Field deployment of a dense wireless sensor network for condition assessment of a bridge superstructure

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1 Feld deployment of a dense wreless sensor network for condton assessment of a brdge superstructure Mchael V. Gangone 1, Matthew J. Whelan 1, Kerop D. Janoyan 1, Ratneshwar Jha 2 1 Clarkson Unversty, Department of Cvl and Envronmental Engneerng 2 Clarkson Unversty, Department of Mechancal and Aeronautcal Engneerng 8 Clarkson Avenue Box 5710, Potsdam, NY Phone: (315) Fax: (315) {gangonmv, whelanmj, kerop, rjha}@clarkson.edu ABSTRACT Wth the ncreased demand placed on agng nfrastructure, there s great nterest n new condton assessment tools for brdges. The routne deteroraton that brdges undergo causes a loss n the ntended performance that, f undetected or unattended, can eventually lead to structural falure. Currently the prmary method of brdge condton assessment nvolves a qualtatve brdge nspecton routne based on vsual observatons. Dscussed n ths paper are methods of nstu quanttatve brdge condton assessment usng a dense wreless sensor array. At the core of the wreless system s an ntegrated network whch collects data from a varety of sensors n real-tme and provdes analyss, assessment and decson-makng tools. The advanced wreless sensor system, developed at Clarkson Unversty for dagnostc brdge montorng, provdes ndependent condtonng for both accelerometers and stran transducers wth hgh-rate wreless data transmsson n a large-scale sensor network. Results from a feld deployment of a dense wreless sensor network on a brdge located n New York State are presented. The feld deployment and testng ad to quantfy the current brdge response as well as demonstrate the ablty of the system to perform brdge montorng and condton assessment. Keywords: Brdge montorng, Stran, Wreless, Ambent, Integral abutment, Brdge management, Condton assessment, Load testng 1. INTRODUCTION Wth the level of demand so hgh on today s nfrastructure, there s a greater level of necessty for determnng the capacty and capabltes of a structure. It s qute common over tme for dfferent brdge components to deterorate, causng a loss n the ntended performance that can lead to falure. For example, structures that are overstressed beyond the desgn loadng, or envronments that cause corroson of steel both lead to fracturng and stffness reducton to the member. Pror to 1968, a mandate to requre state and local government agences to evaluate the condton of brdges was not n place. The Federal Hghway Admnstraton (FHWA) n 1968 developed a Natonal Brdge Inspecton Program after the collapse of the Slver Brdge over the Oho Rver kllng 46 n The level of performance for any structure can be measured n two dstnctvely dfferent ways, through vsual or technologcal methods. An estmated forty percent of the natons 650,000 hghway brdges are structurally defcent. An annual cost of approxmately seven bllon dollars s needed for rehabltaton or replacement of the defcent structures (Ratay, 2005). Ths suggests that the need for nspectons of brdges s necessary and methods to repar and locate the damage are also mportant. The most common method employed by many state transportaton agences n brdge evaluaton s through manual vsual nspecton. Brdge nspecton defned as an educated assessment and extrapolaton of the condton of a brdge (Phares et al., 2000), s genercally stated to mean that brdge assessment s a scence. Many problems arse from manual brdge nspecton because whle there are codes to follow, much of the evaluaton process s judgment based. Wth the advances of technology, the late twenteth century brought the begnnngs to mproved methods of condton assessment. Structural Health Montorng (SHM), once prmarly used n montorng mechancal and aerospace structures, has now been adapted to montor cvl nfrastructure. As a form of Non-Destructve Testng and Evaluaton (NDT&E), SHM allows for an accurate assessment wthout causng addtonal damage to the structure. As mentoned

2 prevously, much of the current level of nspecton of brdges s through local and vsual evaluaton methods. However, these methods are flawed as they are not always true measurements of the brdge performance capabltes. Subjectvty on the part of the nspecton team s one problem when t comes to brdge nspecton. Tme and access to the structure are generally the two addtonal problems when evaluatng brdges. Because not all components are easly accessble to the nspector, such as nternal elements, they cannot contrbute towards the ratng of the structure. Many tmes these elements, whether prmary or secondary, are crtcal to the overall performance level. Wth tme beng of the essence, the level of detal n gvng proper attenton to each and every element s often overlooked. In 2001 the Federal Hghway Admnstraton (FHWA) conducted a study whch found that 56 percent of medum to short span brdges gven an average condton ratng were mproperly assessed (Aktan et al., 2001). One such method to help n overcomng these problems s by mplementng a hghly accurate SHM system. Mantanng structural ntegrty s mportant and s aded by the use of a SHM system. A SHM system has the ablty to detect damage at any pont n tme due to ts real tme contnuous montorng and conssts of two dstnct parts, a network of sensors collectng measurements and software to nterpret the results. Ultmately, there are four goals to an effectve method: (1) Detect damage (2) Accurately locate damage (3) Identfy the magntude of damage (4) Produce a lfe-cycle assessment for the structure (Meadows, 2006). Researchers are workng to develop methods of structural montorng to allow the complete and accurate assessment of structural performance n a non-destructve manner. DeWolf et al., 2006 are currently montorng a seres of brdges wthn the state of Connectcut, ncludng a steel truss brdge usng wreless stran gages and accelerometer. Wreless sensors were chosen due to the long span lengths and dffculty nstrumentng a cabled system over the long span lengths. Lynch et al., 2006 deployed a wreless system on the Geumdang Brdge n South Korea. Test vehcles provded an nput loadng to the concrete box grder brdge where acceleraton measurements supply the modal characterstcs of the structure. Brdge Dagnostc Incorporated (BDI) (2008) has recently developed a wreless stran based system for brdge montorng. Whle varous research and commercal systems have been developed and mplemented, methodologes of detectng and locatng damage are of the utmost mportant to any health montorng system. Currently, gudelnes for brdge nspecton, repar and replacement are covered under the brdge management system (BMS). However, snce most of the data used wth ths system s from vsual nspectons, ncorporatng sensor technology n addton to the current nspecton methods s hghly desrable. Ratonal brdge deteroraton rate nformaton based on measured performance response data would provde a coherent bass for condton assessment of brdges. The measured performance response, such as from brdge load testng, relays nformaton as to the behavor of the brdge whch can be compared to testng prevously conducted. The remander of ths paper dscusses the use of a custom developed wreless sensor system (WSS) for determnng changes n structural performance. Data gathered from a feld deployment of a sngle span ntegral abutment brdge s presented to show the abltes of the system to obtan varous capacty and demand measurements of nterest to be used n the montorng process. 2. CUSTOM DEVELOPED WIRELESS SENSOR SYSTEM Performance montorng methods are crtcal n assessng the functonalty and condton of a structure. Wth the rsng demand for technologes to mprove the accuracy of structural evaluatons, newly devsed methods to effcently and effectvely montor the structure performance s sought. A Wreless Sensor System (WSS) whch ncludes a dual axs accelerometer, stran transducer, and a custom condtonng board has been developed n the Laboratory for Intellgent Infrastructure and Transportaton Technologes (LIITT) at Clarkson Unversty. An accelerometer and stran transducer connected to a sngle condtonng board comprse one unt (node) that attaches to a mote, sendng the sgnal wrelessly n packets to another mote connected to a CPU where the data s collected and processed by a custom software platform. A total of twenty wreless unts were developed and used n the montorng. All unts are transmtted smultaneously n real tme to a sngle mote whch s externally connected to the CPU. For ths partcular paper, only stran measurements from the wreless system usng Brdge Dagnostc Incorporated (BDI) stran transducers (as typcally used for load ratng purposes) were consdered. Fgure 1 shows an mage of the wreless system wth a dual axs accelerometer, stran transducer and the custom desgned system hardware. For more nformaton on the developed wreless system can be found n Whelan et al., 2007.

3 Fg. 1. Custom wreless sensor node wth stran transducer and dual-axs accelerometer (Whelan, et al., 2007). 3.1 Qualtatve nspecton methods 3. BRIDGE CONDITION ASSESSMENT Natonal Brdge Inspecton Standards (NBIS) mplemented by the FHWA n 1968, drect the frequency and personal requred for brdge nspectons. Currently a brdge s requred to undergo routne nspecton at a mnmum of once every two years. NBIS uses a condton ratng for each component on a scale from zero to nne, where zero s faled and nne s excellent. However, many states, such as New York, adopt ther own ratng scale whch s approved by FHWA. Each component of the structure s evaluated usng vsual nspecton technques and assgned a number ratng based on the condton. A weghted method (equaton 1) ncorporatng the lowest ratng for each R = R W W component type (R ) (.e. grders, daphragms, brdge deck, bearngs, etc.) and a correspondng weght (W ) s used by New York State Department of Transportaton (NYSDOT), provdng a condton number of the structure (Ratay, 2005). Usng ths nformaton, the condton number s tracked over tme where extrapolatons and probablstc measures are ncorporated to predct the future condton of the structure. It s mportant to note that a threshold condton ratng s set whch requres acton be taken on the part of the brdge owner to rehabltate or replace the structure. Ths method s a valuable tool n predctng the remanng lfe expectancy of the brdge. NBIS ncorporates n slghtly dfferent approach usng a suffcency ratng whch apples to all brdges n the natonal hghway system. Structural adequacy and safety, servceablty and functonal obsolescence, essentally for publc use, and specal reductons are the four factors that comprse the suffcency ratng (Ratay, 2005). Nonetheless, condton assessments based on vsual nspecton methods contan levels of subjectvty and lack of accessblty n some cases to parts of the structure. Furthermore, t s dffcult to measure the true behavor of the structure usng ths approach. Therefore, ncorporatng sensor technology wth ths process would strengthen the overall ratng system. 3.2 Quanttatve evaluaton methods Feld measurements of n-servce loadng condtons are of utmost mportance. Load ratng provdes the means for measurng a superstructures load capacty. Load demand and capacty are the bass for both ratng and desgn. Mantanng a capacty greater than the load demand s needed to prevent structural collapse. Wth much of the loadng for today s brdges rangng from lght weght vehcles to large tractor traler trucks, ncorporatng safety factors are mportant, partcularly when an unknown loadng condtons beyond the allowable capacty of the structure are possble. Ratng factors are smlar as the rato of the capacty to the lve load demand defnes a safe level of loadng. In any case, equaton 2 must be followed where Φ (factor of safety) s greater than or equal to 1. Preferably, due to the probablstc (1)

4 nature of the loadng and structural deteroraton, a factor of safety greater than 1 s recommended. Methodology mentoned from the feld deployment below, can be used n obtanng measurements and brdge response parameters related to the capacty of the structure. Furthermore, employng addtonal methods wll allow for the demand to be captured. capacty φ demand (2) 3.3 Feld deployment The ntegral-abutment brdge (shown n Fgure 2), constructed n 2004, s m (56 ft) n length and comprsed of a mm (9.5 n) renforced concrete slab on 4-W36x135 steel grders at 2.74 m (9 ft) center-to-center spacng. Two equally spaced C15x33.9 sectons serve as ntermedate daphragms between each grder n addton to MC8x20 end daphragms. For the testng performed n ths study, an approprate loadng vehcle was not avalable and consequently Fg. 2. Sngle-span ntegral abutment test brdge. the relatve magntudes of the measured strans were low. To excte the structure, partcularly for stran response, passenger vehcle and lght truck were used. Snce ths test was completed under permt but ndependent of county assstance or supervson, controlled loadng ndependent from the daly traffc was not feasble. Therefore, random load scenaros wth one to two vehcles were used and ther exact locatons unknown. A more accurate assessment would ncorporate a controlled loadng plan wth test vehcles near or at the servce desgn level smlar to load testng protocols employed by state agences. Stran transducers were placed on the top and bottom flanges along an nteror grder at the md-span and both abutment ends. A sngle transducer was also placed at the md-span of the bottom flange on the remanng three grders for measurng the grder dstrbuton factors. The complete sensor layout along wth an mage of a sngle deployed node contanng an accelerometer and stran transducer can be seen n Fgure 3. Crcled are the locatons along grder 3 where both top and bottom stran transducers were placed to measure neutral axs locaton, secton modulus and bendng moment.

5 (a) Accelerometer Wreless Sensor Node Stran Transducer (b) Fg. 3. (a) Sensor locaton of brdge deployment, (b) Sngle nstrumented node (ncludes accelerometer that s not used for ths paper). 3.4 Feld measurements Capacty Stffness measurements are related to varous structural parameters found as a result of ths feld deployment. Neutral axs locaton, secton modulus, and end fxty defne mportant behavoral characterstcs of the superstructure. As mentoned prevously, an ncrease n the neutral axs locaton and secton modulus ndcate an ncrease n rgdty on account of the stffness contrbuton from the slab to the prmary member (grder). A sgnfcant change n these parameters would lkely suggest a loss of composte acton between the grder and slab, or possbly the presence of Table 1. Neutral axs (NA) and secton modulus measurements along grder 3. Locaton NA (cm) [n] Secton Modulus (cm 3 ) [n 3 ] Md-span (97.07) [38.22] (12,224) [746] South End (105.1) [41.37] (3113) [190] North End (96.39) [37.95] (10,405) [635] Theoretcal (88.9) [35]

6 secton loss. Table 1 llustrates results captured durng the feld deployment. The neutral axs and correspondng secton modulus parameters were computed at each of the three locatons along grder 3 (shown n fgure 3a) from top and bottom stran measurements. As s typcal, plane secton analyss was assumed wth no shear lag effects present. The results ndcate strong composte acton among the slab and grder secton, suggestng greater stffness wthn the superstructure. Montorng the poston of the neutral axs or change n secton loss wth tme provdes a better evaluaton of the change n secton behavor whch may be a result of secton deteroraton. Furthermore, under the assumpton of plane secton analyss and no shear lag effects, the results are ndependent of the loadng condton and therefore can be measured on a contnuous bass wth a permanent montorng system. The presence of negatve strans (depcted n fgure 4) at the supports denotes the presence of rotatonal restrant, ncreasng structural stffness. The combnaton of the above parameters from the measured stran values facltates the determnaton of the structural capacty from the lve load (.e. lve load secton modulus). Comparson of these measurements upon varous testng tmes wll provde nsght nto the change n capacty wth the structures age. For multple contnuous span structures and ntegral abutment desgns, locatons around the pers (or abutments) that are subjected to hgh negatve moments can cause sgnfcant crackng wthn the concrete deck, potentally resultng n reduced structural stffness. Therefore, montorng these locatons s necessary. Demand Fg. 4. Stran output at the north and south abutments of grder 3. Wth varyng loadng scenaros possble, demand levels fluctuate. Statc or sem-statc loadng (approxmately 8.04 km/h or 5 mph) generates the demand as well as capacty lmts from stran readngs. A dynamc allowance factor s appled to the statc moment demand to account for stresses experenced by the grders above statc condtons. Moment dstrbuton factors (DF) descrbe the demand experenced by each of the prmary members. The moment experenced on a partcular grder at a specfed locaton compared to the total moment for all grders at that locaton provdes the fracton of the total moment on that secton. Equaton 3 shows ths n symbolc form where S s the secton modulus, ε s the stran, and E s the elastc modulus of each grder. Most commonly, the mdspan locaton on each span s the most crtcal for montorng as t generally experences hgh moment demand. Fgure 5 s an llustraton of the load dstrbuton at the mdspan from a vehcle travelng between grders 1 and 2. In ths case, when compared to the theoretcal moment DF based on the Amercan Assocaton for State and Hghway Transportaton Offcals (AASHTO) Standard Hghway Brdge Specfcatons (2002) the actual measured values appeared lower. DF = S ε (3) S ε E E

7 As the load s appled to the structure, t s dstrbuted to the nearest prmary members then dstrbuted va the slab and daphragm connectons to the remanng structural elements. Ths would suggest that a shft n the moment dstrbuton factors under a set loadng condton would result from a possble change n the load transfer wthn the superstructure. Addtonally, a change n the dstrbuton factor over the grders could result from a change n secton modulus or end condtons (Hag-Elsaf, 2006). The consequence s a change n the stffness of the grder/slab secton, adjustng the load response. Furthermore, a change would lkely symbolze a loss n the load transfer to other prmary members, potentally as a result of a loose daphragm connecton or nternal damage wthn the slab. Fg. 5. Measured transverse moment dstrbuton factor plots at mdspan based on loadng on the left hand lane Long-term montorng Current methods of brdge evaluaton utlze vsual nspecton technques at a maxmum frequency of every two year. Combnng qualtatve (vsual) and quanttatve (sensor) measurements offer an mproved condton assessment. Upon completon of a brdge nspecton, condton ratngs are awarded on a scale of acceptablty and tracked over the lfe of the structure. A smlar approach wth the above mentoned capacty and demand measurements should be consdered. Wth the ease of nstallaton of the sensor unts, placement n crtcal locatons of the superstructure would ad n provdng a deteroraton model for the structure based on actual quantfable nformaton. Data from Fgure 4 llustrate the ablty to capture the data wrelessly for processng. Ths produces, among others, results show n Table 1 as well as Fgure 5. As the structure deterorates a change n the capacty levels are expected. Calbratng the stran measurements to vehcle weght or usng a wegh n moton (WIM) system s suffcent to measure the level of demand. These performance characterstcs can also be appled to determnng the safety factor, or ratng factor of the brdge. Wth the capacty and demand measurements captured by the wreless system, an overall ratng factor can be establshed. Allowable stress and Load Resstance Factor Ratng (LRFR) are the two current methods for load ratng. LRFR s becomng ever more popular however due to the probablstc nature of the varous potental loadng states. The ratng factor (RF) usng LRFR s gven by Equaton 4 (AASHTO, 2003) where C s the capacty, DC s the dead-load effect due to structural components and attachments, DW s the dead-load effect due to wearng surface and utltes, P s the permanent loads other than dead loads, LL s the lve load effect, IM s the dynamc load allowance and γ DC, γ DW, γ P, and γ L are the correspondng LRFD factors wth ther respectve effect component. The numerator of equaton 4 represents the avalable capacty of the structure, whle the denomnator s the demand. The capacty changes over tme due to RF = C γ DC DC γ γ ( LL + IM ) L DW DW ± γ P p (4) deteroraton. Demand levels also change wth socetes needs, as larger vehcles are travelng the roads and demand level ncreases creatng further stress to the structure. As the ratng factor declnes beyond a certan level, rehabltaton or constructon of a replacement brdge may become warranted. The dea of trackng the safety factor over tme s presented by Wenzel et al., 2005 n Fgure 6. As the state of the brdge reaches a lmt pont, acton by the brdge owner s requred. When comparng ths model to the deteroraton model based on condton ratngs from qualtatve

8 measurements, a smlar trend can be seen. Combnaton of both methods would ultmately mprove the current state of brdge assessment and potentally ad n preventng future brdge collapses Baselne Safety Aganst Falure HEALTHY INTERVENTION Early Warnng Falure Immnent 0.5 FAILURE 0 Tme Fg. 6. Brdge deteroraton model (Wenzel et al., 2005) 4. WIRED VERSUS A WIRELESS SYSTEM Cost s a bg attracton to the use of wreless sensors for brdge montorng. Such cost savngs come n the form of reducng the cablng requred for the sensor as well as the tme requred for deployng a system. Furthermore, the smplcty of sensor nstallaton and the flexblty to place the sensors n any desred arrangement or locaton allows for smpler and proper montorng. Long spannng superstructures can be dffcult to nstrument wth cabled systems as cables are often not long enough or requre extra long lengths to tether to a computer system. From a load testng stand pont, wth the tme savngs of nstallaton alone, one can argue that more tme wll become avalable for montorng addtonal brdges and structures that orgnally had a hgh degree of dffculty to montor and nstrument. Nonetheless, the cost savngs (n ts many forms) s one of the bggest attractons to such a system as mentoned n the paper. Intal skeptcsm of a wreless sensor system was n ts ablty to transfer sgnfcant amounts of data across a network, partcularly at hgher samplng frequences. The real tme transmsson shows the capablty of ths system to be used n long term feld deployments as well as for load testng purposes. The ablty of the real tme data transmsson from ths system allows for contnuous montorng and would be benefcal n knowng the change n structural response at the tme t occurs so any necessary acton can be taken by the approprate agency. Furthermore, for short term feld deployments, such as load testng, montorng the strans nduced to the superstructure n real tme are mportant for determne the safe level of loadng. Ths s of partcular nterest for both dagnostc and/or proof testng of an n-servce deterorated superstructure. For the mentoned feld deployment, 40 channels of both stran and acceleraton were measured wth 100 percent of the data was transmtted successfully thru a sngle network. 5. CONCLUSION Methods for mprovng condton assessment of brdges by ncorporatng quanttatve measurements are of utmost mportance to due to the sgnfcant deteroraton that many of today s brdges exhbt. Dscussed n ths paper are methods for mprovng condton assessment from change n response of varous capacty and demand characterstcs of the superstructure. A full scale feld deployment of a wreless, low-cost and automatc system for structural health management and condton assessment of hghway structures was presented to demonstrate the ablty of obtanng the necessary behavor characterstcs. Stran measurements of a sngle span ntegral abutment composte steel and concrete brdge superstructure were obtaned n real tme wth the wreless system under varous loadng condtons. The results ndcated a stff response, wth capacty measurements from the neutral axs and secton modulus greatly nfluenced by the composte acton between the renforced concrete deck and steel grders. Transverse moment dstrbuton factors

9 ndcated load sheddng was occurrng to all grder members. Integratng such measurements nto brdge nspecton protocols and the brdge management system (BMS) wll ad n provdng a better condton assessment. ACKNOWLEDGEMENTS Ths research has been funded by the New York State Energy Research and Development Authorty (NYSERDA), n collaboraton wth the St. Lawrence Hghway Department, and the New York State Department of Transportaton (NYSDOT). The assstance of Dan Nyanjom and Kevn Cross wth the feld deployment and testng s greatly apprecated. Any opnons, fndngs, and conclusons or recommendatons expressed n ths paper are those of the authors and do not reflect the vews of the agences. REFERENCES Amercan Assocaton for State Hghway Transportaton Offcals (AASHTO)., [Standard specfcatons for hghway brdges], Washngton, DC. (2002). Amercan Assocaton for State Hghway Transportaton Offcals (AASHTO)., [Manual for condton evaluaton and load and resstance factor ratng (LRFR) of hghway brdges], Washngton, DC. (2003). Aktan, A.E., Pervzpour, M., Catbas,N., Grmmelsoman, K., Barrsh, R., Curts, J., Qun, X., Informaton technology research for health montorng of brdge system, London, UK, Structural faults &Repar. July 4-6. (2001). Brdge Dagnostc Incorporated (BDI)., Wreless Structural Testng System (STS-WF), World Wde Web. Onlne. Avalable at: Accessed: 7 January. (2008). DeWolf, J.T., D Attlo, P.F., Feldblum, E.G., and Lauzon, R.G., Brdge Montorng Network - Installaton and Operaton, Connectcut Department of Transportaton, Report No. CT-2217-F December. (2006). Hag-Elsaf, O, Kunn, J., Load testng for Brdge Ratng: Route 22 over Swamp Rver, New York State Department of Transportaton (NYSDOT), Specal Report, February. (2006) Lynch, J.P., Wang, Y., Loh, K.J., Y, J-H., Yun, C-B., Performance Montorng of the Geumdang Brdge Usng a Dense Network of Hgh-Resoluton Wreless Sensors, Insttute of Physcs Publshng, Smart Materals and Structures, IOP publshng, UK, 15(2006), October. Pgs (2006) Meadows, Jeffery., Evaluatng the success of structural health montorng methodologes for cvl nfrastructure, Honors Thess, Clarkson Unversty, Aprl. (2006) Phares, B.M., Rolander, D.D., Graybeal, B.A., Washer, G.A., Studyng the Relablty of Brdge Inspecton, Publc Roads, December. (2000) Ratay, Robert. T., [Structural Condton Assessment], John Wley & Sons, Hoboken, NJ. (2005) Wenzel, H and Pchler, D., [Ambent Vbraton Montorng] John Wley & Sons, Ltd. Hoboken, NJ. pg 18. (2005) Whelan, M.J., Fuchs, M.P., Gangone, M.V., and Janoyan, K.D., Development of a Wreless Brdge Montorng System for Condton Assessment usng Hybrd Technques, Proc. SPIE 6530, San Dego, Calforna. (2007)