclude that the presence of supplementary viscous damping controls excessive displacement at the base level, however it causes additional displacement to the main structure.

describe the system. Four historical strong earthquakes are used to evaluate the structural performance of the system. The four earthquake histories are recommended for benchmark problems for structural control [21] which consist of two near-field and two near-field records: (1) El Centro, the N-S components recorded at the Imperial Irrigation District substation in El Centro California during the Imperial Valley California Earthquake on 18th May 1940; (2) Hachinohe: the N-S component recorded at Hachinohe City during the Tokachi-oki earthquake on 16th May 1968; (3) Northridge, the N-S component recorded at Sylmar County Hospital during the Northridge California earthquake on 17th January 1994; and (4) Kobe, the N-S component recorded by Japanese Meteorological Agency during the Hyogoken-Nanbu earthquake on 17th January 1995. The time history can be downloaded from the web site of Third Generation Benchmark Control Problems for Seismically Excited Nonlinear Buildings [22]. The results are illustrated in Fig. 6(a)-(d) for the four input ground motions. The base drift (lateral movement of mb) and the drift of the main structure relative to base (x-xb) are shown as a function of viscous damper force. The base drift is significantly reduced by the introduction of viscous dampers, with the most drastic decreases when cb/c is 0 and 5. Similar observations can be made across four ground excitations. On the other hand, x-xb tends to increases as cb/c increases, indicating the presence of viscous dampers produces more displacement to the main structure. The numerical study agrees with the experimental studies

3. Conceptual framework of the proposed EEW-enabled base isolation system 3.1. General descriptions The proposed EEW-enabled base isolation system is a combination of a passive isolation system, EEW connectivity, sensors, shear keys and actuators. Fig. 7 shows a schematic diagram of the proposed system. A structure is supported on low-friction cross-linear bearings which permits the main structure to decouple from horizontal ground shaking. However, the base isolation system is locked against movement by horizontal shear keys. The shear keys unlock the base isolation under three conditions: (1) a signal is received from EEW indicating incoming ground motion; (2) averaged acceleration values measured by sensors exceeds a predetermined value; and (3) manual override. The shear keys are operated by hydraulic or electrical linear actuators. Under service condition (Fig. 7a), the shear keys provide the necessary lateral stiffness and strength against design lateral loads (such as wind forces). When one of the three conditions described above is fulfilled, the 4

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Fig. 6. Effects of damping force in base isolation systems.

procedure. The main goal of system initialization is to correctly position the main structure such that shear keys are fully engaged to lock the base isolation system. The controller is programmed to fully extend all actuators and all shear keys will be pushed into their slots. Any misalignment of the main building from its intended original position will be corrected. In this initialization phase, no sensor feedback is required. After initialisation the system proceeds to standby condition in which the communication terminal of the controller continuously monitors signals from the EEW system and its own network of accelerometers.

actuators will disengage the shear keys, allowing the structure to be base isolated (Fig. 7b). The proposed system is fully-automatic and human-intervention is unnecessary. Compared to semi-active or active control systems, the proposed system does not involve any control algorithm or stability issue. Therefore, the system hardware requirements are greatly reduced and long-term maintenance will be easy and inexpensive. It should be noted that, similar to most base isolation systems, the proposed system does not cater for vertical vibrations. The proposed EEW-enabled system operates in several sequences and they will be described below.

3.3. Activation of base isolation 3.2. System initialization and standby condition As described in Section 3.1, the base isolation can be triggered under three situations. When a triggering signal is received, the

When the system is powered-on, it performs an initialization 5

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Fig. 7. Schematic diagram of the EEW-Enabled Base Isolation System: (a) Stand-by mode; (b) Activated mode.

system identification can be found. The test frame is affixed onto two layers of steel linear guide rails arranged orthogonally to allow guided motions in the x-y plane. The linear rails are secured on a timber basebox which is in turn secured on the shake table as shown in Fig. 9(a). The base-box contains a timber frame which forms a rigid boundary for the test frame to move within. The space between the “ground level” plate and the timber frame is the seismic gap. Four linear actuators (Actuonix L12-S) are attached to the bottom side of the base plate, as shown in Fig. 9(b). When the actuators are programmed to fully extend, the timber frame in the base-box provides the reactions such that the main structure will position at the centre or neutral position. Two lowpower, digital accelerometers (ADXL345) positioned on the shake table top, are connected to the controller to detect ground accelerations. In this experiment the threshold accelerations (averaged of the two sensor measurements) to trigger activation of base isolation is set to 0.4 g. Ground motions are simulated by an electric shake table (Quanser Shake Table II). Structural responses of the test frame are independently measured by three 3-axes wireless accelerometers (BeanDevice AX-3D) located at three levels: top of shake table, ground level, and roof level of the frame (see Fig. 8). Sampling rate is set to 100 Hz and the BeanScape Premium Plus software (a real-time WSN supervision software for wireless accelerometers) was used to export data to MATLAB (version R2018b) for further analysis. As shown in Fig. 9(c), initially the measurements from the wireless accelerometers are compared to wired accelerometers to ensure calibration is performed correctly. The wired accelerometers are removed in the tests. Four major historical earthquakes supplied by the courseware of the shake table are simulated through the shake table: (1) 1979 Imperial Valley Earthquake, (2) 1995 Kobe Earthquake, (3) 1994 Northridge Earthquake and (4) 1992 Mendocino Earthquake. In order to demonstrate the effectiveness of the two-dimensional base-isolation system, the building was placed at an angle of 45 degrees with respect to the ground motion. In the experiment, EEW signal is simulated on a local area network. A web-browser is used as a console, and the microcontroller connects to the IP via Ethernet acts as the monitoring server. Three operational modes were tested which correspond to different scenarios in practice, as listed in Table 1. For “Fully fixed”, the main structure is excited with actuators fully extended such that the ground floor plate is locked against movements in x-y directions. For “BI by EEW”, the base isolation system is triggered by the EEW, thus the main structure is fully base-isolated for the entire ground excitation. For “BI by sensors”, the system is triggered when measured accelerations exceed 0.4 g. It should be noted that, the triggering acceleration in practical applications should be structure specific. For example, a facility which houses sensitive equipment may adopt a smaller value.

microcontroller sends a command to the actuators and they will be retracted fully. The shear keys which are connected to the actuators will unlock the base isolation. Depending on the speed of actuators, this process will be completed in seconds. Thus, if the triggering signal is received from EEW system, the base isolation is activated prior to the arrival of ground shaking. On the other hand, if the EEW system fails to signal an incoming earthquake and the triggering signal is initiated by the system's own accelerometers, a delay in unlocking the base isolation will be experienced. This delay will cause the main structure to be excited as a fixed-based structure initially. Its effect will be studied in the experiments and presented in the next section. The triggering acceleration is structure-specific, that is, it is a choice of the design engineer. For example, a building which houses sensitive equipment or made of brittle materials may require a lower triggering value, while a less vibration sensitive structure may allow a higher triggering acceleration. A low-triggering value may ensure activation at low-level of ground acceleration, but it will increase the risk of false-activation, for example, due to a moving heavy vehicle nearby. After a pre-determined time period, the system will recheck the flag of the EEW signal and data from its accelerometers. If it is confirmed that ground motion has ceased and there is not new EEW signal, the system will proceed to reset. The reset process is identical to system initialisation that the actuators are programmed to extend fully. Thus, the main structure is forced to its original position and shear keys will engaged again. The complete system is now reset and ready for the next ground motion, such as an aftershock. In practical applications, the system will require one or more actuator to generate sufficient force to re-center the main structure. Since the main structure is supported on very low friction linear cross bearings (friction coefficient as low as 0.5–1.0%), re-centering by linear actuators can be achieved. In addition, an uninterruptible power supply (UPS) to the actuators will be required due to the uncertainty of power from the main grid in an event of earthquake.

4. Experimental investigation 4.1. Experimental setup and instrumentations A lightweight 6-level test model made of acrylic plastic and aluminium strips is used in the experimental investigation. Columns are made of 4 mm thick clear acrylic plastic strips. Floor plates are made of 3 mm thick timber boards and they are connected to columns via aluminium brackets and steel bolts. Steel masses are positioned centrally on each floor and the total mass of the model frame is 7.5 kg (excluding base plate and base isolation rails). The first three natural frequencies when the test frame model is fixed at its base are: 1.6 Hz, 5.9 Hz and 11.5 Hz; and its viscous damping ratio (ζ) is 2.85%. The same test frame was used in a previous study [5] where a more detail description of 6

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Roof level

Timber frame

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Timber box

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Table 1 Operational modes in experiment. Operational mode

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Nil EEW signal activates the base isolation On-site accelerometers detect vibrations higher than predetermined threshold values of 0.4 g

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Fig. 10. Circuit diagram of experimental setup.

disadvantage is that it is only one-to-one connection and it has low speed (up to 115.2 kbps). So UART communication protocol is not suitable for high-speed, large-scale transmission. Compared with I2C, SPI is full duplex and the speed of transmission is higher, but more wires are required. Since EEW signal is small (less than 1 KB) and the scale is not large, I2C communication protocol is selected for the experiment. The entire system does not require an external power supply, and the power is completely provided by the controller.

4.2. Controller design The controller of the proposed system mainly performs three functions: communication, sensing and execution. The experiments are completed on the Arduino MEGA 2560 control platform, which has 54 sets of digital input/output ports (14 of which can be used for Pulse Width Modulation output), 16 analogue inputs and a 16 MHz crystal oscillator. It possesses a built-in bootloader which can be burned directly via USB. The power supply can be powered by USB or a 5 V DC power supply. To obtain the signal of EEW on the Internet, an Ethernet shield is introduced which makes TCP/IP Protocols (TCP, UDP, ICMP, IPv4 ARP, IGMP, PPPoE, Ethernet) on the hardware circuit to reduce the burden of the microcontroller. An ethernet shield allows up to four connections at the same time and achieve transmission speed up to 100 MB/s. Two digital 3-axis accelerometers (ADXL345) are connected to the microcontroller via I2C connectivity. These sensors are lightweight, ultralow-power 3-axis accelerometers with high resolution (3.9 mg/LSB) and a measurement range of ± 16 g. Its frequency, detection range, sensing mode, and reading and writing are programmable based on the project requirements and conditions. These sensors are factory-calibrated and inexpensive, making them ideal for the experiment. The circuitry used in the experimental setup is illustrated in Fig. 10. In order to enable multiple devices to be used simultaneously on the same platform, it is necessary to select an appropriate communication protocol. There are currently three communication protocols commonly used on controllers: I2C, SPI, and UART. UART is not a comprehensive communication method as it requires different ports and drivers, such as RS-232, RS-422, RS-485. Among them, RS-232 port is more popular whose advantage is that the wiring is simple (only two wires), but the

4.3. Initialization, standby mode and activation of base isolation system When the system initializes, the controller resets the position of the structure by extending the actuators. In the experiment, all four linear actuators perform the system initialization. Once powered on, the linear actuators extend fully and the test frame is forced to its original position. The linear actuators provide feedback signals that the position of structure is known. When the initialisation completes the system enters standby condition which the controller continuously updates the signals of EEW and its accelerometers. Monitoring of EEW is carried out by continuously updating the IP of a simulated EEW signal through its ethernet connection. 4.4. Results and discussions The test setup is robust and undamaged after all tests and the experiment are repeatable. All measured data is filtered by a 6th order Butterworth low-pass filter in MATLAB with cut-off frequency of 25 Hz. Fig. 11 illustrates the building responses (absolute accelerations) measured on ground (G) and roof (R) levels under different operational modes. 8

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Fig. 11. Experimental results - peak absolute accelerations (unit in g).

substantial reduction in structural vibrations. The BI by sensor recording also show significant reduction in structural responses (second row of Fig. 13). It is due to the fact that the on-board sensors detect accelerations exceeds 0.4 g very early in the time history, and when the largest shock which occur at the 3rd second, the base isolation is already released. In Fig. 14, the 1995 Kobe Earthquake is characterized by a period of less severe excitation, followed by a severe shaking at about 18th second. The responses measured under “BI by EEW” (first row of Fig. 14) show great similarity to “BI by sensor” (second row of Fig. 14). The reason for this observation is that under “BI by sensors”, the base isolation has been released prior to the strong ground shaking, hence the similarity between the two modes of operation. In Fig. 15, the 1992 Mendocino Earthquake is presented. Similar observations can be made to those in observed in Imperial Valley and Northridge earthquakes.

Figs. 12–15 show the time histories of measured absolute accelerations under four historical earthquakes. For easy reference, the diagrams are arranged as follows: left to right are measurements on shake table, ground level and roof level respectively. From top to bottom are three operation modes respectively. In Fig. 12, the 1979 Imperial Valley Earthquake is characterized by a strong excitation in the first second. A rapid vibration on the roof level in the “fixed based” test (3rd row in Fig. 12) is recorded. In the first row of Fig. 12, BI by EEW, it is clear that structural responses are significantly reduced. Meanwhile, if the base isolation is triggered by onsite accelerometers (second row of Fig. 12), the main structure is excited as a fixed-base structure initially and structural response quickly diminished as base isolation is released. In Fig. 13, an early strong shaking is also presented in 1994 Northridge Earthquake, but the latest shock occurs at about the 3rd second. The EEW triggered (first row of Fig. 13), as expected, shows

Fig. 12. Structural Responses of 1979 Imperial Valley Earthquake. 9

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Fig. 13. Structural Responses of 1994 Northridge Earthquake.

5. Conclusion

connected to the EEW system. The system consists of a microcontroller which is programmed to continuously refresh the status of EEW through internet connection. The main structure is supported by sliding type bearings with minimal friction and lateral stiffness. This enables maximum effectiveness of the vibration suppression properties of the base isolation. When there is no earthquake warning signal, the moving components of a sliding-type base isolation are locked (this paper refers this to the standby mode), the main structure thus possesses strong

The deployment of Earthquake Early Warning (EEW) system in Japan and other earthquake-prone regions in the world open new opportunities of seismic-risk mitigation for structures. EEW systems are capable of detecting fast traveling earthquake P-waves and send warning signals to the public prior to the arrival of more destructive Swaves. This paper proposes a smart base-isolation system which is

Fig. 14. Structural Responses of 1995 Kobe Earthquake. 10

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Fig. 15. Structural Responses of 1992 Mendocino Earthquake.

lateral resistance and excessive movement of the base isolation due to wind loads is prevented. When a signal is received from EEW indicating incoming ground motion is anticipated, shear keys which lock the base isolation are retracted, allowing the main structure to move freely in the horizontal plane. After ground motion ceases, the actuators recentre the main structure and shear keys are engaged again. The system is fully automatic and repeatable. The proposed system also consists of a backup system which consists of a network of accelerometers to detect ground motions. These sensors will release the base isolation if not enabled prior by the EEW. This paper presents the conceptual design and laboratory-scaled simulations using a shake table. A test frame is mounted on a base-isolation system that allows movement in the x-y plane. The tests are conducted under three conditions: (1) fixed base – when base isolation is locked under ground motion; (2) base isolation released by EEW and (3) base isolation released by the network of sensors. Results have demonstrated that the proposed smart base isolation system significantly reduces floor responses in all tested earthquake histories. The smart system presented in this paper is an IoTenabled seismic-risk mitigation system and it may represent the next generation of earthquake-resistance structure.

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