The Research to Monitoring System of Wuhan Yangluo Yangtze River Bridge s Anchorage System

Transcription

1 The Research to Montorng System of Wuhan Yangluo Yangtze Rver rdge s Anchorage System LIU SHENG-CHUN 1 ASTRACT: It s vtal to montor the condton of suspenson brdge s anchorage and man cable, whch s the key to whole suspenson brdge. In ths paper, a nnovatve Load Cell based on fber ragg gratngs (FG) was researched and developed, and appled to measurng and montorng a brdge s anchorage system for a long term. Ths type of suspenson anchorage s the frst tme to utlze to practcal brdge engneerng system Wuhan Yangluo Yangtze Rver rdge n Wuhan Chna. That s the So-Called the non-cohesve and replacve prestressng anchorage characterzed by alternatve accordng to montorng the tenson state. KEY WORDS: Wuhan Yangluo Yangtze Rver rdge; prestressng anchorage; Load Cell (test-force rng) based on FG; long-term montorng 1 INTRODUCTION The anchorage s very mportant n all of the stress structure parts of suspenson brdge. Traffc load and self-weght of superstructure conducts through the man cables to the man towers and anchorage system, and then to the foundaton. Therefore, suspenson brdge s anchorage system of the man cables s vtal to whole suspenson brdge structure. The Wuhan Yangluo Yangtze Rver rdge s a 250m+1280m +440m rver-span project, whch wll be the crucal segment of both JngZhu and HuRong natonal hghway, and the northeast part of Wuhan cty crcle hghway [1]. At present, man cables anchorage system of suspenson brdge n Chna usually uses coherent antcorrosve prestressng system by pressng cement nto ppelne, whch leads to the structure permanent and un-exchangeable. Moreover, t s not easy to ensurethe constructon qualty of groutng. In order to ensure safety and durablty of anchorage system even the whole brdge servce lfe radcally, the desgner nnovatvely utlze non-cohesve and replacve prestressng anchorage system. It s acorrngly put 1 Lu Sheng-chun, Doctoral Student, Key Lab. of Fber Optc Sensng Technology and Informaton Processng (Wuhan Unversty of Technology), Mnstry of Educaton, Wuhan , CHINA. Emal: lusc@mal.whut.edu.cn

2 forward that a research subject about chroncally montorng prestressng of anchorage system, whch provdes warranty for replacng the prestressng system by montorng the tenson condton of suspenson brdge anchorage system for long term. It s consequently vtal to chroncally montor tenson of prestressng system relably. However, such conventonal methods of measurng tenson of prestress system as electrc resstance stran gauge, vbratng-strng load cell could only be used for short-term measurement. Therefore, the problem developng new-style load cell must be solved for mplementng long-term montorng of prestressng system characterzed by non-cohesve and replacve. Ths paper wll focus on the development and applcaton of load cell based Fber raag Gratng(FG). Ths type of sensng and measurng technology has a lot of remarkable advantages [3],such as tny capacty,good corroson resstance, ant-nterference to electromagnet, long-term relablty, hgh-accuracy dgtal transducng and dstrbuted measurng, whch have hugely contrbuted to cvl engneerng, and become mportant foundaton of developng the new-style load cell. In practcal applcaton, used experence of exstng vbratng-strng load cell for reference, The nnovatve load cell based FG wll be developed for montorng prestressng anchorage system long-term and on lne. The changes of magntude and dstrbuton of prestress wll accorngly reflect safety of whole brdge structure drectly.the data chroncally gathered wll be sgnfcant parts for knowng health condton of the suspenson brdge n Health Montorng Syetem and Dgtal Management of Wuhan Yangluo Yangtze Rver rdge. 2 THE STRUCTURE OF SUSPENSION OF RIDGE ANCHORAGE SYSTEM At present, the suspenson brdge anchorage system that has been establshed n the world usually adopts prestressng steel-stranded-wre anchorage. Accordng to desgn and constructon experence of anchorages of many establshed suspenson brdges, prestressng anchorage system usually uses groutng that bumpng cement nto ppelne, whch s cohesve prestressng system leadng to the structure permanent and un-exchangeable.sometmes,the rust-proof and durablty of prestressng steel stranded wre was badly affects due to constructon qualty. In order to ensure safety and durablty of anchorage system of Wuhan Yangluo Yangtze Rver rdge, desgner developed the non-cohesve and replacve prestressng anchorage system [1]. As shown n Fgure 1. The component of anchorage system s composed of the jont structure of cable strands anchorage and the structure of prestressng steel bundle anchorage. The jont structure of cable strands anchorage s composed of couplng, drawbars and ther components, whle the structure of prestressng steel bundle anchorage s composed of ppelnes, prestressng steel stranded wre, anchor devce, antseptc grease, anchorage helmets and so on. The upper end of drawbar s connected wth anchor plate on cable strand anchorage, and the other end s connected wth the couplng that s fxed on front anchor by prestressng steel bundle anchorage. The couplng s made up of couplng flat plate and couplng reel, whch makes couplng desgned by components, makes anchorage stress defntely, and makes t convenent to product, transport and nstall.

3 The prestressng steel bundle between the front sde and the back sde of anchor s non-cohesve prestressng system.the reason for why hs type of anchorage was called non-cohesve and replacve prestressng anchorage system s that the steel stranded wre wll not be grouted n the ppelne flled wth antseptc grease, and characterzed by non-cohesve.the stretchng end of anchor was remaned addtonal length whch suffce the replacement of stell stranded wre, Accordng to desgn concepton of the anchorage system, we need to nstall pressure sensors on anchorage lnk parts of the anchorage system to the effect that engneer can montor stress state of the cable strand at any tme n rope of cable anchor deck anchor cup jont deck test-force rng pad deck of anchor screwy renforcng steel bar back guardng cap pull rod Fgure 1: chart for Wuhan Yangluo Yangtze Rver rdge brdge s servce perod. In practcal applcaton, The load cell(test-force rng) based FG s regarded as alternatve of gasket, whch s nstalled between the front sde of anchor and couplng flat plate. So, measurng surplus prestressng of anchorage system and calculate stretchng force of cable strand can be deal wth n once wthout accessory structure to the anchorage system. fore plane of anchor steel wre back plane of anchor 3 THE DEVELOPMENT OF FIER GRATING LOAD CELL [2] 3.1 The Theory of Fber Gratng Sensng Fber ragg Gratng (FG) s the commonest fber gratng sensor, ts basc prncple as shown n Fgure2. When the temperature, stran or other physcal quantty whch can be transferred to temperature or stran change, the gratng perod Λ(we call t raster unt) wll change, so wave center length of FG wll excurson. And through the montorng of wavelength excurson, we can apperceve change of physcal parameter beng measured. The change of FG wavelength caused by stran can be expressed. Δ / = (1 ε = K ε (1) P e) ε Where P e s the elasto-optcal coeffcent of fber, s the center wavelength of FG wthout stran, Δ s the wavelength dsplacement caused by stran, ε s

4 the appled axal stran and The change of FG wavelength caused by temperature. K ε s the relatve stran senstvty coeffcent of FG. Δ / = ( α + ξ) ΔT = K ΔT (2) where α s the thermal expanson coeffcent of FG, ξ s the t thermo-optcal coeffcent, Δ T s varance of temperature and K t s the relatve temperature senstvty coeffcent of FG. From the analyss above, we can know that there s good lnear relatonshp wrap layer FG ncdent spectrum penetrated spectrum reflected spectrum Λ core layer Δ (a) ncdent spectrum (b) reflected spectrum (c) penetrated spectrum (d) wavelength excurson of reflected spectrum Fgure 2: sensng prncple of FG between wavelength, stran and temperature change. Accordng to the reference lterature, t s approxmately ndependent between the temperature and the stran nfluence of FG wavelength when the temperature ranges between 0 C and 100 C, and the stran s n the range of 0 to με. Another advantage of FG sensor s the wavelength dvson multplex (WDM) technology actualzed dstrbuted sensng measure. In other words, we can use the same fber concatenated many FG to measure stran, temperature and other parameters smultanety. Fgure 3 shows multplex sensng measure prncple of FG.( 1, 2,, n )s the FG center wavelength, whch s correspondng wth dfferent measurng ponts. Each FG s nfluenced by temperature or stran, and wavelength shft occurs nearby of each FG.In stran measurng, the effect to measurng result caused due to temperature change can be elmnated by usng the prncple of multplex measurng and lnear supermposed relatonshp of sensors.

5 Sensng prncple above of the FG sensor and characterstc of fber materal tself brng about long-term relablty of sensng system. Therefore, FG sensor s generally regarded as deal devce that can realze fber smart structure and fber smart materal, and t s wdespread used n safety montorng of aerospace vehcle, brdge structure, shp structure, buldng structure, nuclear power plant and so on. gratng ( ) stran feld gratng ( ) 1 n ncdent spectrum wde band source coupler - decode for wavelength excurson n return of wavelength code Fgure 3: multplex sensng prncple of FG 3.2 Load Cell ased on FG [2] Load cell(also called test-force rng) s the man method of measurng tenson of brdge cable drectly. There are three structural styles of force cell whch are spoke-type, rng-type and hydraulc. Sensng prncples used for tradtonal force cell are manly dfferental resstance type, vbratng-strng-type, electrc resstance stran type and so on. ecause tradtonal measurng technology s defcency n tolerance to abomnable envronmental and long-term relablty, a nnovatve Load cell based on FG was developed on the purpose of brdge health montorng system. elastomer pressure at the end of anchor fber optcal gratng Fgure 4: Prototype of Load cell based FG The prototype of Load cell as shown n Fgure 4. Four FG stran sensors connected n seres wth a temperature sensor are evenly arranged axally on the annular alloy elastomer (materal 40CrNMo). The whole structure s formed an ndependent unt n order to afford facltes wth constructon. Accordng to the sensng prncple of FG, we evenly dvde elastomer and n FGs nto n parts correspondngly. Equaton (3) determnes the relatonshp between test load stress of load cell of FG and wavelength of each stran sensor.

6 Where parameter equaton (5) respectvely: Δ F = F = K + KT ΔT (3) K and parameter K T can be expressed by equaton (4) and E ΔS K = (4) 1 p e K T = K ( α ξ ) (5) + Where E s Young s modulus, Δ S s area of stress secton of elastomer s correspondng part. Accordng to what we explaned above, we can work out the total load of elastomer f we know the wavelength dsplacement of each FG and ts varance of work temperature. We can get parameter K and parameter K whch are related to Young s modulus and thermal expanson coeffcent of elastomer by calbratng. To sum up, Load cell based FG characterzed by: (1) The excellent Sgnal-to-Nose of measurement, hgh senstvty and accuracy,and good long-term relablty; (2) The temperature measurng and self-compensaton by multplex sensng;(3) The ndependent part and convenent nstallaton ; (4)The remote montorng of tenson convenently. T 4 FIELD MEASURING AND INTERPRETATION OF LOAD CELL 4.1 Constructon and Installaton of Load Cell In man cable anchorage system of Wuhan Yangluo Yangtze Rver rdge, there are 94 anchorage cables(nclude 34 sngle-cable and 60 double-cable anchorage cables) both upstream and downstream on south bank. Whle 98 anchorage(nclude 34 sngle-cable and 64 double-cable anchorage cables) both upstream and downstream on north bank. Accordng to the overall desgn scheme of long-term health montorng system by experts repettous evaluaton, Some 12 of them representatv test-force rng Fgure 5: feld nstallaton of test-force rng of FG prestressng anchor were chosen to measure the cable tenson of anchorage system by nstallng test-force rngs. Namely, 8 for sngle cable anchor (seral number from 1 # -8 # ) and 4 for double cable anchor (seral number from9 # -12 # ). The nstallaton of load cell on-ste as shown n Fgure 5. The test-force rngs were nstalled between the fore plane of anchor

7 and jont flat plate before beng tensoned. Accordng to desgn crteron, the control tenson( T 1 ) of sngle cable anchor of s ton, whle the control tenson( T 2 ) of double cable anchor s ton. The full procedure of tenson nclude three levels (15%T, 30%T, 103%T). The Load cell of anchorage system of Wuhan Yangluo Yangtze Rver rdge had been nstalled n June 2005, and the data was tested when each prestressng anchorage was tensonng. In ths paper, we manly analyze data acqured before spanng all of man cable strands.the test process contan when prestressng anchorage cables are tensoned and perodc tests of temperature nfluence after nstallaton. 4.2 Interpretaton of Test Result Tenson load s mposed on anchorage by the hydraulc jack, and the load s controlled by ndcaton readng of ol pressure guage. The test result of cable tenson was read and processed on ste by transmtng sgnal from load cell to portable demodulator. Once collectng wavelength sgnal of sensors, the computer calculated load of test-force rng accordng to calbraton data. Whle noted document readng of ol pressure gauge, extenson length of anchorage cable and measurng data of load cells after each level have been fnshed. The the contrast between test data for 9 # load cell and theoretcs controllng readng durng tensonng process as shown n Table 1. The relatve dfference between cable tenson acqurred by load cell and controllng readng s -1.8%, whch s the largest dfferent n all 12 load cells. The result ndcates that the measurng accuracy of load cells s very hgh, whch can entrely satsfy the measurng error(less than ±2.0%). Table 1: the result data of 9 # test-force rng tenson level theoretc value hydrau pressure (Mpa) cable force (t) hydrau pressure (Mpa) measurng value test-force rng value(t) FG1 FG2 FG3 FG4 aver. dfference cable force (%) 15% T % 30%T % 103%T % reverso % After nstallaton data measured lastng for two month was processed between September and November n order to understand the relatonshp between cable tenson and anchorage temperature. We choose test data of sngle-cable-strand(1 # and 3 # ) and double-cable-strand(9 # and 11 # ) load cell to analyze the contrast relatves between cable force and temperature of load cells.the test data curve as shown n Fgure 6. Where cable force s the total prestress after calculatng average measurng data of 4 FG stran sensors,and temperature s the measurng value of temperature FG sensor capsuled n load cell. We can explan that the relatonshp between cable force and temperature s reverse from ths charter. In other words, the hgher temperature s, the tenson of cable s smaller, and vce versa. Ths rule s consstent wth the analyss results

8 when appled theory of thermodynamcs,and must be taken account for when we measure tenson of cable under dfferent temperature condtons. Fgure 6: Curve of cable tenson wth temperature 5 CONCLUSIONS The load cell based FG developed specally had been successfully utlzed n anchorage prestressng montorng system of Wuhan Yangluo Yangtze Rver rdge. The tenson process test ndcates that the accuracy of the load cell of FG s very hgh(less than ±2.0%), and the temperature nfluence test ndcates that cable tenson changes regularly along wth the temperature changes.these results verfed that the type of load cell s sutable for chroncally montorng anchorage prestressng system. In subsequent constructon of man cable of ths brdge, we can further montor varance regulaton of anchorage cable force by utlzng these load cells nstalled.wth the accumulaton of long term montorng data the replacve warranty of new-type non-cohesve and replacve prestressng anchorage system of suspenson montorng wll be setup. ACKNOWLEDGEMENTS Thanks to all colleagues n Key Lab. of Fber Optc Sensng Technology and Informaton Processng (Wuhan Unversty of Technology) Mnstry of Educaton and the Natonal Scence Fund Project (No ) for ther assstance on the research. REFERENCES 1. Xu Guopng, Lu Huatu. The Desgn of Anchorage System n Wuhan Yangluo Yangtze Rver rdge. Hghway [J]. 12(2004): Jang Desheng, Lu Shengchun.The Applcaton and Research of FG Sensors ased Load Cell to owstrng Arch rdge Health Montorng System Proceedngs of IWSHM: S.C.Tjjn, Tjjna s. Rupalb, P.MOyO. Applcaton of fbre ragg gratng based stran sensors for cvl nfrastructure health montorng Proceedngs of SPIE Vol (2003):