sensors Review

Health Monitoring and Evaluation of Long-Span Bridges Based on Sensing and Data Analysis: A Survey Jianting Zhou *, Xiaogang Li, Runchuan Xia, Jun Yang and Hong Zhang College of Civil Engineering, Chongqing Jiaotong University, Chongqing 400074, China; [email protected] (X.L.); [email protected] (R.X.); [email protected] (J.Y.); [email protected] (H.Z.) * Correspondence: [email protected]; Tel.: +86-23-6265-2702 Academic Editor: Simon X. Yang Received: 21 January 2017; Accepted: 11 March 2017; Published: 16 March 2017

Abstract: Aimed at the health monitoring and evaluation of bridges based on sensing technology, the monitoring contents of different structural types of long-span bridges were defined. Then, the definition, classification, selection principle, and installation requirements of the sensors were summarized. The concept was proposed that new adaptable long-life sensors could be developed by new theories and new effects. The principle and methods to select controlled sections and optimize the layout design of measuring points were illustrated. The functional requirements were elaborated on about the acquisition, transmission, processing, and management of sensing information. Some advanced concepts about the method of bridge safety evaluation were demonstrated and technology bottlenecks in the current safety evaluation were also put forward. Ultimately, combined with engineering practices, an application was carried out. The results showed that new, intelligent, and reliable sensor technology would be one of the main future development directions in the long-span bridge health monitoring and evaluation field. Also, it was imperative to optimize the design of the health monitoring system and realize its standardization. Moreover, it is a heavy responsibility to explore new thoughts and new concepts regarding practical bridge safety and evaluation technology. Keywords: sensor; bridge health monitoring; safety evaluation

1. Introduction As lifeline engineering, bridges are in a stage of rapid development at present, and the community has attached great importance to the operational safety of bridges. Therefore, how to ensure the healthy operation of long-span bridges has become a hot topic that is difficult to address in this field. Through the service period that ranges from decades to even a hundred years, bridges will be affected by external loads like cars, climate, and rivers. Bridges will also be affected by internal effects like material aging, fatigue, and other factors [1]. These factors will result in inevitable damage and serious accidents. For example, on 1 August 2007, the I-35W Mississippi River Bridge in the United States collapsed [2]. On 14 July 2011, the Wuyishan Mansion Bridge in the Fujian province of China collapsed [3]. The lessons taught by a series of disasters have made health monitoring and evaluation for bridges a major issue. Sensing technology belongs to modern science and its purpose is to obtain and identify the information from natural sources by sensors. With the innovation of technology, sensors that are original, intelligent, and reliable have been widely used for health monitoring of long-span bridges. For instance, sensors were installed on the Foyle Bridge [4], which is a continuous steel bridge with

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variable heights and a total length of 522 m. Their purpose was to monitor the response of vibration, deflection, and strain under the vehicle and wind load in the operation stage. Numerous acceleration sensors and strain gauges were attached to the Tsing Ma Bridge with a set of GPS (Global Position System) devices, which were used for the long-term monitoring of the service safety [5]. Monitoring systems have also been installed in numerous bridges. For example, in Japan, the Akashi Kaikyo Bridge [6] has a main span of 1991 m. In Denmark, there exists the Great Belt East Suspension Bridge and the Faroe Sea-Crossing Cable-Stayed Bridge, of which the main spans are 1624 m and 1726 m, respectively. In Norway, the Skarnsundet Cable-Stayed Bridge has a main span of 530 m. Additionally in China, there exists the Su Tong Yangtze River Highway Bridge and the Run Yang Yangtze River Bridge, of which the main spans are 1088 m and 1490 m, respectively. In this paper, the Caijia Jialing River Bridge for the subway in Chongqing city, China, will be taken as an example. Its total length is 1250 m. Also, constant, on-line, and dynamic health monitoring is conducted so as to achieve the objective of real and effective evaluation of its structural state [7–9]. With the progress and development of technology, the current health monitoring program focuses on smart sensors and methods of reliability and safety evaluation. The implementation relies on the monitoring system [10]. A set of complete monitoring systems has functions such as acquisition, transmission, processing, management, evaluation, early warning, etc. The whole process is completed by six systems [11]. (1) (2) (3) (4) (5) (6)

The sensor system is aimed at obtaining load, environment, and structural response information. The data acquisition and transmission system is aimed at signal acquisition, caching, and transmission. The data processing and control system is aimed at routine data processing and analysis, system status monitoring, remote control, and structural warning. The evaluation system is aimed at executing the original monitoring data analysis, structural status evaluation, diagnosis, and prediction. The data management system is aimed at storage and extraction of the monitoring data and analysis results. The inspection and maintenance system is aimed at sensors for inspection and maintenance, data acquisition unit, data transmission network and display equipment, etc.

With respect to bridge safety evaluation technology, the applied theories mainly include the reliability theory, analytic hierarchy process, fuzzy theory, etc. In summary, the health monitoring of long-span bridges has achieved great progress. To obtain standardized management, many countries have promulgated relevant guidelines and procedures. For example, ISIS (Intelligent Sensing for Innovative Structures) in Canada released “The Structural Health Monitoring Guide” [12]. Drexel University in America completed “A Case Study on the Health Monitoring Guide for Significant Bridges” [13] with the FHWA (Federal Highway Administration). SAMCO (Structural Assessment, Monitoring and Control Organization) released “The Structural Health Monitoring Guide” [14] with the European Union. In China, the department of transportation released “Technical specification for structural safety monitoring systems of highway bridges” [15]. However, when they were applied to engineering practices, there were still some obvious shortcomings, such as the mechanism and technology of monitoring and sensing, monitoring theory and technology of the service lifetime, the processing of monitoring information [16], structural status evaluation, etc. Also, the combined effects of the complex service environment and disaster loads can accelerate the damage evolution of long-span bridges, which will have negative results on the health and safety of the overall structure. This should be considered with the planning and construction of super bridge projects across long-span seas or rivers, such as the Shanghai Railway Yangtze River Bridge, the Egongyan Special Track Bridge in China, the Pulau Muara Besar Bridge, etc. The monitoring and evaluation tasks will become more burdensome and difficult. As a result of the health monitoring and evaluation of long-span bridges based on sensing technology, the research status, positive innovation,

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and the scientific grasp of damage evolution laws need to be recognized to ensure the safety of the structures [17]. 2. Health Monitoring Contents Via physical and mechanical properties, the health monitoring of long-span bridges is aimed at monitoring the overall structures non-destructively and constantly. Then, the location and degree of structural damages can be diagnosed. Also, the service situation, reliability, durability, and bearing capacity can also be evaluated. If the structure has an emergency or severely abnormal usage, warning signals will be triggered to provide a basis and guidance for maintenance, management, and decision-making [18]. Due to different types of long-span bridges combined with differences in the operating environment as well as the need for monitoring functions, the monitoring contents should be chosen appropriately, which are shown in Table 1. Table 1. The Health Monitoring Contents of Long-Span Bridges. Girder Bridge

Arch Bridge

Cable-Stayed Bridge

Suspension Bridge

load and environment

moving load earthquake impact temperature humidity wind speed and direction rainfall

F F F N N 3 F

F F F N N F F

F F F N N N F

F F F N N N F

the overall response of the structure

vibration deformation rotation angle

N N 3

N N F

N N F

the local response of the structure

stress cable force crack fatigue corrosion changing and the reaction of supports foundation erosion

N N 3

N N F F F F F

N N F F F F F

Bridge Type Categories

N # F # F F F

N N F F F F F

Illustrating: N indicates the item which shall be monitored. F indicates the item which is selected to be monitored if the condition permits. 3 indicates the item which can be monitored. # indicates the item which is unnecessary to be monitored.

3. Sensing Technology 3.1. Sensor The sensor is a device that can detect certain specified parameters and convert them into available signals. According to the principle of operation, it can be classified by resistance, raster, piezoelectric, laser [19], etc. As the leading edge of health monitoring systems, the selection of the sensor should follow the principle of “stable and reliable performance which is cost-effective” and should be convenient for the integration of the health monitoring system. Also, some evaluation indexes, such as sensitivity, linearity, signal-to-noise ratio, and resolution should also meet the monitoring needs. Before the installation of the sensor, it is necessary to carry out its calibration. During the installation process, damage to the structure should be reduced. With the rapid development of science and technology, there has been some breakthroughs in the long-term stability, reliability, durability, and other aspects of sensors. Meanwhile, some new types of practical sensors have emerged, such as the multi-directional dynamic stress monitoring sensor for concrete [20]. However, the information acquisition of bridge health monitoring and evaluation are greatly restricted because of the complexity of the working environment, the mismatch between the long life of the bridge and the short life of the sensors, and the monitoring signals which result from

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multi-effects. Therefore, by using new theories and new effects, in the future it will be a hot topic to study and develop new types of long-life sensors with stronger adaptability. 3.2. Optimum Layout of the Sensors Due to the limited number of measuring points in health monitoring systems, the full freedom data of long-span bridges cannot be obtained. Therefore, it is necessary to optimize the sensor configuration so as to achieve effective damage identification. Considering the comprehensiveness, accuracy, and economic factors regarding constant response information, when sensors are arranged, the controlled sections should be selected by following multiple principles of analyzing internal forces and deformation, vulnerability, and geometric dimensions. Methods include technology of static control analysis, dynamic control analysis, and monitoring overall bridge state acquisition via finite points. According to the criterion of minimum errors of identification (transmission), model reduction [21], interpolation fitting, and MAC (Modal Assurance Criterion) [22], some methods can be used to optimize the layout of sensors. The methods include genetic algorithms, data fusion, inversion theory, and optimization which is combined with construction monitoring. 4. Sensing Information 4.1. Data Acquisition According to the structure type of long-span bridges, the number of health monitoring points, the type of sensor, the acquisition mode of sensing information, etc., should be rationally designed: (1) (2)

If the measuring points are relatively far apart and scattered, the distributed acquisition mode should be adopted. If the measuring points are relatively close to each other and concentrated, the centralized acquisition mode or that combined with the distributed mode should be adopted.

The acquisition device can be divided into hardware and software. The hardware device should follow the standard protocol and interface with the function of constant acquisition, automatic storage, and instant display. The software device can achieve manual intervention acquisition and adjust parameters, preliminarily dealing with the output contents of sensing information and the identification of abnormal information. Sensor devices with special software to measure and control should be provided with the software running platform, communication protocol, and response interface. As for the acquisition software of the development platform, the language should meet the requirements of health monitoring visualization where LabVIEW [23] and LabWindows [24] can be adopted. 4.2. Data Transmission Combined with the transmission distance, network coverage, communication facilities, and other factors, reasonable transmission modes of sensing information are respectively selected, which are then followed. (1) (2) (3) (4)

(5)

If the monitoring range is small, it shall be based on the signal synchronization technology. If the monitoring range is large, it shall be based on the time synchronization technology [25]. If the electromagnetic interference cannot be shielded in bridge monitoring, cable transmission shall be adopted. If it is in surroundings where the layout and the maintenance of wires are difficult, or if it is necessary to build the monitoring structure which can temporarily transmit network data, wireless transmission [26] shall be adopted. According to the needs of the projects, one or more modes can be combined.

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4.3. Data Processing Based on the sensing information, distinguishing the abnormity caused by structural and nonstructural damage is a hot topic in this field of research. Due to the sensor deviation, noise, system failures, and other coupling factors, abnormal sensing information caused by nonstructural damage must be obtained so as to provide effective information for structural damage identification and safety evaluation. In engineering applications, as for the processing of sensor information, the methods are aimed at sensing signals. Specifically, the time domain, frequency domain, and time-frequency domain method can be adopted for noise reduction and filtering the gross error and accidental error. Also, the distorted information can be reconstructed by the trend curve method or the neural network method. Then, the missing information can be repaired by way of interpolation, substitution, and weight adjustment [27]. 4.4. Data Management The management of sensing information is carried out on a server which requires a fast display, efficient storage, report generation, and other functions. With an effective connection between information sensing devices and the Internet, system technologies regarding the Internet of Things can be adopted for management. This practice has proven that a modular structure in the form of the database can achieve the hierarchical and classified management of sensing information. This database mainly includes static and dynamic databases. (1) (2)

The static database includes information on design and construction, daily management and maintenance, system software, and hardware. The dynamic database includes information on monitoring, analysis results, and maintenance recommendations.

5. Bridge Safety Evaluation Based on Sensing Information Based on the response information, theoretical calculation analysis, mathematics, and mechanics, the safety evaluation of structures can be achieved via the changing characteristics indexes of the structure itself, its response, and trends. Moreover, the individual control index can be used for a comprehensive evaluation. Some advanced methods of safety evaluation are briefly introduced as follows. (1)

Safety Evaluation Based on Reliability Theory

The structural function Z is constructed and the sensing information is used to modify the structural load effect model S and resistance model R. Via the reasonable statistical analysis method, the probability of exceeding the limit state during the period of usage can be obtained, which can implement the structural safety evaluation. When Z = R – S > 0, it indicates that the structure is in a state of reliability. When Z = R – S < 0, it indicates that the structure has been deactivated or destroyed. When Z = R – S = 0, it indicates that the structure is in a limited state. (2)

Safety Evaluation Based on the Deterioration Effect

According to numerous historical monitoring data, the random process Zi is constructed, searching for and extracting the characteristic information which reflects the structural state. Then, the deterioration law of the structural performance is determined. Therefore, the following Equation (1) is used to implement the structural safety evaluation. Zi = µ + ξ i ,

(1)

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In Figure 1, when the structure is in aand normal state, μ randomly will be unchanged and ξi will randomly where: µ indicates the process mean value ξ i indicates varied parameters. vary In with no obvious throughisthe process. or safety occur,vary the Figure 1, whentrend the structure in awhole normal state, µWhen will bedamage unchanged and ξproblems i will randomly changing amplitude of Z i will continue to increase and μ will sustain an irreversible monotonic with no obvious trend through the whole process. When damage or safety problems occur, the changing trend. amplitude of Zi will continue to increase and µ will sustain an irreversible monotonic trend.

Figure Figure 1. 1. Schematic Schematic Diagram Diagram of of the the State State of of the the Bridge Bridge Structure. Structure.

(3) Safety Evaluation Based on Envelope Theory (3) Safety Evaluation Based on Envelope Theory According to the limit state principle of bearing capacity based on the plastic theory, the design to the limit state principle of bearing based on the plastic theory, the design valueAccording of the unfavorable combination of the loading capacity effect must be less than or equal to that of the value of the unfavorable combination of the loading effect must be less than or equal to that of the structural resistance. It can be expressed as Equation (2). structural resistance. It can be expressed as Equation (2).  0 S  R( f d ,  c adc ,  s ads ) Z1 (1  e ) , (2) γ0 S ≤ R( f d , ξ c adc , ξ s ads ) Z1 (1 − ξ e ), (2) where:  0 indicates the importance factor of the structure, S indicates the loading effect function, R where: γ0the indicates theeffect importance factor of the the structure, indicates the loading effect function, indicates resistance function, fd indicates design Svalue of material strength, adc indicates R indicates the resistance effect function, f indicates the design value of material strength, a d dc indicates the geometric parameter of concrete, ads indicates the geometric parameter of rebar, Z1 indicates the the geometric parameter of concrete, ads indicates the geometric parameter coefficient of rebar, Z1ofindicates the calculating coefficient of the bearing capacity, ξe indicates the deterioration the bearing calculating coefficient of the bearing capacity, ξ indicates the deterioration coefficient of the bearing capacity, ξc indicates the cross section reductione coefficient of the reinforced concrete structure, and ξ c indicates the crossreduction section reduction coefficient of the reinforced concrete structure, and ξ s ξcapacity, s indicates the cross section coefficient of rebar [28]. indicates the cross coefficient of section rebar [28]. Therefore, the section loadingreduction effect diagram of each reflected by the sensing information can be Therefore, the loading effect diagram of each section reflected by the sensing information can be compared and analyzed with the modified limit resistance effect diagram of the bridge. The purpose compared theevaluation. modified limit resistance effect diagram of the bridge. The purpose is to obtainand the analyzed structuralwith safety is to obtain the structural safety evaluation. (4) Safety Evaluation Based on Dynamic Response (4) According Safety Evaluation on information, Dynamic Response to the Based sensing the dynamic characteristics parameters [29] of the structure can be obtained as well as the judgment, localization, and quantification [30,31] of the According to the sensing information, the dynamic characteristics parameters [29] of the structure structural damage. Then the finite element model can be corrected via the reanalysis of the ultimate can be obtained as well as the judgment, localization, and quantification [30,31] of the structural bearing capacity, implementing the structural safety evaluation. damage. Then the finite element model can be corrected via the reanalysis of the ultimate bearing Regarding the fatigue damage evaluation for a steel box girder based on sensing information, capacity, implementing the structural safety evaluation. the S-N curve (fatigue curve) of the construction details can be adopted to evaluate the fatigue status Regarding the fatigue damage evaluation for a steel box girder based on sensing information, according to the Miner principle. Under special meteorological conditions (such as the wind, the S-N curve (fatigue curve) of the construction details can be adopted to evaluate the fatigue status rainfall, etc.) regarding natural factors which will affect the normal service of the bridge, it is according to the Miner principle. Under special meteorological conditions (such as the wind, rainfall, necessary to carry out this special evaluation. etc.) regarding natural factors which will affect the normal service of the bridge, it is necessary to carry Based on the sensing information, although bridge safety evaluation has obtained some great out this special evaluation. achievements which have been successfully applied to some long-span bridges, most of them focus Based on the sensing information, although bridge safety evaluation has obtained some great on the overall evaluation of historical data accumulation and accidents. So we are confronted with a achievements which have been successfully applied to some long-span bridges, most of them focus technical bottleneck [32] for early damage warning and online safety evaluation. on the overall evaluation of historical data accumulation and accidents. So we are confronted with a technical bottleneck [32] for early damage warning and online safety evaluation.

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6. Application Analysis 6. Application Analysis

6.1. Engineering Outline 6. Application Analysis 6.1. Engineering Outline The Caijia Jialing River Bridge is located in the Liangjiang New Area of the Chongqing 6.1. Engineering Outline The Caijia Jialing River Bridgeinisthe located in the Liangjiang New Area of the Chongqing Municipality in China. It is designed Rail Transit Line 6 (second-phase project) between the The Caijia Jialing BridgeStation. is the located intotal the length Liangjiang Area of the Chongqing Municipality in China. ItRiver is designed in Rail Transit Line 6 of (second-phase between Jinshan Temple Station and Caojiawan The theNew bridge isproject) 1250 m, and thethe main Municipality in China. It is designed in the Rail Transit Line 6 (second-phase project) between the Jinshan Temple Station and Caojiawan Station. The total length of the bridge is 1250 m, and the main structure is a cable-stayed concrete bridge with twin towers and double cable planes where tower-beam Jinshan Temple Station and Station. Theistotal length the and main structure isis aadopted. cable-stayed concreteofbridge with twin towers and double cable consolidation TheCaojiawan layout the spans 60 m + 135ofm +bridge 250 mis+1250 135 m, mplanes + 60the mwhere = 640 m, structure is consolidation a cable-stayed concrete The bridge with twin towers double cable tower-beam is adopted. layout of the spans is 60 and m + 135 m + 250 m +planes 135 m +where 60 m and the transverse layout is 1.5 m (cable area) + 1.4 m (maintaining road) + 4.6 m (carriageway) + 4.6 m consolidation is adopted. layout of the area) spans +is 1.4 60 mm+ 135 m + 250 m road) + 135 m 60 m =tower-beam 640 m, and the transverse layout The is 1.5 m (cable (maintaining + +4.6 (carriageway) + 1.4 m (maintaining road) + 1.5 m (cable area). The shape of the tower is a diamond, = 640 m, and+ the transverse layout is m 1.5(maintaining m (cable area) (maintaining road) + 4.6 m (carriageway) 4.6 m (carriageway) + 1.4 road)+ +1.4 1.5 m m (cable area). The shape of the and tower the auxiliary piers are rectangular cross-sectional hollow piers. Cables adopt the steel strand and (carriageway) + 4.6 m (carriageway) + 1.4 m (maintaining road) + 1.5 m (cable area). The shape of the is a diamond, and the auxiliary piers are rectangular cross-sectional hollow piers. Cables the standard fpk and =and 1860 which areare protected an HDPE (High Density tower the is astrength diamond, the auxiliary piers cross-sectional hollow piers. adopt steel strand theMPa, standard strength fpkrectangular = 1860by MPa, which are protected byPolyethylene) an Cables HDPE tube.(High The single cell box girder with a constant height is adopted as the main girder, and theHDPE adopt Density the steelPolyethylene) strand and the standard strength fpk =girder 1860 MPa, areheight protected by an tube. The single cell box with awhich constant is adopted asconcrete the grade is C55. The layout of the main bridge is shown in Figure 2. (Highgirder, Density Polyethylene) single cell box girder constant height in is adopted main and the concretetube. gradeThe is C55. The layout of the with mainabridge is shown Figure 2.as the main girder, and the concrete grade is C55. The layout of the main bridge is shown in Figure 2.

Figure TheLayout Layoutof of the the Main Main Bridge Figure 2.2.The Bridge(unit: (unit:m). m). Figure 2. The Layout of the Main Bridge (unit: m).

6.2. Layout and Installation of Measuring Points and Sensors 6.2. Layout and Installation of Measuring Points and Sensors 6.2. Layout and Installation of Measuring Points and Sensors According to the principle and method of selecting controlled sections and the layout According toofthe and method ofoverall selecting controlled sectionsinsections and layout optimization According toprinciple the principle and the method oflayout selecting controlled andJialing the layout optimization the measured points, of the sensors thethe Caijia River of the measured points, the overall layoutthe of overall the sensors inof thethe Caijia Jialing River Bridge is shown optimization of in theFigure measured points, layout sensors in the Caijia Jialing River in Bridge is shown 3. Figure 3. is shown in Figure 3. Bridge ④（1） ⑥（1） ④（11） ） ⑦（ ⑥（11） ） ⑧（ ） ⑦（11） ⑨（ ⑧（1） ⑨（1）

Jinshan Temple Jinshan Temple ①（6） ③（2） ①（6） ③（2） ⑤（2） ⑤（2）

④（1） ⑥（1） ④（1） ⑥（1）

⑩（2） ⑩（ ⑩（22） ） ⑩（2） ⑩（2） ⑩（ ⑩（22） ）

Caojiawan Caojiawan

⑩（2）

①（6） ①（6） ②（1） ①（26） ） ①（6） ③（ ②（1） ③（2）

①（6） ①（6） ②（4） ②（1） ①（6） ③（ ） ①（26） ②（4） ②（1） ③（2）

①（6） ②（1） ①（26） ） ③（ ②（11） ） ④（ ③（2） ④（1）

①（6） ①（6） ①（6） ①（6） ②（1） ②（1） ①（26） ） ①（6） ①（6）③（ ） ①（26） ③（ ②（1） ②（1） ③（2） ③（2）

①（6） ③（2） ①（6） ③（2） ⑤（2） ⑤（2）

Figure 3. The Overall Layout of the Sensor Measuring Point. ① the stress sensor which can also measure temperature of theof structure ② the acceleration sensor static level gauge GNSS Figure 3.the The Overall Layout the Sensor Measuring Point. ①③ thethe stress sensor which ④ can also 1 the stress sensor which can also Figure 3. The Overall Layout of the Sensor Measuring Point. (Global Satellite System) ⑤ the meter ⑥③ thethe tiltmeter ⑦ the temperature measureNavigation the temperature of the structure ② displacement the acceleration sensor static level gauge ④ GNSS 2 the acceleration sensor 3 the static level gauge 4 GNSS measure the temperature of the structure and humidity sensorSatellite ⑧ the pluviometer ⑨ the dogvane and anemoscope ⑩ the anchor cable meter (Global Navigation System) ⑤ the displacement meter ⑥ the tiltmeter ⑦ the temperature 5 the displacement meter 6 the tiltmeter 7 the temperature (Global Navigation Satellite System) through the cable. Also, in parentheses indicate number of⑩ each and humidity sensor ⑧ the the numbers pluviometer ⑨ the dogvane andthe anemoscope thesensor. anchor cable meter 8 the pluviometer 9 the dogvane and anemoscope 10 and through humidity

anchor cable meter thesensor cable. Also, the numbers in parentheses indicate the number of eachthe sensor.

through the cable. Also, the numbers in parentheses indicate the number of each sensor.

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Combined with the construction progress of the bridge, the health monitoring sensors were appropriately installed at the corresponding corresponding positions, positions, as as follows. follows.

(1) On thethe longitudinal direction of the the vibrating wire strain (1) On the themain maingirder. girder.Along Along longitudinal direction of bridge, the bridge, the vibrating wire sensors were attached to the lower edge of the top slab and the upper edge of the bottom strain sensors were attached to the lower edge of the top slab and the upper edge ofslab, the which shown in are Figure 4. ItinisFigure worth 4.mentioned the sensor could also measure the bottomare slab, which shown It is worththat mentioned that the sensor could also temperature of the structure and modify the initial temperature [33]. In addition, the correlation measure the temperature of the structure and modify the initial temperature [33]. In addition, function [34] of the stress-time relation curve could established by monitoring the the correlation function [34] of the stress-time relation curvebecould be established by monitoring corresponding embedded sensors in the construction process. The purpose was to obtain stress the corresponding embedded sensors in the construction process. The purpose was to obtain monitoring of theofbridge structure under the the dead load and the level stress monitoring the bridge structure under dead load and thelive liveload. load.Also, Also, static static level gauges based on the pipe principle = were installed to monitor the static long-term deformation gauges based on the pipe principle = were installed to monitor the static long-term deformation of girder. Meanwhile, installed to to of the the girder. Meanwhile, the the GNSS GNSS (Global (Global Navigation Navigation Satellite Satellite System) System) was was installed monitor the acceleration acceleration monitor the the spatial spatial mid-span mid-span deformation. deformation. Also, Also, the the displacement displacement sensors sensors and and the sensors were installed to measure the width of the expansion joint and to test the dynamic sensors were installed to measure the width of the expansion joint and to test the dynamic characteristics, respectively. characteristics, respectively.

Strain Sensor

1

2

3

4

5

6

Figure 4. The Layout Diagram of Strain Sensors on the Surface of Concrete.

(2) On the the cables. cables. Before Before the the cable cable tension tension in in the the construction construction stage stage and and anchorage anchorage of of the (2) On the cable cable and and tower, intelligent anchor cable meters with temperature measuring functions were embedded tower, intelligent anchor cable meters with temperature measuring functions were embedded to monitor monitor the thecable cableforce. force.ToTo accurately measure cable force under eccentric to accurately measure thethe cable force under the the eccentric load,load, the the intelligent six-string meters should be adopted. intelligent six-string typetype meters should be adopted. (3) On the the main main tower. tower. The Thetiltmeters tiltmetersand andGNSS GNSSwere wereinstalled installedononthe thetop topofofthe the tower. Also, (3) On tower. Also, a a monitoring stationaimed aimedatatmeteorological meteorologicalfactors factors was was set set up. The monitoring monitoring contents monitoring station up. The contents included the the wind wind speed, speed, wind wind direction, direction, rainfall, rainfall, and and humidity. humidity. included 6.3. Safety Evaluation Evaluation System 6.3. Safety System Based Based on on Envelope Envelope Theory Theory The Jialing River Bridge considered loading factors including live The health healthmonitoring monitoringfor forthe theCaijia Caijia Jialing River Bridge considered loading factors including train load, transverse rocking force, braking force, temperature force, wind load, water pressure [35,36], live train load, transverse rocking force, braking force, temperature force, wind load, water pressure etc. Viaetc. theVia finite simulation analysis by theby professional program Midas/Civil, the sub [35,36], theelement finite element simulation analysis the professional program Midas/Civil, the loads werewere combined to establish the upper lower the envelope interval. For example, sub loads combined to establish the and upper andbound lower of bound of the envelope interval. For for the main girder of girder the bridge, vertical and the longitudinal stress ofstress pointof 2 example, for the main of thethe bridge, thedeformation vertical deformation and the longitudinal (middle the topof slab) (middle of the bottomofslab) measured. point 2 position (middle of position the and top point slab) 5and pointposition 5 (middle position the were bottom slab) were These results are shown in Figures 5–7, where Section A–Section G successively represent: L/2 measured. Secondary Sidespan next to Jinshan Temple, L/2 Sidespan next to Jinshan Temple, L/4 Mid-span, These results are shown in Figures 5–7, where Section A–Section G successively represent: L/2 L/2 Mid-span, 3L/4 next Mid-span, L/2 Temple, SidespanL/2 next to Caojiawan, and L/2Temple, Secondary Secondary Sidespan to Jinshan Sidespan next to Jinshan L/4 Sidespan Mid-span,next L/2 to Caojiawan. Mid-span, 3L/4 Mid-span, L/2 Sidespan next to Caojiawan, and L/2 Secondary Sidespan next to

Caojiawan.

Sensors 2017, 17, 603 Sensors 2017, 17, 603 Sensors 2017, 17, 603 Sensors 2017, 17, 603 Envelope Upper Bound Envelope Upper Bound Lower Bound Bound Envelope Upper Envelope Lower Bound Envelope Lower Bound

Vertical Deformation/mm Vertical Deformation/mm Vertical Deformation/mm

150 150 150 100 100 100 50 50 50 0 0 0 -50 -50 -50 -100 -100 -100 -150 -150 -150 -200 -200 -200 -250 -250 -250

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Section A Section A Section A

Section B Section B Section B

Section C Section D Section E Section C Section D Section E SectionMonitoring C Section Section D Section E

Section F Section F Section F

Section G Section G Section G

Monitoring Section Monitoring Section Figure 5. Envelope Upper and Lower Bounds of the Vertical Deformation Deformation for for the the Main Girder. Girder. Figure 5. 5. Envelope Envelope Upper Figure and Lower Bounds of the Vertical the Main Main Girder. Figure 5. Envelope Upper and Lower Bounds of the Vertical Deformation for the Main Girder. 0 0 0

Envelope Upper Bound Envelope Upper Bound Lower Bound Bound Envelope Upper Envelope Lower Bound Envelope Lower Bound

Stress/MPa Stress/MPa Stress/MPa

-5 -5 -5 -10 -10 -10 -15 -15 -15 -20 -20 -20

Section A Section A Section A

Section B Section B Section B

Section C Section D Section E Section C Section D Section E Section Monitoring C Section Section D Section E

Section F Section F Section F

Section G Section G Section G

Monitoring Section Monitoring Section Figure 6. Envelope Upper and Lower Bounds of the Stress of Point 2 for the Main Girder. Figure 6. Envelope Upper and Lower Bounds of the Stress of Point 2 for the Main Girder. Girder. Figure 6. Figure 6. Envelope Upper and Lower Bounds of the Stress of Point 2 for the Main Main Girder. 5 5 5

Envelope Upper Bound Envelope Upper Bound Lower Bound Bound Envelope Upper Envelope Lower Bound Envelope Lower Bound

Stress/MPa Stress/MPa Stress/MPa

0 0 0 -5 -5 -5

-10 -10 -10 -15 -15 -15 -20 -20 -20

Section A Section A Section A

Section B Section B Section B

Section C Section D Section E Section C Section D Section E Section Monitoring C Section Section D Section E

Section F Section F Section F

Section G Section G Section G

Monitoring Section Monitoring Section Figure 7. Envelope Upper and Lower Bounds of the Stress of Point 5 for the Main Girder. Figure 7. Envelope Upper and Lower Bounds of the Stress of Point 5 for the Main Girder. Figure 7. Envelope Bounds of of the the Stress Stress of of Point Point 55 for for the the Main Main Girder. Girder. Figure 7. Envelope Upper Upper and and Lower Lower Bounds

Based on the upper and lower bounds of the envelope interval, the safety coefficient r = 0.75, Based on the upper and lower bounds of the envelope interval, the safety coefficient r = 0.75, Based on theThen upper and lower bounds of[37] the of envelope interval, the safety coefficient r = 0.75, was determined. the envelope intervals a normal state, critical state, and degradation on theThen upperthe and lower bounds of[37] the of envelope interval, the safety coefficient r = 0.75, was Based determined. envelope intervals a normal state, critical state, and degradation was determined. Then the envelope intervals [37] of a normal state, critical state, and degradation evaluation were established. For instance, for the vertical stiffness ofcritical the mid-span, the calculation was determined. Then the envelope intervals of a normal state,of state, and evaluation were established. For instance, for [37] the vertical stiffness the mid-span, thedegradation calculation evaluation were established. For instance, for(DZ) the vertical stiffness of 2. the mid-span, the calculation results for the deformation in the Z-direction are shown in Table evaluation were established. For instance, for the vertical stiffness of the mid-span, the calculation results for the deformation in the Z-direction (DZ) are shown in Table 2. results for the deformation in the Z-direction (DZ) are shown in Table 2. results for the deformation in the Z-direction (DZ) are shown in Table 2.

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Table 2. Deformation in the Z-direction (DZ) for Mid-span of the Main Girder (unit: mm). Table 2. Deformation in the Z-direction (DZ) for Mid-span of the Main Girder (unit: mm).

Load

DZ min −154 Load DZ train live load max 31 min −154 train live load max + 31 0 transverse rocking force − 0 + 0 transverse rocking force − + 0 0 braking force − 0 + 0 braking force system temperature rising − 0 21 system temperature dropping system temperature rising 21−21 component temperature rising system temperature dropping −21−41 temperature load [38] component temperature rising component temperature dropping −4141 temperature load [38] component temperature dropping 41 10 gradient temperature rising gradient temperature rising 10 gradient temperature dropping −5 −5 gradient temperature dropping transverse wind + 0 transverse wind + 0 transverse wind − transverse wind − 0 0 longitudinal + longitudinal windwind + 0 0 wind wind loadload longitudinal wind wind − 0 0 longitudinal − traintrain windwind + 0 + 0 train wind − 0 train wind − 0 + 0 0 + water pressure water pressure − 0 − 0 Illustrating: down-warping is negative, otherwise, positive.

rDZ −115 rDZ 23 −115 023 0 0 00 00 160 −16 16 −31 −16 − 3131 731 7 −4 −4 0 0 00 00 00 00 0 0 0 0 0 0

Illustrating: down-warping is negative, otherwise, positive.

So in main girder, thethe envelope of the stiffness was [was −166[−166 mm, mm, 77 mm] in the themid-span mid-spanofofthe the main girder, envelope of vertical the vertical stiffness 77 in the normal state and was [ − 221 mm, − 166 mm) ∪ (77 mm, 103 mm] in the critical state. mm] in the normal state and was [−221 mm, −166 mm) ∪ (77 mm, 103 mm] in the critical state. Similarly, Similarly, the the envelope envelope intervals intervals of of other other physical physical parameters parameters such as the stress and cable forces could be established. The time-varying time-varying effect effect among among the the material material characteristics characteristics was was considered. considered. established. The With operation time, the results of the of daily were alsowere organically integrated. With the thecontinuous continuous operation time, the results theinspection daily inspection also organically Then, the coefficient of coefficient checking and thecalculation, deterioration of thecoefficient bearing capacity, integrated. Then, the of calculation, checking and thecoefficient deterioration of the the reduction coefficient of the sections, andofthe of the train live load bearing capacity, the reduction coefficient theinfluence sections, coefficient and the influence coefficient of[28] the were train introduced and the introduced envelope interval be reasonably revised. live load [28] were and thecould envelope interval could be reasonably revised. Meanwhile, based on the finite element model, the static and and dynamic dynamic characteristics characteristics were analyzed. The natural vibration characteristics were taken as an example [39]. The first-order vibration analyzed. The natural vibration characteristics were taken as an example [39]. The first-order mode is shown the 8, first five of the frequency vibration are shown vibration mode in is Figure shown 8, inand Figure and theorders first five orders of the and frequency andmode vibration mode in 3. in Table 3. areTable shown

Figure 8. The First-order Vibration Mode. Figure 8. The First-order Vibration Mode.

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Table Table3.3.Analysis AnalysisofofNatural NaturalVibration VibrationCharacteristics Characteristicsofofthe theBridge. Bridge. Order Order 1 2 3 4 5

1 2 3 4 5

Calculated Calculated Frequency Frequency(Hz) (Hz) 0.686 0.686 0.752 0.752 0.8702 0.8702 0.8852 0.8852 1.1352 1.1352

Period (s) Period (s) 1.457 1.457 1.330 1.330 1.149 1.149 1.130 1.130 0.881 0.881

Description of Vibration Mode

Description of Vibration Mode

Transverse Vibration of Bridge Tower + Transverse Bending of Beam Transverse Vibration of Bridge Tower + Transverse Bending of Beam Anti-symmetric TransverseVibration Vibration Bridge Tower + Transverse Bending of Beam Anti-symmetric Transverse of of Bridge Tower + Transverse Bending of Beam Longitudinal Drift of the Beam Longitudinal Drift of the Beam First-order Symmetric Vertical Bending First-order Symmetric Vertical Bending First-order Transverse Bending First-orderSymmetric Symmetric Transverse Bending

6.4.Monitoring Monitoringand andEvaluation Evaluation 6.4. Wheninstalled installedcompletely, completely,the thehealth healthmonitoring monitoring system system should should be be debugged debugged as as aa whole. whole. When Beforethe thepassage passageofofthe thetrain, train,the thetime time02:00 02:00ininthe theearly earlymorning morningwould wouldbe beregarded regardedasasthe the Before referencetime, time, which would commence the acquisition of monitoring information constantly, reference which would commence the acquisition of monitoring information constantly, online, online, and dynamically. For the example, the GNSS automatically collected data at 10Meanwhile, s intervals. and dynamically. For example, GNSS automatically collected data at 10 s intervals. Meanwhile, the sensors acceleration sensors automatically collected data continuously. Other monitoring the acceleration automatically collected data continuously. Other monitoring contents contents the including the stress andforce the were cable collected force were collected at 10 min intervals. Then, the including stress and the cable at 10 min intervals. Then, the monitoring monitoringcould information could be the of server bytransmission. way of wiredUltimately, transmission. Ultimately, information be transmitted totransmitted the server bytoway wired the processing the evaluation processingof and evaluation of data was implemented. and data was implemented. Oneday dayisistaken takenasasan anexample. example.The Themonitoring monitoringand andevaluation evaluationofofsome somemeasured measuredparameters parameters One areexplained. explained. are

Longitudinal Deformation /mm

(1) Displacement: Displacement: via via the the baseline baselinecalculation calculation software software(spider) (spider)and andanalysis analysissoftware software(GeoMoS), (GeoMoS), (1) thespatial spatialdeformation deformationinformation informationof ofthe themid-span mid-spancollected collectedby byGNSS GNSScould couldbe bechanged changedinto into the the longitudinal, transverse, and vertical deformation with respect to the reference time. After the longitudinal, transverse, and vertical deformation with respect to the reference time. After thecorrection correctionof ofthe thetemperature temperaturefactor, factor,the themonitoring monitoringinformation informationwould wouldenter enterthe theevaluation evaluation the system,and andthe theresults resultsare areshown shownininFigures Figures9–11. 9–11. system, (2) Stress: the strain information would be collected by the vibrating wire strain sensor (including (2) Stress: the strain information would be collected by the vibrating wire strain sensor (including the function of temperature). Then, according to the constitutive relationship between stress the function of temperature). Then, according to the constitutive relationship between stress and and strain, the stress of the structure would be calculated under the loads. Via the evaluation strain, the stress of the structure would be calculated under the loads. Via the evaluation system, system, the evaluation results of the top slab and bottom slab stress along the longitudinal the evaluation results of the top slab and bottom slab stress along the longitudinal direction in direction in the mid-span are shown in Figures 12 and 13. the mid-span are shown in Figures 12 and 13. (3) Cable force: the measured information from the embedded and intelligent anchor cable meter (3) Cable force: the measured information from the embedded and intelligent anchor cable meter would be corrected by the temperature, and the real cable force would be obtained. Then via the would be corrected by the temperature, and the real cable force would be obtained. Then via evaluation system, the monitoring and evaluation results of the longest cable are shown in the evaluation system, the monitoring and evaluation results of the longest cable are shown in Figure 14. Figure 14. (4) Dynamic characteristics: when the temperature was stable and no train was passing, the (4) Dynamic characteristics: when the temperature was stable and no train was passing, the acceleration acceleration signal was sampled. Then, the signal was transformed by FFT (Fast Fourier signal was sampled. Then, the signal was transformed by FFT (Fast Fourier Transform), and the Transform), and the natural frequency of vibration was obtained. The monitoring result is natural frequency of vibration was obtained. The monitoring result is shown in Figure 15. shown in Figure 15. 30 20 10

Envelope Upper Bound Envelope Upper Bound ×0.75 Monitoring Value Envelope Lower Bound ×0.75 Envelope Lower Bound

0 -10 -20 -30

1

21

41

61

81

101

121

141

161

181

Acquisition Times of Monitoring Information

Figure9.9.Longitudinal LongitudinalDeformation Deformationofofthe theMid-span. Mid-span. Figure

Transverse Deformation /mm Transverse Deformation /mm Transverse Deformation /mm Transverse Deformation /mm Transverse Deformation /mm

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12of of16 16 12 12 of 16 12 12 of of 16 16 12 of 16

Envelope EnvelopeUpper UpperBound Bound Envelope Upper Bound Envelope Envelope Upper Bound EnvelopeUpper UpperBound Bound×0.75 ×0.75 Envelope Upper Bound Envelope Upper Bound Monitoring Value Envelope Upper Bound ×0.75 ×0.75 Monitoring Value Envelope Upper Bound ×0.75 Monitoring Value Envelope Lower Bound ×0.75 Monitoring ValueBound ×0.75 Envelope Lower Monitoring ValueBound Envelope Lower Bound Envelope Envelope Lower Bound ×0.75 EnvelopeLower Lower Bound×0.75 Envelope Envelope Lower Bound EnvelopeLower LowerBound Bound×0.75 Envelope Lower Bound

21 21 21 21 21

41 41 41

61 61 61

81 81 81

101 101 101

121 121 121

141 141 141

41 61 81 101 121 141 Acquisition Information Acquisition Times81of ofMonitoring Monitoring Information 41 61 Times 101 121 141 Acquisition Times of Monitoring Information Acquisition Times of Monitoring Information

161 161 161 161 161

181 181 181 181 181

Vertical Deformation /mm Vertical Deformation /mm Vertical Deformation /mm Vertical Deformation /mm Vertical Deformation /mm

Figure Figure 10. 10. Transverse Transverse Deformation Deformation of of the the Mid-span. Mid-span. Figure 10. Transverse Deformation of the Mid-span. Figure Figure10. 10.Transverse TransverseDeformation Deformationof ofthe the Mid-span. Mid-span. 100 100 100 100 100 00 00 0 -100 -100 -100 -100 -100 -200 -200 -200 -200 -200 1 1 11 1

Envelope EnvelopeUpper UpperBound Bound Envelope Upper Bound Envelope Envelope Upper Bound EnvelopeUpper UpperBound Bound×0.75 ×0.75 Envelope Upper Bound Envelope Upper Bound Monitoring Value Envelope Upper Bound ×0.75 ×0.75 Monitoring Value Envelope Upper Bound ×0.75 Monitoring Value Envelope Lower Bound ×0.75 Monitoring Value Envelope Lower Bound ×0.75 Monitoring ValueBound Envelope Lower Bound Envelope Envelope Lower Bound ×0.75 EnvelopeLower Lower Bound×0.75 Envelope Envelope Lower Bound EnvelopeLower LowerBound Bound×0.75 Envelope Lower Bound

21 21 21 21 21

41 41 41

61 61 61

81 81 81

101 101 101

121 121 121

141 141 141

41 61 81 101 121 141 Acquisition Information Acquisition Times81of ofMonitoring Monitoring Information 41 61 Times 101 121 141 Acquisition Times of Monitoring Information Acquisition Times of Monitoring Information

161 161 161 161 161

181 181 181 181 181

Figure Figure 11. 11. Vertical Vertical Deformation Deformation of of the the Mid-span. Mid-span. Figure VerticalDeformation Deformationofofthe theMid-span. Mid-span. Figure 11.11.Vertical Figure 11. Vertical Deformation of the Mid-span. Top Sab Stress /MPa Top Sab Stress /MPa Top Sab Stress /MPa Top Sab Stress /MPa Top Sab Stress /MPa

00 00 0 -1 -1 -1 -1 -1 -2 -2 -2 -2 -2 -3 -3 -3 -3 -3 -4 -4 -4 -411 -4 11 1

Envelope EnvelopeUpper UpperBound Bound Envelope Upper Bound Envelope Envelope Upper Bound EnvelopeUpper UpperBound Bound×0.75 ×0.75 Envelope Upper Bound Envelope Upper Bound Monitoring Value Envelope Upper Bound ×0.75 ×0.75 Monitoring Value Envelope Bound Monitoring Value Envelope Lower Bound ×0.75 Monitoring Value EnvelopeUpper Lower Bound×0.75 ×0.75 Monitoring ValueBound Envelope Lower Bound Envelope Envelope Lower Bound ×0.75 EnvelopeLower Lower Bound×0.75 Envelope Envelope Lower Bound EnvelopeLower LowerBound Bound×0.75 Envelope Lower Bound

21 41 61 81 101 121 21 41 61 81 101 121 21 41 61 81 101 121 21 41 61 81 101 121 Times Information Acquisition Times of Monitoring Monitoring Information 21 Acquisition 41 61 of 81 101 121

Acquisition Times of Monitoring Information Acquisition Times of Monitoring Information

141 141 141 141 141

Figure 12. Figure 12. Top Top Slab Slab Stress Stress of of the the Mid-span. Mid-span. Figure 12. Figure 12.Top TopSlab SlabStress Stressofofthe theMid-span. Mid-span. Figure 12. Top Slab Stress of the Mid-span. Bottom Slab Stress /MPa Bottom Slab Stress /MPa Bottom Slab Stress /MPa Bottom Slab Stress /MPa Bottom Slab Stress /MPa

22 22 2 11 11 1 00 00 0 -1 -1 -1 -1 -1 -2 -2 -2 -211 -2 11 1

Envelope EnvelopeUpper UpperBound Bound Envelope Upper Bound Envelope Envelope Upper Bound EnvelopeUpper UpperBound Bound×0.75 ×0.75 Envelope Upper Bound Envelope Upper Bound Monitoring Value Envelope Upper Bound ×0.75 ×0.75 Monitoring Value Envelope Bound Monitoring Value Envelope Lower Bound ×0.75 Monitoring Value EnvelopeUpper Lower Bound×0.75 ×0.75 Monitoring ValueBound Envelope Lower Bound Envelope Envelope Lower Bound ×0.75 EnvelopeLower Lower Bound×0.75 Envelope Envelope Lower Bound EnvelopeLower LowerBound Bound×0.75 Envelope Lower Bound

21 41 61 81 101 121 21 41 61 81 101 121 21 41 61 81 101 121 21 41 61 81 101 121 Times Information Acquisition Times ofMonitoring Monitoring Information 21 Acquisition 41 61 of 81 101 121

Acquisition Times of Monitoring Information Acquisition Times of Monitoring Information

141 141 141 141 141

Figure 13.13. Bottom Figure Bottom Slab Stress of the Mid-span. Figure 13. BottomSlab SlabStress Stressof ofthe theMid-span. Mid-span. Figure 13. Bottom Slab Stress of the Mid-span. Figure 13. Bottom Slab Stress of the Mid-span. Force ofof the Longest Cable/kN Force of the Longest Cable/kN Force of the Longest Cable/kN Force the Longest Cable/kN Force of the Longest Cable/kN

3200 3200 3200 3200 3200 3100 3100 3100 3100 3100 3000 3000 3000 3000 3000 2900 2900 2900 2900 2900 2800 2800 2800 280011 2800 11 1

Envelope EnvelopeUpper UpperBound Bound Envelope Upper Bound Envelope Envelope Upper Bound EnvelopeUpper UpperBound Bound×0.75 ×0.75 Envelope Upper Bound Envelope Upper Bound Monitoring Value Envelope Upper Bound ×0.75 ×0.75 Monitoring Value Envelope Upper Bound ×0.75 Monitoring Value Envelope Lower Bound ×0.75 Monitoring Value Envelope Lower Bound ×0.75 Monitoring ValueBound Envelope Lower Bound Envelope Envelope Lower Bound ×0.75 EnvelopeLower Lower Bound×0.75 Envelope Envelope Lower Bound EnvelopeLower LowerBound Bound×0.75 Envelope Lower Bound

21 21 21 21 21

41 41 41

61 61 61

81 81 81

101 101 101

121 121 121

41 61 81 101 121 Acquisition Times Information Acquisition Times ofMonitoring Monitoring Information 41 61 of 81 101 121 Acquisition Acquisition Times Times of of Monitoring Monitoring Information Information Acquisition Times of Monitoring Information

Figure 14. Figure 14. Force Force of of the the Longest Longest Cable. Cable. Figure 14. Figure 14.Force Forceofofthe theLongest LongestCable. Cable. Figure 14. Force of the Longest Cable.

141 141 141 141 141

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Figure Figure15. 15.Dynamic DynamicCharacteristics. Characteristics.

In order to clearly describe the results of the static characteristics, the evaluation coefficient F In order to clearly(3). describe the results of the static characteristics, the evaluation coefficient F is is defined in Equation defined in Equation (3). M FF= M, (3) (3) NN where: M the measured value of of thethe loading effects, N indicates the theoretical value value of the M indicates where: indicates the measured value loading effects, N indicates the theoretical loading effects. WhenWhen F is less orthan equal 1, it to indicates that the working performance of the F than is less orto equal 1, it indicates that the working performance of the loading effects. structure is good. When F is greater than 1, it indicates that the working performance is not ideal and of the structure is good. When F is greater than 1, it indicates that the working performance is not some defects may exist. ideal and some defects may exist. Therefore, based on Figures 9–15 and combined with the evaluation coefficient F, the conclusions Therefore, based on Figures 9–15 and combined with the evaluation coefficient F , the can be drawn as follows. conclusions can be drawn as follows. (1) Based Basedon onFigure Figure9,9,which whichwas wasaimed aimedat at longitudinal longitudinaldeformation deformationin inthe the mid-span mid-span section, section, the the (1) measuredextreme extreme M Mmax1 was −5 mm which meant the bridge was deviating to the river bank, measured max1 was −5 mm which meant the bridge was deviating to the river bank, and it was located in the normal state envelope interval [−17 mm, 17 mm]. The theoretical and it was located in the normal state envelope interval [−17 mm, 17 mm]. The theoretical extreme Nmax1 was −17 mm, so F1 was less than 1, which indicated that the longitudinal stiffness extreme N max1 was −17 mm, so F1 was less than 1, which indicated that the longitudinal was normal. was normal. (2) stiffness Based on Figure 10 which was aimed at the transverse deformation in the mid-span section, (2) Based on Figure 10 which was was aimed at the transverse in the mid-span section, the the measured extreme Mmax2 4 mm which meantdeformation the bridge was deviating from upstream measured extremeand M itmaxwas 4 mminwhich meant the was deviating from upstream to to downstream, located the normal statebridge envelope interval [− 63 mm, 63 mm]. 2 was

(3)

(3)

(4)

(4)

The theoretical Nmax2 was 63normal mm, sostate F2 was less than 1, which indicated thatThe the downstream, andextreme it was located in the envelope interval [−63 mm, 63 mm]. transverse extreme stiffness was theoretical N maxnormal. 2 was 63 mm, so F2 was less than 1, which indicated that the Based on Figure 11 which was aimed at the vertical deformation in the mid-span section, transverse stiffness was normal. the measured extreme Mmax3 was −24 at mm which meant the bridge in was deviating from upstream Based on Figure 11 which was aimed the vertical deformation the mid-span section, the to downstream, and it was located in the normal state envelope interval [−166 mm, 77 mm]. measured extreme M max 3 was −24 mm which meant the bridge was deviating from upstream The theoretical extreme Nmax3 was −166 mm, so F3 was less than 1, which indicated that the tovertical downstream, was located in the normal state envelope interval [−166 mm, 77 mm]. stiffnessand wasitnormal. The theoretical extreme N max −166 so slab F3 was less(point than 1, the 3 was Based on Figure 12 which was aimed atmm, the top stress 2) which in the indicated mid-spanthat section, vertical stiffnessextreme was normal. the measured Mmax4 was −2.56 MPa which indicated compressive stress, and it was Based oninFigure 12 which was aimed at the top[−slab in theThe mid-span section, the located the normal state envelope interval 2.79stress MPa,(point −0.78 2) MPa]. theoretical extreme Nmax4 wasextreme −2.79 MPa, less than whichindicated indicatedcompressive that the stiffness of the slab measured M maxso4 Fwas MPa 1,which stress, andtop it was 4 was−2.56 was normal. located in the normal state envelope interval [−2.79 MPa, −0.78 MPa]. The theoretical extreme

N max 4 was −2.79 MPa, so F4 was less than 1, which indicated that the stiffness of the top slab was normal. (5) Based on Figure 13 which was aimed at the bottom slab stress (point 5) in the mid-span section, the measured extreme M max 5 was −0.87 MPa which indicated compressive stress, and it was

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(6)

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Based on Figure 13 which was aimed at the bottom slab stress (point 5) in the mid-span section, the measured extreme Mmax5 was −0.87 MPa which indicated compressive stress, and it was located in the normal state envelope interval [−1.43 MPa, 1.35 MPa]. The theoretical extreme Nmax5 was −1.43 MPa, so F5 was less than 1, which indicated that the stiffness of the bottom slab was normal. Based on Figure 14 which was aimed at the cable force of the longest cable, the measured extreme Mmax6 was 2953.6 kN and it was located in the normal state envelope interval [2843.9 kN, 3107.3 kN]. The theoretical extreme Nmax6 was 3107.3 kN, so F6 was less than 1, which indicated that the cable force was normal.

In a word, when combined with the results above, the evaluation coefficient F = { F1 , F2 , F3 , F4 , F5 , F6 } was less than 1. So the static characteristics of the bridge were normal. Meanwhile, based on Figure 15, the measured first mode frequency (0.835 Hz) was greater than the calculated frequency (0.686 Hz). It indicated that the dynamic characteristics were normal. In summary, the static and dynamic characteristics of the Caijia Jialing River Bridge were all normal. The bridge structure was also in a normal state. 7. Conclusions (1)

(2)

(3)

The rapid development of sensor technology has provided strong technical support for the health monitoring and evaluation of long-span bridges. It is urgent to adopt new theories and new effects so as to study and develop bridge monitoring sensing devices which are reliable, stable, and durable. The health monitoring and evaluation projects of long-span bridges have been spawned by many committed companies. They are immersed in this smooth and prosperous situation. Moreover, the monitoring system including the sensors and the collection, transmission, processing, and management of sensing information, also has its own unique characteristics, advantages, and disadvantages, which are difficult to evaluate effectively. So following the maximization principle of the “cost-benefit ratio”, the optimization of the design can be carried out, and the monitoring standardization system can be established to realize standardization. The analysis of the engineering application demonstrates that the comprehensive evaluation can be effectively combined with individual control indicators. Also, the objective evaluation of the bridge state can be conducted. However, for long-span bridges, using new ideas and concepts to achieve practical safety evaluation techniques based on massive sensor information are still great challenges for engineers to overcome, including conducting structural damage prognosis and safety prognosis studies [40].

Acknowledgments: This work was supported by the National Science Fund for Distinguished Young Scholars (51425801), the National Key Research and Development Program of China (2016YFC0802202), the Chinese Academy of Engineering Consulting Project (2015-XZ-28, 2016-XY-22), the National Natural Science Foundation of China (51278512, 51508058, 11404045), the Social Livelihood Science and Technology Innovation Special of Chongqing (cstc2015shmszx30012), the Science and Technology Project of Guizhou Provincial Transportation Department (2016-123-006, 2016-123-039, 2016-123-040), the Science and Technology Project of Yunnan Provincial Transportation Department (2014 (A) 27), and the Communications Science and Technology Project of Guangxi Province of China (20144805). Author Contributions: J.Z. arranged all the work in the project and provided keen insight for this manuscript; X.L. and H.Z. investigated the current situation in this research field; H.Z. established the finite element model and obtained the calculated results; J.Y. analyzed the data; X.L. and R.X. wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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