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SmartSync: An Integrated Real-Time Structural Health Monitoring and Structural Identication System for Tall Buildings Tracy Kijewski-Correa, A.M.ASCE 1 ; Dae Kun Kwon, M.ASCE 2 ; Ahsan Kareem, D.M.ASCE 3 ; Audrey Bentz, M.ASCE 4 ; Yanlin Guo 5 ; Sarah Bobby, S.M.ASCE 6 ; and Ahmad Abdelrazaq, A.M.ASCE 7 Abstract: This study introduces a unique prototype system for structural health monitoring (SHM), SmartSync, which uses the buildings existing Internet backbone as a system of virtual instrumentation cables to permit modular and largely plug-and-play deployments. Within this framework, data streams from distributed heterogeneous sensors are pushed through network interfaces in real time and seamlessly synchronized and aggregated by a centralized server, which performs basic data acquisition, event triggering, and database management while also providing an interface for data visualization and analysis that can be securely accessed. The system enables a scalable approach to monitoring tall and complex structures that can readily interface a variety of sensors and data formats (analog and digital) and can even accommodate variable sampling rates. This study overviews the SmartSync system, its installation/operation in the worlds tallest building, Burj Khalifa, and proof-of-concept in triggering under dual excitations (wind and earthquake). DOI: 10.1061/(ASCE)ST.1943-541X.0000560. © 2013 American Society of Civil Engineers. CE Database subject headings: High-rise buildings; Structural health monitoring; Wind loads; Earthquakes. Author keywords: Tall buildings; Structural health monitoring; System identication. Introduction Long before the term structural health monitoring (SHM) was coined, buildings were instrumented and monitored in situations where their performance was suspect, e.g., the John Hancock Tower in Boston (Durgin 1990). These unsavory origins made the concept of proactive monitoring, particularly for buildings, quite foreign, as owners were naturally reluctant to install instrumentation systems for fear that this may tarnish the reputation of their buildings. With growing need to assure life safety and rapid reoccupation in regions of high seismicity, full-scale monitoring applications began to ourish in the western United States as part of coordinated strong motion instrumentation programs. With such exposure, these early perceptions and attitudes have gradually changed with the advent and acceptance of smart building systems, which rely on in- strumentation to assure and optimize the performance of elevators, various mechanical systems, and, in some cases, auxiliary dampers installed to limit building movements. In addition to some owners embracing the feedback from real-time monitoring, a number of research efforts have sought to validate structural performance and various design principles in full scale, not only for earthquakes but also wind events, as summarized in Kijewski-Correa and Cycon (2007). One class of structure that is especially well suited to benet from continuous monitoring is tall buildings. These structures affect the safety and comfort of a large number of people in both residential and ofce environments, and as state-of-the-art structural analysis software and wind tunnel testing used in the design of these struc- tures are advancing rapidly, the accuracy and validity of their results need to be calibrated with respect to actual performance. This be- comes particularly important to insure the economy and efciency of future designs with increased complexity and height generally governed by serviceability and habitability limit states under wind that are especially sensitive to the amount of inherent damping in these systems. Unfortunately, only limited studies have pursued full- scale investigations related to habitability performance of structures (Ohkuma 1991, 1996; Denoon et al. 1999) and validation of their design practice (Littler 1991; Li et al. 2004), whereas most published full-scale damping observations under service conditions are derived from midrise buildings, associated largely with the well-known Japanese database (Satake et al. 2003). For this reason, the authors initiated the Chicago Full-Scale Monitoring Program in 2002 to 1 Associate Professor, Dept. of Civil and Environmental Engineering and Earth Sciences, Univ. of Notre Dame; Notre Dame, IN 46556 (correspond- ing author). E-mail: [email protected] 2 Research Assistant Professor, Dept. of Civil and Environmental Engineering and Earth Sciences, Univ. of Notre Dame; Notre Dame, IN 46556. 3 Professor, Dept. of Civil and Environmental Engineering and Earth Sciences, Univ. of Notre Dame; Notre Dame, IN 46556. 4 Structural Engineer, Magnusson Klemencic Associates, 1301 5th Ave., Suite 3200, Seattle, WA 98101; formerly, Graduate Student, Dept. of Civil Engineering and Geological Sciences, Univ. of Notre Dame; Notre Dame, IN 46556. 5 Graduate Student, Dept. of Civil and Environmental Engineering and Earth Sciences, Univ. of Notre Dame; Notre Dame, IN 46556. 6 Graduate Student, Dept. of Civil and Environmental Engineering and Earth Sciences, Univ. of Notre Dame; Notre Dame, IN 46556. 7 Executive Vice President, Highrise and Complex Building Division, Samsung C&T Corp., 12th Floor, Samsung C&T Corp. Building 1321-20, Seocho2-Dong, Seocho-Gu Seoul, Korea 137-965. Note. This manuscript was submitted on April 18, 2011; approved on December 29, 2011; published online on January 2, 2012. Discussion period open until March 1, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Structural Engi- neering, Vol. 139, No. 10, October 1, 2013. ©ASCE, ISSN 0733-9445/ 2013/10-16751687/$25.00. JOURNAL OF STRUCTURAL ENGINEERING © ASCE / OCTOBER 2013 / 1675 J. Struct. Eng. 2013.139:1675-1687. Downloaded from ascelibrary.org by UNIVERSITY OF NOTRE DAME on 10/08/13. Copyright ASCE. For personal use only; all rights reserved.

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Page 1: SmartSync: An Integrated Real-Time Structural Health Monitoring … · 2017-09-16 · tallest building, Burj Khalifa (formerly Burj Dubai), whose basic structural details will be

SmartSync: An Integrated Real-Time Structural HealthMonitoring and Structural Identification System

for Tall BuildingsTracy Kijewski-Correa, A.M.ASCE1; Dae Kun Kwon, M.ASCE2; Ahsan Kareem, D.M.ASCE3;

Audrey Bentz, M.ASCE4; Yanlin Guo5; Sarah Bobby, S.M.ASCE6;and Ahmad Abdelrazaq, A.M.ASCE7

Abstract: This study introduces a unique prototype system for structural health monitoring (SHM), SmartSync, which uses the building’sexisting Internet backbone as a system of virtual instrumentation cables to permit modular and largely plug-and-play deployments. Within thisframework, data streams from distributed heterogeneous sensors are pushed through network interfaces in real time and seamlesslysynchronized and aggregated by a centralized server, which performs basic data acquisition, event triggering, and database managementwhile also providing an interface for data visualization and analysis that can be securely accessed. The system enables a scalable approach tomonitoring tall and complex structures that can readily interface a variety of sensors and data formats (analog and digital) and can evenaccommodate variable sampling rates. This study overviews the SmartSync system, its installation/operation in theworld’s tallest building, BurjKhalifa, and proof-of-concept in triggering under dual excitations (wind and earthquake).DOI: 10.1061/(ASCE)ST.1943-541X.0000560.© 2013 American Society of Civil Engineers.

CE Database subject headings: High-rise buildings; Structural health monitoring; Wind loads; Earthquakes.

Author keywords: Tall buildings; Structural health monitoring; System identification.

Introduction

Long before the term structural health monitoring (SHM) wascoined, buildings were instrumented and monitored in situationswhere their performance was suspect, e.g., the John Hancock Towerin Boston (Durgin 1990). These unsavory origins made the conceptof proactive monitoring, particularly for buildings, quite foreign, asowners were naturally reluctant to install instrumentation systemsfor fear that this may tarnish the reputation of their buildings. With

growing need to assure life safety and rapid reoccupation in regionsof high seismicity, full-scale monitoring applications began toflourish in the western United States as part of coordinated strongmotion instrumentation programs. With such exposure, these earlyperceptions and attitudes have gradually changed with the adventand acceptance of smart building systems, which rely on in-strumentation to assure and optimize the performance of elevators,various mechanical systems, and, in some cases, auxiliary dampersinstalled to limit building movements. In addition to some ownersembracing the feedback from real-time monitoring, a number ofresearch efforts have sought to validate structural performance andvarious design principles in full scale, not only for earthquakes butalso wind events, as summarized in Kijewski-Correa and Cycon(2007).

One class of structure that is especially well suited to benefit fromcontinuous monitoring is tall buildings. These structures affect thesafety and comfort of a large number of people in both residentialand office environments, and as state-of-the-art structural analysissoftware and wind tunnel testing used in the design of these struc-tures are advancing rapidly, the accuracy and validity of their resultsneed to be calibrated with respect to actual performance. This be-comes particularly important to insure the economy and efficiencyof future designs with increased complexity and height generallygoverned by serviceability and habitability limit states under windthat are especially sensitive to the amount of inherent damping inthese systems. Unfortunately, only limited studies have pursued full-scale investigations related to habitability performance of structures(Ohkuma 1991, 1996; Denoon et al. 1999) and validation of theirdesign practice (Littler 1991; Li et al. 2004), whereasmost publishedfull-scale damping observations under service conditions are derivedfrom midrise buildings, associated largely with the well-knownJapanese database (Satake et al. 2003). For this reason, the authorsinitiated the Chicago Full-Scale Monitoring Program in 2002 to

1Associate Professor, Dept. of Civil and Environmental Engineering andEarth Sciences, Univ. of Notre Dame; Notre Dame, IN 46556 (correspond-ing author). E-mail: [email protected]

2Research Assistant Professor, Dept. of Civil and EnvironmentalEngineering and Earth Sciences, Univ. of Notre Dame; Notre Dame,IN 46556.

3Professor, Dept. of Civil and Environmental Engineering and EarthSciences, Univ. of Notre Dame; Notre Dame, IN 46556.

4Structural Engineer, Magnusson Klemencic Associates, 1301 5thAve., Suite 3200, Seattle, WA 98101; formerly, Graduate Student, Dept.of Civil Engineering and Geological Sciences, Univ. of Notre Dame;Notre Dame, IN 46556.

5Graduate Student, Dept. of Civil and Environmental Engineering andEarth Sciences, Univ. of Notre Dame; Notre Dame, IN 46556.

6Graduate Student, Dept. of Civil and Environmental Engineering andEarth Sciences, Univ. of Notre Dame; Notre Dame, IN 46556.

7Executive Vice President, Highrise and Complex Building Division,Samsung C&TCorp., 12th Floor, Samsung C&TCorp. Building 1321-20,Seocho2-Dong, Seocho-Gu Seoul, Korea 137-965.

Note. This manuscript was submitted on April 18, 2011; approved onDecember 29, 2011; published online on January 2, 2012. Discussion periodopen until March 1, 2014; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Structural Engi-neering, Vol. 139, No. 10, October 1, 2013. ©ASCE, ISSN 0733-9445/2013/10-1675–1687/$25.00.

JOURNAL OF STRUCTURAL ENGINEERING © ASCE / OCTOBER 2013 / 1675

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permit the response of three tall buildings in Chicago to be comparedagainst design predictions, including their levels of inherent damping(Kijewski-Correa et al. 2006a).

The SHM system historically used in many buildings, includingthe Chicago Full-Scale Monitoring Program, has been a wiredsystem, often termed hub and spoke because of the fact that thesensors are located throughout the structure and then wired toa central data acquisition unit (datalogger). This is a proven tech-nology that has been applied to awide range of structures, but suffersfrom two significant issues associated with its cables: (1) instrumentcables are very costly and difficult to deploy and maintain and (2)lengthy cables essentially serve as antennas, allowing noise to in-filtrate the system. In the aforementioned applications to tall build-ings, where response at the highest floor is generally the soleobserved quantity, the wired hub-and-spoke systems have proven tobe exceptionally reliable, e.g., have performed very well in nearlya decade of continuous monitoring in the aforementioned project inChicago (Kijewski-Correa et al. 2006a); however, because this typeof system originates from the small-scale laboratory setting, itbecomes increasingly less practical in full scale when sensors aredistributed over large, complex structures. Perhaps more impor-tantly, the use of cables inherently creates a rigid system that cannotbe easily redeployed or expanded.

Thanks to a number of technology advancements recognizingthese limitations and taking advantage of wireless communications,multihop radio or cellular relays have replaced cables in some recentapplications to ease installation, relocation, and maintenance bur-dens (Kijewski-Correa et al. 2008). However, the passage of largeamounts of data wirelessly increases the likelihood of packets beinglost or other radio frequencies interfering. Further, although theseradios may perform well in outdoor environments with line of sight,like bridges, communications within buildings have proven espe-cially challenging, particularly on mechanical floors where thesesensors are often placed. Moreover, the distributed nature of thesesystems requires some form of network synchronization.

In response to this need, this study presents a unique system theauthors termed SmartSync, which leverages the strengths of theaforementioned architectures while minimizing their limitations, assummarized in Fig. 1. The SmartSync system uses the building’s

existing Internet backbone as a system of virtual instrumentationcables. Given the reliability of modern local area networks (LANs),the issues of packet loss and synchronization are nullified, withoutthe need for lengthy instrumentation cables that increase cost andnoise. Furthermore, because this modular system is packaged to beplug-and-play, the units can be moved to any location with access topower and an Internet connection, removing an important tech-nology adoption barrier to put SHMhardware directly in the hands ofthose best suited to advocate for it (building owners and manage-ment). Within this framework, data streams from distributed sensorsare pushed through network interfaces in real time and seamlesslysynchronized and aggregated by a centralized server, which per-forms basic data acquisition, event triggering, and databasing whilealso providing a powerful interface for data visualization. Thisenables a completely modular and scalable approach to structuralhealthmonitoring and can readily interface awide variety of sensors,data formats (digital and analog), and even variable sampling rates.Furthermore, hardware costs are reduced because all acquisitionfunctions are centralized at the server, and only basic network in-terfacing is required at the location of each sensor. Also, the flex-ibility of this modular system and the fact that all of its configurationand triggering operations are software enabled uniquely allows thesystem to be used concurrently for wind or seismic monitoring, withdual triggering mechanisms (top down versus bottom up) designedto activate the sensor arrays in case of either hazard. This paper willpresent the hardware and software used to realize the SmartSyncconcept in the context of its prototype application in the world’stallest building, Burj Khalifa (formerly Burj Dubai), whose basicstructural details will be first presented.

Structural Overview

Burj Khalifa, located in Dubai, United Arab Emirates, and knownformerly as Burj Dubai, was completed in 2010 and crowned theworld’s tallest building, measuring 828 m in height (Abdelrazaq2010). The primary structural system, as described in Baker et al.(2007), is a 606-m buttressed RC core topped by a.200-m di-agonally braced steel structure supporting the tower’s spire/pinnacle. The building’s Y-shaped plan consists of three wings,

Fig. 1. Summary and assessment of three architectures used in monitoring civil structures

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each connected to the central hexagonal core to achieve this buttresseffect. Walls extend from the central core through these wings toform hammer head walls that provide lateral resistance and createa torsionally stiff structure. In addition, outriggers connect the in-terior concrete core to the perimeter columns in each wing at themechanical levels to decrease the overturning moment at the base.

As further discussed in Baker et al. (2007), extensive iterativewind tunnel testing was used in the design stage to achieve anaerodynamic profile that reduced the wind loads significantly. Thisincluded a setback scheme in a spiraling pattern to not only ef-fectively modify the structural cross section with height in an effortto disorganize vortex shedding, but also achieve a desired aestheticeffect. This was enabled by the choice of structural system thatprovides a seamless means to transition to these reduced floor plateswhile maintaining a continuous load path to the foundation. Fig. 2shows the site, including the main tower with its three wings, as wellas pool and office annexes. The inset image provides further detail ofthe main tower’s cross section and its variation with height accord-ing to the continuous setback scheme, as well as the orientation ofprimary structural axes.

Given the unprecedented nature of this project, a real-timemonitoring program capable of integrating measurements from avariety of sensors was requested by the owners, engineers, and con-struction team (Abdelrazaq 2010). Because the monitoring programwas to be initiated during the later stages of construction and then beextended into a permanent systemon commissioning of the building,a robust and rugged yet flexible and scalable system was requiredthat would allow units to be deployed, redeployed, and augmentedwith ease tomeet evolvingmonitoring needs. The following sectionsdiscuss the SmartSync modular hardware and software that weredeveloped and deployed on this structure in two stages in response tothis need.

Hardware and System Architecture

In SmartSync, sensor options are presented in a modular format thatpermits the customization of a monitoring package uniquely suitedfor the target application, whichwas particularly important given theevolving needs of building ownership from construction-stageto permanent monitoring in the case of Burj Khalifa. Each sensormodule is housed in a ventilated, National Electrical Manufac-turers Association (NEMA)-certified enclosure, which can be wallmounted at any location with access to building power and Internet.Within each enclosure, there are various supporting electronics,

including uninterruptible power supplies (UPSs) and remote powerregulators to provide control of power and reboot functions in eventof a system failure to mitigate the need for on-site interventionsfollowing installation [Figs. 3(a–d, and f and g)]. The followingsections discuss a number of sensing modules developed in thisresearch that use sensors previously vetted in other tall buildingmonitoring programs (Kijewski-Correa et al. 2006a, b) and discusstheir physical deployment on Burj Khalifa.

Acceleration Module

Basic dynamic response measurements are achieved using an an-alog accelerometer module [Figs. 3(a and b)] featuring low-noise,force-balance analog accelerometers from Columbia ResearchLaboratories. These devices are highly sensitive to even theslightest motions, which are ideal for capturing the low-amplitude,low-frequency responses of a tall building. In the case of a tor-sionally stiff structure like Burj Khalifa, measurements are generallyfocused on the two lateral directions, and the biaxial sensors aremounted within the enclosure and installed in the service core,insuring accessibility for maintenance and installation, with thepower cord and network line being the only external cabling. Toprovide a stable interface for long-term monitoring in especiallyharsh desert environments, in lieu of a lower-cost data acquisitionover LAN interface, the accelerometers and their supporting pro-grammable antialiasing filters interface with a Campbell ScientificDatalogger that samples data at 20 Hz. The acceleration module issuitable not only for monitoring the response of the structure, but byincorporating the triaxial version of these sensors, can serve asa seismograph to detect potential ground motions.

Displacement Module

The displacement response of any structure can be characterized bythree components: mean, background (quasi-static), and resonant.When monitoring the wind-induced response of structures, all threecomponents are present and quite important. In fact, studies haveshown that the background response contributions can be as high as20–80% for some structures in certain wind events (Williams andKareem 2003). The only way to recover these components is bydirectly measuring displacements in full scale, a task that proveddifficult until the advancement of global positioning systems(GPSs). This is an important addition to any monitoring programfor tall flexible structures that exhibit significant mean and quasi-static deflections under winds, which cannot be recovered by

Fig. 2. Site plan of Burj Khalifa with inset detail of cross-sectional shape setting back with height

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accelerometers alone (Kijewski-Correa et al. 2006b). It should benoted that GPS is not an off-the-shelf technology, as often perceived.In fact, the continuous variation in satellite visibility and orientation,as well as the potential for multipath distortions, requires consid-erable signal processing and compensation to achieve consistentlyreliable measurements (Kijewski-Correa and Kochly 2007). For thismodule, the Leica AT504 GG choke ring GPS antenna [Fig. 3(e)]must be placed external to the enclosure and connected by a shortlength of coaxial cabling with in-line surge protection to the LeicaGRX1200 GG receiver within the displacement module enclosure[Figs. 3(c and d)], acquiring data at 10 Hz. Additionally, the need toproduce high accuracy displacement measurements (subcentimeter

accuracy) requires a second displacement module to be installed ona stationary nearby structure (termed the reference site). Measure-ments taken at this location as well as on the primary structure beingmonitored are processed against one another on a dedicated GPSserver to reduce atmospheric interferences that degrade the accuracyof GPS displacements. The corrected displacements are thenstreamed from this server at 10 Hz in near real time.

Meteorology Module

This module offers a direct measurement of on-site weather con-ditions, with wind speed and direction being the most relevant to

Fig. 3. Accelerometer module (a) before and (b) after UPS installation; displacement module (c) before and (d) after UPS installation with(e) externally installedGPS antenna; meteorological module (f) before and (g) after UPS installationwith (h) externally installedmeteorological station,as deployed in Burj Khalifa

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evaluate dynamic responses of tall buildings against wind tunnelpredictions. In this case, the sensor again must be external to itselectronics enclosure, requiring a relatively short cable runwith in-linelightning protection. The instrumentation should have sufficient du-rability to withstand the harsh extreme winds; thus, it is desirable forthe anemometer to be as compact as possible with no moving parts,whichmakes ultrasonic devices like the VaisalaWXT520Met Station[Fig. 3(h)] a viable option even in tenant-occupied areas. This digitaltechnology streams data, including temperature, humidity, andbarometric pressure, at a rate of 1 Hz through the same CampbellScientificDatalogger [Figs. 3(f andg)] used in the accelerationmodule.

Construction Stage Module Deployment

Thefirst SmartSync prototypewas deployed inBurjKhalifa during thefinal stages of its construction in July 2008. As discussed in Abdel-razaq (2011), this system included one analog accelerometer module,a pair of GPS modules (reference and rover), and a meteorologicalmodule to monitor the structure’s responses and environmental

conditions at Level 138. In addition, a digital accelerometer modulewas included at the prototype stage for the purposes of validating thestability and sensitivity of a Summit Industries digital sensor withdirect-LAN interfacing capabilities. Fig. 4 shows the GPS antennaand meteorological station on the balcony at Level 138 of the tower,aswell as the internally installed enclosures anchored to the corewallat that level. Because of the negligible torsional response anticipated,the biaxial accelerometers are installed only at the core to monitorlateral response in the building’s two primary axes, denoted pre-viously in Fig. 2.At this stage, the referenceGPSwas installed on theroof of the adjacent pool annex, also shown as deployed in Fig. 4.

Permanent Module Deployment

This system was later expanded in the summer of 2010, with re-location of these units (with the exception of the prototype digitalaccelerometer) and addition of other hardware as part of thepermanent building movement monitoring system (BMMS)(Abdelrazaq 2010). As further described in Abdelrazaq (2011), this

Fig. 4. Construction stage SmartSync deployment in Burj Khalifa at single level defined in Abdelrazaq (2011); note: object linking and embeddingfor process control (OPC) is used for communication between the data loggers and the server

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Fig. 5. Screenshots of user selection of real-time data display (inset) and resulting interface to view (a) real-time data streams and (b) power spectraldensities of accelerations based on current buffered data at multiple levels defined in Abdelrazaq (2011)

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expansion was executed in partnership with Cermak PeterkaPetersen (CPP), Inc. and included acceleration monitoring at sixpoints along the height of the tower: Levels 73, 123, 155 and 160M2,Tier 23A, and the top of the Pinnacle—all focused near the buildingcenterline to capture only lateral responses in the primary axes of thestructure and including displacement andmeteorologicalmonitoringon the balcony of Level 160M3. These locations represent readilyaccessible levels that still had significance for the dynamics of thestructure or mark transition points between various aspects of thelateral system. In addition, triaxial accelerations are monitored ineach of the wings at the base of the structure. The GPS referencestation was also relocated to the roof of the Office Annex as part ofthis final phase of deployment.

Software-Enabled Data Delivery and Management

The realization of a LAN-based architecture significantly dependson the software development operating on the system’s centralizedserver housed within the building being monitored. In the case ofBurj Khalifa, the adoption of a software-enabled data delivery andmanagement system was essential to not only effectively integratedistributed, heterogeneous sensors belonging to the authors but also

to effectively integrate the uniquely formatted data streaming froma completely different set of hardware installed by theCPP team. Theserver is responsible for coordinating five essential functions: (1)data receipt and processing, (2) statistical analysis and archiving, (3)triggering and archiving, (4) real-time display, and (5) online dataanalysis, with the robustness to integrate both analog and digitalsensing elements with variable data formats and sampling rates, toprovide an efficient means to archive and retrieve the large volumesof data anticipated over the life of the deployment, and to supportreal-time, secure remote access, and visualization of data streams.The extent to which these processes, as well as basic data curationand analyses, could be automated to minimize human interventionswas an equally important design parameter.

All data acquisition operations in SmartSync are programmed inLabVIEW at the central server, whereas the higher-level processingand analyses are facilitated byweb-based onlinemodules running onan additional analysis server with results pushed to web-accessibleuser interfaces that are streamlined using the authors’ e-technologyscheme (Kwon et al. 2008). This allows an automated system thatcollects data in various formats, transforms all voltages to physicalengineering units, adjusts all inclined sensors to yield results on theprimary building axes (shown previously in Fig. 2), and correctsfor any bias or drift in the sensor outputs. In the case of GPS

Fig. 6. Screenshots of user selection of real-time statistics (inset) and resulting interface to view the latest 10-min statistics from sensors at multiplelevels defined in Abdelrazaq (2011)

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measurements, this includes removal of the GPS baseline position aslocked down by international positioning services using multipleregional reference stations. One challenge that arises when usingdistributed sensormodules, similar to the situation inwireless sensornetworks, is the challenge of synchronization. Each module hasa different clock with its own accuracy, and the precision of theseclocks would make strict synchronization across devices a chal-lenging and ongoing concern. SmartSyncmakes this issue irrelevantby not relying on the clocks and time stamps at individual devices butinstead ties everything to the clock of the server.When the LabVIEWcode executes a call for sampling data, it is collected simultaneouslyacross all the distributed devices. All data captured at the time of thisrequest are thereby tied to the same time stamp, that of the centralserver itself. Certainly there exists some potential network latency,usually less than 1 ms in a reliable LAN environment, but this hasbeen observed to affect more the data delivery to the server and notthe call to transmit data. Because the unified time stamping occurs atthemomentwhen data delivery is requested across the network, evenif the data are slightly delayed in its arrival at the sever, this is of noconsequence. To insure reliability of this data delivery, transmission

control protocol/internet protocol (TCP/IP) is invoked as a stablestream delivery service that guarantees transmission of data sentfrom one host to another without duplication or loss, ideally suitedfor data from variable sensors coordinated over LAN. It is noted thatthe Nagle algorithm is used by default in TCP communications inmost operating systems, such as Windows, as one of the basicalgorithms that helps reduce internet traffic by combining severalsmall packets into a single large one. However, the algorithm canintroduce some latency for situations where small amounts of in-formation are being sent back and forth, such as in instrumenthandshaking. Accordingly, in SmartSync, the Nagle algorithm isforcibly turned off to remove a potential TCP delay, which can becontrolled in the LabVIEW code.

As the data are received from each module, at variable samplingrates, buffers accumulate the data over predefined intervals (10 min)for real-time dissemination to authorized users, and statistics fromall sensing modules are stored in terms of a database managementsystem (DBMS), e.g., MySQL (MySQL AB). Even though largevolumes of data storage are relatively inexpensive nowadays, forlong-term monitoring programs such archiving of statistics, proves

Fig. 7. Screenshot of user selection of archived statistics (inset) at multiple levels described in Abdelrazaq (2011) and resulting interface to querystatistical archive by date; pop-up inset depicts calendar date selection tool

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to be far more efficient than continuously storing time histories thatoften are nothing more than low-level ambient vibration and thenhaving the burden of mining those massive stores of data later.

However, detailed analysis must be conducted using data fromsignificant events. For this reason, a server-level triggering meth-odology is incorporated using a moving windows analysis of thebuffered data to determine whether the response has exceededa predefined threshold at a master module in the network, in whichcase continuous time histories would be archived synchronously forall the sensing modules. For wind events, this master module wouldgenerally be the accelerometer at the uppermost floor, whereas inseismic events, the accelerometers at the ground level or free fieldwould be queried. It should be emphasized that because triggeringis handled by the server, which is based only on the response ofa particular master accelerometer module and not designed a prioriinto the hardware, the system can be easily expanded and readilytriggered for either excitation source, as the trigger level and evendata stream used as the master feed for triggering can be regulatedwith ease, including reducing it to zero to create continuous re-cording of data. Both the running 10-min statistics and triggeredtime histories collected by the central server are also backed up dailyon the off-site server via file transfer protocol to mitigate potentialdata losses caused by a system failure.

In addition to data acquisition and triggering, SmartSync alsosupports various data visualization and analysis features througha secure web portal: real-time modules such as streaming data

visualization and the latest 10-min statistical display, and on-demand modules such as triggered data archive, daily statisticalarchive query, and spectral analysis. By selecting the real-time datastream option (Fig. 5), authorized users are capable of viewing datain two formats. The first tab accesses the time histories [Fig. 5(a)],with the right two panels providing real-time feeds of accelerationsand displacements at the highest levels of the building as a functionof time or plotted spatially in the x,y-plane. The left top panel of theinterface has dials to display the real-time wind speed, direction,humidity, and temperature, whereas the bottom left panel displays thestatistics of the acceleration anddisplacement responses.The second taballows users to view the latest buffered 10-min data as a power spectraldensity at multiple levels [Fig. 5(b)]. On the other hand, selecting thereal-time 10-min statistics interface (Fig. 6) outputs a quick summary ofthe most current wind field and response statistics at all locations in theinstrumented building, including the three wings at the base of thestructure. Similarly, the daily statistics can be accessed (Fig. 7) ondemandsuch that thevalues (maximum,minimum,SD)at each level aremined from the archival statistics, using a calendar tool to select the dateof interest. In addition, triggered data sets can be viewed and down-loaded by authorized users through a web library as shown in Fig. 8.

In addition to the basic spectral information available from10-min records in real time [Fig. 5(b)], a higher-quality spectralanalysis can be requested by selecting a triggered 1-h time history.This results in a multitab web display [Fig. 9(a)], whose first tabshows the overview of all upper level accelerometers with the time

Fig. 8. Screenshots of user selection of archived triggered data (inset) and resulting interface to show lists of triggered data to date

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histories and the power spectra for the selected triggered data. Sub-sequent tabs allow users to select a specific location along thebuilding height and view the time history, corresponding powerspectra, and basic system identification (natural frequency anddamping estimates) for each accelerometer from a triggered event

[Fig. 9(b)]. The final tab on the interface provides wind velocitytime histories and wind roses for the selected event [Fig. 9(c)].

To automate the basic system identification provided by thismodule, a reliable method to identify the peaks corresponding to theactual modes of the structure was required. Because of the variance

Fig. 9.Screenshots of user selection of on-demand spectral analysis (inset): (a) user interface to select a triggeredfile to analyze; (b) example of selectedaccelerometer time history, power spectral density, estimated properties; (c) wind speed and direction

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of power spectra from limited amounts of data, more than a hundredpeaks (local maxima) can be identified in a given spectrum, many ofwhich are a cluster of several local maxima. Particularly in the caseof Burj Khalifa, this phenomenon is evenmore complicated becauseof the number of closely spacedmodes in the structure. Therefore, analgorithm was developed that first identifies all local maxima in thespectrum [Fig. 10(a)] and then executes the average linkage method(Morse 1980) to classify all potential clusters among the identifiedpeaks [Fig. 10(b)]. To then determine whether the clustered spectralvalues are physicallymeaningful, a repeatability check is executed toverify that an identified cluster frequency recurs in other spectra inthe array,which onewould expect for any physicalmodes. The resultof this process causes all nonrecurring spectral values within a givencluster to be averaged together.

Even after this process, some spurious peaks with nonnegligibleamplitude persist and are then eliminated by a subsequent thresh-olding operation based on the statistics of the peak distribution overa moving window of 0.1 Hz, to adjust the threshold accordingly forthe relative participation of various modes in the spectrum. In somecases, coupled modes will display a pair of peaks with one havingrelatively larger amplitude in a given axis of the building. Thecompanion mode (with smaller spectral value in that direction) maybe eliminated during the thresholding process. Thus, a final re-peatability check is conducted to reinstate any of these companionpeaks in coupled modes that were erroneously removed in thethresholding process. The end result, shown in Fig. 10(c), is dis-played in the user interface in Fig. 9(b). Each of the identifiedclustered peaks is used to estimate of the frequency and damping ofthat mode using the half-power bandwidth approach for simplicity,although any other approach can be implemented in the future. This

result is displayed on the right side of Fig. 9(b). Even with thesemeasures, the level of variance in the spectra generated from a singlehour of ambient vibration data are far too significant to be used forany reliable system identification, as only one raw low-bias spectracan be generated from an hour of data for this very flexible structure.Accordingly, more accurate data processing requires the use ofa series of triggered time histories (Kijewski-Correa et al. 2006a),downloaded via the triggered data archive as shown previously inFig. 8.

Proof of Concept

SmartSync has been deployed on Burj Khalifa since late 2008.Given its unique software-enabled triggering functions, a verifica-tion of the system’s ability to effectively trigger under eitherearthquake or wind excitations is now presented as proof of concept.Since 2008, a number of earthquakes have originated from southernIran and sufficiently excited the building to trigger the system, oneof those being the September 10, 2008, magnitude 6.1 earthquakethat struck the region of Bandar Abbas, approximately 1,368 km(850mi) south of Tehran. Fig. 11 displays the acceleration responsesalong the two primary axes of the structure as measured by theconstruction stage accelerometer module. The variations of themodal participation with time are explored using the waveletanalysis framework discussed in Kijewski and Kareem (2003) andconfirmed the participation of approximately five modes in each ofthe response axes. The results affirm the dominance and persistenceof the fundamental mode in the x-axis responses, with intermittentcontent at higher frequencies during the strongest shaking between250 and 350 s. Two of these contributions correspond to the pre-dicted third and fifth sway modes in that direction. The y-directionresponse does not show the same dominance of the fundamentalmode, with the most significant energy again being associated withthe frequency of 0.6 Hz, although for only about 1 min. Two of thehigher modes participating correspond to the predicted second andfourth sway modes in that direction. Fig. 11 also shows the resonantcomponent of the GPS displacements during this event, dominatedby the fundamental modes, reasonably agreeing with the waveletanalyses of the accelerations during this event. Additional systemidentification results verified that the observed frequencies werewithin 2–3% of finite-element model predictions (Abdelrazaq 2011).

The first substantial triggers of the system underwind occurred inMarch 2009; a sequence of triggered records from March 30, 2009,is provided in Fig. 12. This figure displays the wind speed anddirection over a 24-h period. Inset images provide snapshots of thetriggered building responses correlated to their time of occurrence onthe wind speed plot. The peak accelerations associated with eachinset plot are reported in the figures, as the axis scales can be hard toview given the image size. During the most sustained response, thewind speed intensifies to a mean value of 37 km=hð23mi=hÞ, pri-marily from the east-southeast. Airport records indicate that thun-derstorms were in the area at this time, which have been shown inother studies to produce noteworthy responses in similar structuralsystems (Bentz andKijewski-Correa 2011), particularly because thiswind direction is known to be one of the higher-impact angles fromwind tunnel testing of Burj Khalifa (Baker et al. 2007).

Concluding Remarks

This paper presented a modular structural health monitoring frame-work interlinked by SmartSync technology that enabled the flexi-bility to tailor a customized global monitoring system that can beexpanded and deployed with plug-and-play ease in the world’s

Fig. 10. Three-stage peak identification algorithm by clusteringanalysis: (a) all identified peaks; (b) all peaks identified by clusteranalysis; (c) identified dominant modes of vibration

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Fig. 11. Measured acceleration and displacement responses in September 10, 2008, earthquake

Fig. 12.Measured wind-induced accelerations on March 30, 2009: inset accelerations are shown indicating on the wind speed and direction plots thetimes at which they were triggered with peak accelerations reported for each axis; inset cross section of building indicates the angle of attack for thetriggered wind events and the primary axes against which responses are measured

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tallest building, Burj Khalifa. In particular, because the SmartSynccommunications infrastructure is local area network based, it canadapt easily to the evolving needs of building use without hardwareredesign and the need for lengthy cable runs, which would not onlybe costly but incredibly inefficient in tall and complex buildings.This proved particularly important in the application case studyherein, where monitoring both during construction and after com-missioning by a heterogeneous array of sensors was required. Thispaper overviewed three different hardware modules, as well as theautomated data acquisition, curation, and archiving functionalities ofthe system. Various web-based online modules were presented todemonstrate the range of data mining, postprocessing, and systemidentification capabilities available through a combination of real-time and on-demand services. Two proof-of-concept demonstrationsare provided to verify the ability of a software-enabled triggeringframework to effectively monitor both seismic and wind events fromdifferent master modules to offer a truly multihazard platform formonitoring of Burj Khalifa.

Acknowledgments

The authors acknowledge the financial support of the National Sci-ence Foundation Grant No. CMMI 06-01143 and the Samsung Cor-poration. The authors also thank former graduate students JenniferCycon and Jeffrey Loftus for their efforts on this project and CPPfor their collaboration on the permanent monitoring system.

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