design and implementation of a monitoring system applied to a long-span prestressed concrete bridge

82 © 2011 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Structural Concrete 12 (2011), No. 2

Articles

Helder SousaCarlos FélixJoão BentoJoaquim Figueiras

DOI: 10.1002/suco.201000014

Currently, long-term monitoring systems are mandatory for majorcivil engineering structures such as bridges, tunnels and dams.Generally, they monitor a set of physical, chemical and mechani-cal parameters in critical sections of the structure by incorporat-ing appropriate sensors. The set of data collected demonstratesgreat potential in the prevention of damage and contributes tomore efficient maintenance of the structures monitored.This work presents the long-term monitoring system installed onthe new Lezíria Bridge over the River Tagus in Portugal. The sys-tem was developed to control some aspects of the constructionprocess and to survey the service life of the structure. A set ofstructural, durability and environmental parameters defining thebridge condition are remotely assessed in real-time via a fibre-optic network. Aspects such as architecture, installation andfunctionality of the monitoring system are discussed and the in-novative aspects of the implementation are highlighted.In this context, the main goal of this work is to present the long-term monitoring system of Lezíria Bridge, sharing the experiences, the solutions and the procedures adopted, giventheir potential usefulness in the implementation of similarprojects.

Keywords: long-term surveillance, concrete bridges, monitoring systems,system implementation, project management

1 Introduction

One current major concern related to large infrastructuresis their increasing age and the implied inspection andmaintenance costs. A major focus regarding this matterhas been awarded to bridges and high-rise structures [1].Bridges, in general, are experiencing accelerated deterio-ration and are becoming more and more exposed to wearand tear as time progresses because they were designedwhen the demand for transportation facilities was not ashigh as it is today. The weights of vehicles and the in-crease in traffic are critical aspects [2]. Maintenanceworks related to structural problems such as joints andbearings are critical because experience has shown thatthese are the items that suffer from premature wear, thusrequiring careful and regular maintenance procedures [3].Another common problem in bridges is the loss of sedi-ments around and under the bridge footings due to scour,which can lead to excessive pier movement, creating un-wanted stresses in the bridge structure that may eventu-ally lead to failure or collapse [2]. Human error is also a

critical issue in a structure’s health. In Korea severalman-made disasters were registered in the 1990s as a con-sequence of the country’s modernization without a corre-sponding integrative moral basis [4].

In the case of long-span bridges, the effectiveness ofvisual inspections in reaching all the critical locations andfinding all the possible defects becomes especially ques-tionable. In the United States, a study by the FederalHighway Administration (FHWA) revealed that at least56 % of the average condition ratings were incorrect, witha 95 % probability from the visual inspection (2001). Itfollows that if health monitoring could be designed andimplemented as a complement to visual inspection, to en-hance its effectiveness and mitigate its shortcomings,bridge owners would decide to take advantages of thisnew paradigm [5].

Structural health monitoring (SHM) is a subject ofmajor international research. While in the past this topicwas mainly addressed from the angle of sensors, now thepractical implications regarding the acquisition, collectingand processing of data are being addressed [1]. Today it ispossible to monitor highly instrumented structures contin-uously and remotely, with a high degree of automation.Present solutions are versatile enough to allow for surveil-lance tasks to be realized remotely with sound cost-effec-tiveness [6]. This is performed by measuring a set of phys-ical and chemical parameters with appropriate sensors,which allow the permanent control of critical parametersthrough a compatible acquisition and communication sys-tem, allowing automatic and remote storage in a database,often accessible through the Internet. In general, continu-ous measurement at low frequencies over a long time (e.g.hourly measurements) would be needed to capture thetrends in climate- and weather-related inputs, changes inground and soil, the movements of the foundations andthe superstructure. Programmed, as well as triggered, in-termittent measurements would be needed for shorter pe-riods at higher frequencies for capturing the operationaland corresponding structural parameters [5]. Over thepast decade there have been several full-scale demonstra-tion projects that have involved varying degrees of SHMtechnologies for short- and long-span bridges [5]. In aEuropean research project (Smart Structures [7]), innova-tive and inexpensive probes for monitoring existing con-crete structures were developed, tested and integrated intoa monitoring system to reduce inspection and mainte-

Design and implementation of a monitoring systemapplied to a long-span prestressed concrete bridge

83

H. Sousa/C. Félix/J. Bento/J. Figueiras · Design and implementation of a monitoring system applied to a long-span prestressed concrete bridge

Structural Concrete 12 (2011), No. 2

nance costs and traffic delays [1]. In Hong Kong and Chi-na, SHM is currently included as a standard mechatronicsystem in the design and construction of most large-scaleand multi-disciplinary bridge projects [8].

In this context it is important that new bridges areequipped with monitoring systems from the beginning oftheir operation. It is envisaged that the cost of the moni-toring system and the perpetual cost of its maintenanceare expected to protect the much higher investment in thebridge construction and its operating costs [5]. Monitor-ing the condition of an existing highway bridge structurehelps to ensure its safety with regard to life extension andreplacement strategies [6].

This article presents the monitoring system imple-mented in a recent bridge built in Portugal: Lezíria Bridge.The main scope is to show how a complex process, relatedto the implementation of a monitoring system, was guidedto obtain the intended solution. After a general descrip-tion of the bridge, the monitoring system is detailed inthree main parts:1. The process, where issues related to sensor type, acqui-

sition systems and communication with a remote data-base are described.

2. The installation, with reference to laboratory worksand some of the most complex and particular fieldworks associated with the sensors’ installation.

3. The records, referring the reading procedures adoptedand how this information is organized and delivered tothe bridge owner.

2 Lezíria Bridge2.1 The socio-economic context

Lezíria Bridge, built between 2005 and 2007, forms part ofthe A10 highway from Bucelas to Carregado (A1)/IC3(A13). With a total length of 39.9 km, this road forms anouter boundary to the Lisbon metropolitan area. It bene-fits those who wish to travel to or from Alentejo or the Al-garve (A2 highway) and Spain (A6 highway) without hav-ing to cross the Portuguese capital. In addition, this newbridge will improve accessibility between Vila Franca deXira and the Samora Correia/Benavente locations, sub-stantially relieving traffic on the national roads EN10 andEN118 [9].

2.2 The structure

The 11670 m total length of Lezíria Bridge is realized bythree substructures:1. The north approach viaduct, 1700 m long.2. The main bridge substructure, crossing the River Tagus,

with a total length of 970 m.3. The largest substructure, the south approach viaduct,

with a total length of 9160 m.

Fig. 1 illustrates the construction of the three substruc-tures.

2.2.1 North approach viaduct

The north approach viaduct provides the connection tothe A1 highway. It has three elementary girder viaducts,

with spans of 33 m, except where it crosses a railway line,where the largest span is 65 m and is partially formed by abox girder. The viaduct deck is supported by piers on pilesthat in some cases are 40 m deep. The railway line thatcrosses the north viaduct and the existence of an electricpower plant and a neighbouring substation were con-straints considered in the project [9].

2.2.2 Main bridge

The main bridge structure is formed by eight spans andseven piers supported on pilecaps over the river bed. Thespans are 130 m long except the end spans, which are

c) south approach viaduct

Fig. 1. Lezíria Bridge

a) north approach viaduct

b) main bridge

84



95 m, and two of the middle spans that differ in length by5 m due to a change in a pier implementation, leading tospans of 125 m and 135 m. The bridge deck is realized bya box girder of variable inertia which is about 10 m wideand has a depth varying between 4 and 8 m. The box gird-er core construction was made by segmental constructionusing a movable scaffolding, whereas the side consoleswere constructed subsequently, supported by a differentmovable scaffolding and metal struts fixed in the bottomslab of the box girder, as it is illustrated in Fig. 1b. Theconcrete piers are formed by four walls with constantthickness and variable width, and are supported on pile-caps (8 piles in general and 10 in the two pile caps adja-cent to the navigation channel).

2.2.3 South approach viaduct

The south approach viaduct has 22 elementary viaductswith extensions ranging from 250 to 530 m. It has a spanlength of 36 m, with exceptions due to the existence of ir-rigation canals and dykes in the Lezíria fields. The deckfloor is supported by precast beams cast on site, where aindustrial precasting unit was specifically set up, being theprecast elements monolithically linked to the piers. Theviaduct deck is formed by precast slabs, supported on theprecast beams, and serves as formwork for the in situ topconcrete layer. As with the north viaduct, the viaduct deckis supported by piers on piles varying from 35 to 60 mdeep, given the need for crossing alluviums with variableproperties.

2.3 The main concepts of the monitoring system

A project such as the long-term monitoring system forLezíria Bridge is complex and has a broad scope. Never-theless, it may be unfolded in a sequence of three mainstages:1. The process, which includes all the development stages

up until execution, ending with the document for exe-cution – Executive Project.

2. The installation, which includes all the work that al-lows the full implementation of the Executive Project.

3. The records, organized as a database representing thefinal product.

In the area of bridge monitoring, the long-term monitor-ing system of Lezíria Bridge presents a number of innova-tive aspects in comparison with other bridge monitoringprojects. The structural and durability monitoring projectis part of the bridge design tasks from the beginning,through a specific project volume entitled “Structural andDurability Monitoring Plan” [10]. As part of that projectvolume, the long-term monitoring system was subjectedto successive versions with the participation of differententities, such as the bridge owner, the designer, the con-tractor, various consultants and SHM experts in order tobring together and coordinate a variety of interests andpoints of view. The final version of the monitoring systemproject, the Executive Project, through its organization,contents and objectives, is a reference document forstructural and durability bridge monitoring in Portugal.Following the specification and process definition phase,

the installation of the monitoring system began. Over aperiod of approx. 18 months, a highly specialized teamhad many installation tasks. The key to the successful in-stallation of the monitoring system resulted from theknow-how, dynamism, flexibility, adaptability and com-mon understanding qualities of the team.

The results obtained are an essential source ofknowledge which in its present state provides a valuablebasis for further research in the domain of structural mon-itoring. That source of knowledge represents a great po-tential both for the damage prevention and managementof the monitored bridge.

3 The monitoring system of Lezíria Bridge – the process

The Executive Project of the Structural and DurabilityMonitoring Plan was fully defined as a set of structureddocuments. Those documents, although prepared in ad-vance, were finished during the implementation phase,following an “opening drawers” process [11]. Each docu-ment has a defined objective and a chronological relation-ship with the other documents as specified in Table 1.

Following this concept, the process evolved in atimely manner and it was completed step by step, with aset of clear intermediate objectives, until final handover tothe owner. The process organization took into accountsome special features of these systems, especially issuesrelated to its conclusion. The finalization of such aprocess does not necessarily coincide with the installa-tion of all equipment, cables and devices. Operationalconclusion effectively occurs only after a certain periodafter the physical installation. In that time, the installedmonitoring system is submitted to a meticulous valida-tion process.

The monitoring system integrates all the electrical/electronic components, sensors, automatic acquisitionsystem and data treatment/management through an opti-cal fibre communication network that also enables remoteaccess. Such a system has an high degree of complexity,with three main components:a) sensorial component,b) communication component, andc) data treatment and management component (Fig. 2).

This architecture offers the client a set of continuous andsimultaneous records of the parameters observed, with ca-pabilities for surveillance and prevention of structuralsafety and durability [11].

The “A – Project brief” document [12] contains a de-tailed description of the monitoring system adopted, withspecial attention paid to the selection of sensors and ac-quisition systems, the communication network, the inte-gration of all systems, the data treatment and the manage-ment software (Table 1).

3.1 Sensorial component

Considering the structure to be monitored and what it isintended to measure, a number of critical points are select-ed for monitoring a set of parameters. In this context, thesensorial component is based on the installation of, on theone hand, appropriate sensors to perform the measure-

85



ments and, on the other, compatible acquisition systems toperform the signal processing and store the readings.

In the case of Lezíria Bridge, the instrumentationconsisting of those sensors and acquisition systems is dis-tributed over a number of structure zones as follows: twozones on the north approach viaducts, the whole of themain bridge length and four zones on the south approachviaducts. A set of instrumented sections defines a moni-tored zone and the corresponding acquisition system tointerrogate the sensors is called an Acquisition Node(AN). A set of sensors is installed in each section to mea-sure the intended parameters. Suitable cables connect thesensors to the acquisition systems, with the connectionnodes made in Junction Boxes (JB) and Signal ConnectionBoxes (SCB) as shown in Fig. 3 [12].

Fig. 4 illustrates the sensorial component. It identi-fies the parameters measured, shows the project symbolsand abbreviations used and the types of acquisition sys-tems adopted to interrogate the signals from the sensorialcomponent.

3.1.1 Static acquisition system

The static acquisition system interrogates 80 % of the totalnumber of installed sensors. Consequently, the monitoringsystem adopted for Lezíria Bridge is oriented towardslong-term monitoring. Strains, rotations, displacements,corrosion, scour and environmental parameters are thoseconsidered for static monitoring (Fig. 4). The records ob-tained can be used to analyse the structure from the point

Table 1. Executive project organization (Figueiras et al., 2007)

Document Objective System installation

Before During After

Presentation Document Executive project organization, description of objectives of ✓

each document

Project brief Monitoring system definition and specifications, namely: ✓

sensors, acquisition systems, communication network, data treatment and management software

Contract drawings Plan and section drawings of monitoring system ✓

implementation, namely: instrumented sections, sensors,acquisition nodes, cables path

Specifications and procedures Definition, sequence and description of a set of tasks to ✓

consider during monitoring system installation

Observation reports – Bi-weekly reports with the records obtained during the bridge ✓

bridge construction construction through time series graphs and summary tables with the main statistical results

Final report Verification of compliance of the monitoring system as ✓

installed, including: detailed location of the sensors installed in each section, table of calibration constants by sensor to convert the electrical or optical signal to the physical para-meters intended to measure, sensor reference readings on which all measurements will be based

Technical compilation Detailed technical specifications of each type of sensor, their ✓

guarantee and certificates of conformity provided by the manufacturers

Operations manual Software and hardware descriptions of the monitoring system ✓

with: alert levels defined by the designers, operational mode in terms of service, maintenance plan, recommendations regarding good practice, procedures to detect and correct possible failures

Observation reports – Semestral reports with the records obtained during the service ✓

service life life of the bridge through time series graphs and summary tables with the main statistical results

Sensorialcomponent

Communicationcomponent

Data treatmentand management

component

Fig. 2. Components of the monitoring system for Lezíria bridge

86



of view of its behaviour with respect to environmental ef-fects (e.g. temperature, relative humidity), time-dependenteffects (e.g. shrinkage, creep, loss of prestress) and the in-teraction between the structure and the surrounding soil.

All these sensors are interrogated by the same acqui-sition system group, where it is possible to define readingprocedures, with options available for defining the acquisi-tion frequency of each sensor, for ordering the records se-quence and storing those records in data files.

3.1.2 Dynamic acquisition system

The number of sensors interrogated by the dynamic acqui-sition system represents about 5 % of the total number ofsensors installed. The main goal of these sensors is tomonitor the accelerations induced in the structure andsurrounding soil caused by earthquakes or boat collisions.

The configuration adopted means it will be possible toanalyse the energy transmitted from the soil to the struc-ture and its dissipation effects on the structural elements.The possibility of identifying the occurrence of thoseevents with the dynamic system can provide valuable in-formation for the interpretation of the long-term behav-iour when changes in the pattern of evolution over timecan be justified by those events.

Triaxial accelerometers are used to measure the ac-celerations at a specific point (structure or soil). Thosesensors measure and record the accelerations in three or-thogonal directions and are connected to the acquisitionsystem by armoured cables specially manufactured for thistype of device. An acquisition system supplied by the samemanufacturer performs the sensor interrogation. The soft-ware managing the dynamic acquisition system allows analarm levels definition for each sensor and/or measure-ment axis, and such alarms can be sent to a particular ad-dress as an IP message.

3.1.3 Optical acquisition system

The last decade has witnessed huge developments in theapplication of optical fibre sensors, in particular Bragggrating sensors in civil engineering structures [13], [14],[15]. With recognized advantages such as immunity toelectromagnetic fields and low signal losses, and also dueto the application of multiplexing techniques – where sig-nals of multiple sensors can be sent through a single opti-cal fibre –, Bragg sensors represent one of the mostpromising sensing technologies for use in civil engineeringstructures [6]. With this technology it is possible to encap-sulate long lengths of optical fibres carrying the signals ofvarious sensors to one acquisition system located at a spe-cific point [12].

Considering the properties referred to and the socio-economic importance of Lezíria Bridge, it was decided toinstall an optical acquisition system based on the afore-mentioned Bragg grating sensors. The sensors, speciallydeveloped for this purpose [16], are an integral part of thestructural and durability monitoring system. They accountfor the remaining 15 % of the total number of sensors in-stalled for measuring vertical displacements, strains andtemperatures [12].

The sensor interrogation is performed by a compati-ble acquisition system. The management software for the

NORTH VIADUCT MAIN BRIDGE SOUTH VIADUCT

AN1V2N

AN2V2N

CANAN1PT AN2

PTAN3PT

AN1V1S

AN1V2S

AN1V14S

AN2V14S

SCB

Sensor 2.1

Sensor 2.2

Sensor ...

Sec

tion

2

Sec

tion

1

Sec

tion

1

Sec

tion

1

Sec

tion

1

Sec

tion

1

Sec

tion

1

Sec

tion

1

Sec

tion

2

Sec

tion

2

Sec

tion

2

Sec

tion

2

Sec

tion

2

Sec

tion

2

Sec

tion

...

Sec

tion

...

Sec

tion

...

Sec

tion

...

Sec

tion

...

Sec

tion

...

Sec

tion

...

Sec

tion

...

Sec

tion

...

Sec

tion

2

Sec

tion

2

Sec

tion

1

Sec

tion

1

JB

Fig. 3. Architecture of sensorial component

durability

scou

r

dynamic

parameters

para

met

ers

para

met

ers

parameters

parameters

structural

electric(dynamic) optic

Acquisitionsystem type

electric (static)

ACSC

DV

EX

DA

SO

HR

ST

RO DJ

HR Relative Humidity

RO Rotation

DA Bearings displacements

DV Vertical displacement

SC Corrosion

ST Temperatute

DJ Joints displacements

EX Strain

AC Aceleration

SO Scour

envi

ronm

ent

Fig. 4. Constituents of sensorial component

87



readings of the optical sensors was developed based onthe software of the manufacturer’s acquisition system [17].The optical acquisition system aims to expand the infor-mation about the structural behaviour of the main bridgeand also to compare the efficiency of this system with theelectrical monitoring system [12].

3.2 Communication component

Because of the length of this bridge, the Acquisition Nodes(AN) of the monitoring system are physically distant fromeach other. For example, the distance between the two ex-treme ANs is about 8 km. Consequently, a local communi-cation network was installed, allowing for the integrationand centralization of the information recorded by the dif-ferent ANs in a single place, the Central Acquisition Node(CAN). Opting for a local communication network hassimplified remote access to the monitoring system by al-lowing access to every device in the system through a sin-gle CAN. The communication network, also using opticalfibres, has two rings with nine nodes (one for each of thenine instrumented zones) matching the various ANs andis shown schematically in Fig. 5.

Lezíria Bridge is part of the motorway network oper-ated by BRISA. It is, accordingly, included in BRISA’s so-phisticated communication network covering the whole ofits motorway system. In order to enable remote access tothe bridge monitoring system, a link was established be-tween the local communication network and BRISA’scommunication network. A dedicated server installed atBRISA’s Operations Control Centre manages this link andallows for direct and permanent communication with theCAN (Fig. 5).

3.3 Data treatment and management

A dedicated software module was developed for datatreatment and management purposes. It also provides themain database updating functions and enables the visual-ization of results. In addition, the system has a consulta-tion module covering the technical information about theinstalled system [12]. In terms of data treatment, the dataupdating module distinguishes durability and dynamic pa-rameters from the remaining ones, with a specific proce-dure available for each case [12]. After selecting the sen-sors, the visualization module delivers results in bothtabular and graphic forms. The graphics allow the obser-

vation of the time pattern of the sensors selected, individ-ually or grouped by monitored sections. The consultationmodule provides all the technical information about themonitoring system installed, such as the location of the in-strumented sections, and a description of the sensors in-stalled in each one. It is also possible to create data files intext format for external processing as well as to generatereports automatically with the intended graphical results.Moreover, the software is prepared to notify the bridgeowner by e-mail if the values measured by the sensors ex-ceed the threshold values previously defined by the bridgedesigner [12].

4 Monitoring system of Lezíria Bridge – the installation

The guidelines for the installation of the monitoring sys-tem were the documents of the Executive Project: “A – Project Brief” [12], “B – Contract Drawings” [18] and“C – Specifications and Procedures” [19]. Document Cwas specifically developed to guide the installation works,taking into account the size and complexity of the moni-toring system. This project document, prepared to antici-pate and organize a set of tasks to be carried out duringthe installation works, covered aspects such as:1. Organization of laboratory tasks in order to minimize

the field works.2. Sequence and interdependence of in situ work to mini-

mize repetition of procedures.3. Phasing of construction tasks in order to anticipate

scenarios, optimize allocation of resources and mini-mize human input.

4.1 Preparation and organization of laboratory works

The success of the in situ installation heavily depends onthe preparatory work carried out in the laboratory. Toprepare the equipment and organize cables and acces-sories, it is essential to have a strong laboratory team. Aset of normalized verification procedures has to be ap-plied to all equipment, cables and accessories, and nomaterial should leave the laboratory without passingthrough these. Fig. 6 illustrates normalized proceduresconsidered in the preparation and organization of labora-tory work, including (a) calibration and verification ofsensors, (b) setup of interfaces for the in situ installationof sensors, and (c) preparation, identification and protec-tion of cables.

NORTH VIADUCT MAIN BRIDGE SOUTH VIADUCT

CAN

CONNECTINGBOX

AN1V2N

AN2V2N

AN1PT AN2

PTAN3PT

AN1V1S

AN1V2S

AN1V14S

AN2V14S

Fig. 5. Communication network integrating the various ANs in the CAN

88



4.2 Bridge instrumentation

To monitor a concrete bridge during its service life, thesystem installation may be implemented in two separatesteps: firstly, all the embedded sensors are installed duringthe concreting phases, and, secondly, upon completing ofthe concreting works, all the complementary work to con-clude the installation. However, in the presence of a mon-itoring requirement during the bridge construction, the in-stallation of Provisional Acquisition Nodes (PAN), inaddition to the embedded sensors, is compulsory. More-over, cabling rails and connections for the constructionphase are also provisionally placed. Thus, a truly tempo-rary monitoring system is put in place, as part of the firstinstallation step.

Given the importance of Lezíria Bridge, the monitor-ing process covered the construction period. With thatproposal, three of the nine instrumented zones were mon-itored during the bridge construction. This fact requiredgreat commitment from the installation team to follow therhythm imposed by the construction works (often

24h/day) and, at the same time, to obtain successfulrecordings. From the above it becomes evident that moni-toring projects addressing construction phases implygreater complexity and demand higher commitment fromthe installation teams than when monitoring service lifeonly. Fig. 7 illustrates some of the first monitoring works,performed during the construction of Lezíria Bridge, inparticular (a) the embedded sensors installation, (b) theprovisional Acquisition Node installation, (c) the provi-sional cables path, and (d) provisional connections. It ismandatory for the anticipation and preparation of all nec-essary procedures to guarantee a robust installation thatwill stand up to the aggressiveness of the concreting oper-ations.

Independently of the bridge being monitored duringconstruction, the installation of the embedded sensors isfollowed by a short period for the implementation of thepermanent monitoring system, which generally coincideswith the finishing works of the bridge. As shown in Fig. 8,the monitoring system implementation requires, addition-ally, (a) the installation of the external sensors, (b) the de-

Fig. 6. Laboratory preparation work

a) sensor verifications b) sensor holders c) cable preparation

Fig. 7. First installation step of the monitoring system

a) embedded sensors b) Acquisition Node c) cables path d) provisional conections

Fig. 8. Second installation step of the monitoring system

a) external sensors b) permanent Acquisition Node c) permanent cables path d) permanent conections

89



finitive installation of the acquisition nodes, (c) the cablespassing through the technical rails and pipes, and (d) theprovision of the connection boxes.

Bridge finishing is the most intensive working peri-od, with the simultaneous presence of multiple workteams to fulfil all sorts of diversified works that must befinished before the inauguration date. This fact leads toincreased pressure upon the installation work, thus forc-ing longer daily working periods. At this stage, capabilitiessuch as dynamism, flexibility, adaptability and integrationare crucial to the success of the system installation.

Given their particular complexity or special features,the installation of some sensors deserves a special men-tion. That is the case of (1) pile strain gauges, (2) soil ac-celerometers, (3) sonar in pile heads, and (4) the verticaldisplacement system.

4.2.1 Pile strain gauges

Twelve strain gauges were installed inside a pile of themain bridge in a rather remarkable fashion. After the piledriving, the installation was carried out in three mainphases:1. Installation of the vibrating wire strain gauges using

steel bars with an appropriate fixing system to place thetransducers inside the holes left for cross-hole acoustictests at three different levels (1, 5 and 35 m).

2. After the initial works for the pile head execution, thestrain gauges were tested immediately before the holeswere sealed.

3. The cables previously placed at the pile top (to avoidconnections inside the concrete) were routed throughthe pile head up to the pier base. Fig. 9a shows theplacement of one of these sensors in the hole.

4.2.2 Soil accelerometers

The accelerometers installed in the soil were inserted in aborehole through an inclinometer tube and positioned atdifferent depths (over a range of 1 to 40 m), with final ce-ment sealing in order to achieve a good connection withthe surrounding soil [12], [18], [20]. This installation wasparticularly difficult because the sensors were installed

with the total cable length necessary to reach the AN on the bridge deck. This option had the advantage ofavoiding additional wire connections, but required thecables to be passed through the inclinometer tube to theacquisition system node beforehand. To illustrate theeffort involved, the longest cable was > 300 m long and itspathway developed, sequentially, through (1) a cabletrench, (2) a pipe installed over the height of a pier, (3)along the technical paths of the border deck, and, finally,(4) inside the box girder of the main bridge up to the ac-quisition system. Fig. 9b illustrates the placement of oneof the inclinometer tubes in the borehole, with theaccelerometer rigidly positioned at the end of the tube inadvance.

4.2.3 Sonar devices

The sonar devices were installed after the bridge construc-tion due to the particular conditions during the construc-tion period in the pile head (placement of scaffolding andthe backrest of boats to support the construction). In or-der to prevent the impact of objects floating in the river orboat collisions, the mechanical protection of the sensorand its maintenance were crucial aspects considered inthe installation. The sonar devices were installed at thebottom of the pile head on the upstream side. To place itin its resting position, the sonar device was fixed to the ex-tremity of a metal tube inserted in a stainless steel sectionspecially designed to protect the sonar setup. Fig. 9cshows the positioning of one of the sonar devices insidethe stainless steel tube.

4.2.4 Vertical displacement system

A liquid levelling system was installed along the entirelength of the main bridge to allow the measurement ofvertical displacements (deflections and settlements). Forthat purpose, a specialized team installed a hydraulic cir-cuit after the main bridge box girder was finished. After in-stallation, the pile system was filled with water and purgedfor possible air inside the hydraulic circuit. Finally, thesensors were installed and connected to the hydraulic cir-cuit, after which a calibration routine was performed by

Fig. 9. Particular tasks of the monitoring system installation

a) strain gauges in pile b) soil accelerometer c) sonar protection d) hydraulic circuit

90



varying the circuit water level to ensure the adequate per-formance of the system. Fig. 9d illustrates one of the con-tainers fixed to the girder wall as well as the hydraulic cir-cuit and the reference sensor.

4.3 Testing and final checks

Before delivering the monitoring system to the owner, aseries of final tests and checks were performed in order toverify its full performance. Those tests and final checksare crucial for the identification and correction of anyanomalies that might have occurred during installation,given the well-known aggressiveness of construction envi-ronments. Several tests are performed:1. Signal verification of all sensors.2. Verification of cable integrity.3. Verification of the acquisition systems up-state.4. Verification of communication and data transmission

to the Operations Centre.

After the conclusion of the tests and final verification, themonitoring system was considered ready to operate in fullmode.

4.4 Image manual, waterproofing and sealing

At this stage it was possible to produce an image-basedmanual. The layout of this manual aims to offer acomprehensive view and provide an easy understandingof all equipment in the monitoring system. Moreover,it facilitates future intervention in the system, as de-scribed in [18]. The so called Image Manual is in fact a set of identification plates, plastic sheets and user man-uals.

The connection boxes were all waterproofed to max-imize system up state and durability. Finally, the entiresystem was sealed to prevent and trace any unauthorizedintervention that would otherwise not be easily de-tectable. Fig. 10 illustrates some finishing works, in partic-ular (a) the identification plates provided for an externalsensor protected by a protection box, (b) plastic sheetswith useful information about the monitoring system in-side an acquisition node box, (c) waterproofing the tubesentering an acquisition node, and (d) sensor sealing toprevent unauthorized access.

5 Monitoring system of Lezíria Bridge – the records5.1 Reading procedures

The records obtained so far are defined by reading proce-dures previously established according to the project ob-jectives. For the static parameters resulting from interro-gation by the electrical acquisition system, the samplingrate adopted is one sample every 3 hours in normalmode. In alarm mode the acquisition frequency can beincreased to one sample per minute. In the case of thedynamic parameters, the accelerometer sampling rate isestablished at 200 Hz by default and the system is perma-nently alert, with the sensors’ readings continuouslysaved in a ring buffer. If an alarm level is reached, anevent occurs and the system creates a set of files with therespective sensor readings and a warning message. Forthe optical sensors the sampling rate adopted is one sam-ple every 3 hours in normal mode. In alarm mode the ac-quisition frequency can be increased up to 500 Hz in thecase of the fibre-optic strain sensors [17]. Table 2 showsthe parameters monitored and also sensor type, acquisi-tion system type, sample rate acquisition, bridge zoneswhere sensors are installed and their purpose, and thethresholds for surveillance and alarm levels defined by the bridge designer [10]. The threshold values forsome parameters have not been defined yet (n/d inTable 2): their evaluation depends on the first years ofobservation.

5.2 Monitoring records

As previously mentioned, the Lezíria Bridge monitoringsystem has been operating since the installation of the firstsensors, thus enabling monitoring of the structural behav-iour during construction. During this period it was possi-ble to monitor some construction operations, such as con-creting operations, prestressing, falsework disassemblyand movements of the movable scaffolding used for seg-mental construction (main bridge), application of forcesat the closing sections (main bridge) and load tests forstructural behaviour conformity upon at completion ofconstruction.

After the bridge was finished, and with the monitor-ing system in full operation, some effects concerned withthe service life of the structure have been monitored,

Fig. 10. Image manual, waterproofing and sealing

a) identification plates b) user manuals c) waterproofing d) system sealing

91



namely: environmental effects, shrinkage and creep evolu-tion, and traffic load effects.

The contract called for a set of periodic observationreports to be delivered to the bridge owner twice a weekduring the construction phase. Those reports includeddrawings with the positioning of all installed sensors, themain events organized as a schedule, time series charts ofthe recordings and summary tables with the main statisti-cal results. Fig. 11 illustrates the information included inthe periodic observation reports delivered twice a monthduring the bridge construction. This task allowed for acloser check of the monitoring system during its installa-tion and has proved to be useful in evaluating the struc-tural response during the construction process, which isone of the most important stages of a structure’s life. Sinceopening to traffic, an observation report, including all thesensor records and main statistical information, is deliv-ered every semester in order to be analysed and accountedfor.

Although the bridge maintenance includes compre-hensive visual inspections every six years, the sensor read-ings represent extra knowledge to help in the interpreta-tion of damage identified in the visual inspections.Moreover, the monitoring system provides informationabout the bridge performance permanently, thus enablingthe owner to organize an extraordinary visual inspectionto facilitate an interpretation of the situation if abnormalvalues are read at any time between those campaignsevery six years. Likewise, if during a regular visual inspec-tion any given pathology is noticed, the owner can resortto the model and to the history of measured data in orderto promote a better interpretation of the actual situation.

6 Conclusions

The present paper described in detail the procedures relat-ed to the design and installation of a concrete bridge mon-itoring system spanning from construction to life cycle

Table 2. Characteristics and locations of measured parameters

Parameter Acquisition Measuring Objective Instrumented zones Thresholdsystem frequency

Soil Piles Piers Bearings Deck Surveillance*a) Alert*b)

Strain Electrical/ Static/ Concrete ✓ ✓ ✓ n/dOptical Dynamic deformation

Relative Electrical Static Relative ✓ 275 mm 350 mm*c)horizontal displacement displacement between pier 520 mm 675 mm*d)

and deck and at expansion 315 mm 380 mm*e)joints

Rotation Electrical Static Rotation of ✓ n/dstructural elements

Temperature Electrical/ Static Environment ✓ ✓ ✓ n/dOptical and concrete

temperatures

Relative Electrical Static Environment ✓ n/dHumidity relative humidity

Scour Electrical Static Scouring ✓ –9.8 m –13.5 m*f)

Durability Electrical Static Corrosion potential ✓ ✓ *g)in reinforcing steel near concrete surface

Acceleration Electrical Dynamic Accelerations in three ✓ ✓ ✓ 0.05 g 0.10 gorthogonal directions in soil and structure

Vertical Optical Static Vertical displacements ✓ 50 mm 100 mm*h)displacement of main bridge

*a) The surveillance levels are determined for the frequent combination of actions, with a limit of L/2500 [10].*b) The alert levels are determined for the characteristic combination of actions, with a limit of L/1200 for the main bridge and L/1000 to

L/600 for the approach viaducts [10].*c) Maximum values allowing for joint expansion in north approach viaduct.*d) Maximum values allowing for joint expansion in main bridge.*e) Maximum values allowing for joint expansion in south approach viaduct.*f) Maximum values, considering as a reference the riverbed elevation at the end of the bridge construction.*g) The alarm is triggered when the penetration of aggressive agents can predict that the depassivation of the reinforcement will occur in half

of the remaining lifetime of the structure, with a minimum of 10 years.*h) Maximum value allowing for the longest spans of the main bridge.

92



surveillance. The project’s complexity and its scale werethoroughly illustrated adopting a hands-on approach andreflecting an understanding of implementation. Severalhierarchical stages had to be crossed to turn this systeminto a physical and manageable reality, with emphasis onthree fundamental phases:1. A conceptual design based on a set of structured docu-

ments. Due to the system’s complexity, these docu-ments were crucial for the subsequent work stages. Thedefinition of intermediate objectives was an efficientstrategy, with full detail of all work steps involved fromthe preparatory works to the desired measurements inthe form of graphs and tables. It is fundamental to havea full advance vision of the system that integrates dif-ferent systems (static, dynamic and optical systems)and components to anticipate potential difficultiesand/or problems at the implementation stage.

2. Installation works that were performed during thebridge construction. Document “C – Specifications andProcedures” [18], elaborated in the previous phase, wasan important guide for the installation works, i.e. for abetter mutual understanding between the contractorand the monitoring team, providing all the conditionsnecessary for the implementation of the system. Themonitoring requirement during the bridge constructionled to the installation team exploring capabilities suchas dynamism, flexibility, adaptability and integration tofollow the rhythm imposed by the construction works(often 24h/day). After installation, several tests were

needed to consider the system ready and operational infull mode. In a monitoring system like that of Lezíriabridge, it is vital to waterproof and seal all the connec-tion boxes and sensors in order to maximize the systemrobustness and durability in a long-term managementprocess.

3. Data acquisition and treatment was conceived to sup-ply the management authority with the graphs and sta-tistical tables required. The reading procedures for nor-mal and alarm modes were established according to theproject requirements, and the measurements collectedare stored in a remote database linked to the field sys-tem via fibre-optic cables. The fact that the monitoringsystem has been operating since the installation of thefirst sensors has the advantage of a closer check of theconstruction process as well as the evaluation of thestructural response from the beginning of construction.Since being opened for traffic, the monitoring systemhas been working in full mode, and periodic reports aredelivered to the owner. The possibility of combining in-formation with the visual inspections can certainlybenefit the surveillance and management of the bridge.

Acknowledgements

As usual for in situ works, there were many relevant, if notdecisive, personal contributions. It would be impossibleand inappropriate to mention them all here. Nevertheless,the authors wish to thank to all those who contributed to

Fig. 11. Information included in the periodic observation reports during bridge construction

93



the success of the implementation of this system, includ-ing the LABEST team, NewMENSUS, the contractorTACE and the bridge owner BRISA SA. The first authorwould also like to express gratitude to the PortugueseFoundation for Science and Technology for the PhD grantSFRH/BD/29125/2006.

References

1. Auweraer, H. V., Peeters, B.: International Research Projectson Structural Health Monitoring: An Overview. Journal ofStructural Health Monitoring, 2 (4), 2003, pp. 341–358.

2. Minchin, Jr., et al.: Best Practices of Bridge System Manage-ment – A Synthesis. Journal of Management in Engineering,22 (4), 2006, pp. 186–195.

3. Brito, J., et al.: ’Inspectionability’ of bridges. Structural Con-crete, the official Journal of the fib, 3 (1), 2002, pp. 29–34.

4. Chang, S. P., et al.: Necessity of the bridge health monitoringsystem to mitigate natural and man-made disasters. Struc-ture and Infrastructure Engineering, 5 (3), 2009, pp.173–197.

5. Pines, D., Aktan, E.: Status of structural health monitoringof long-span bridges in the United States. Progress in Struc-tural Engineering and Materials, 4 (4), 2002, pp. 372–380.

6. Bergmeister, K., Santa, U.: Global monitoring concepts forbridges. Structural Concrete, the official Journal of the fib, 2(1), 2001, pp. 29–39.

7. BRITE/EURAM, SMART STRUCTURES – Integrated Mon-itoring Systems for Durability Assessment of Concrete Struc-tures Online. Project No. BRPR-CT98-0751, 1997. Availablefrom: http://smart.ramboll.dk/smart_eu/index.htm [ac-cessed October 2008]

8. Wong, K. Y.: Design of a structural health monitoring systemfor long-span bridges. Structure and Infrastructure Engi-neering, 3 (2), 2007, pp. 169–185.

9. Oliveira, C. B.: New crossing of the Tagus river from Car-regado to Lezíria through A10. Ingenium, No. 95, 2006 (inPortuguese).

10. COBA-PC & A-CIVILSER-ARCADIS – Construction of theTagus Crossing in Carregado Subsection A1/Benavente, ofA10 Highway Bucelas/Carregado/IC3; Volume XV – Struc-tural and Durability Monitoring Plan. Contract of Concep-tion, Design and Construction of Crossing the Tagus in Car-regado, Portugal, 2006 (in Portuguese).

11. Figueiras, J., et al.: Construction of the Tagus Crossing inCarregado Subsection A1/Benavente, of A10 Highway Buce-las/Carregado/IC3; Structural and Durability MonitoringExecutive Project 0 – Presentation. LABEST, Faculty of En-gineering, University of Porto, 2007 (in Portuguese).

12. Figueiras, J., et al.: Construction of the Tagus Crossing inCarregado Subsection A1/Benavente, of A10 Highway Buce-las/Carregado/IC3; Structural and Durability MonitoringExecutive Project A – Project Brief. LABEST, Faculty ofEngineering, University of Porto, 2007 (in Portuguese).

13. Masskant, R., et al.: Fiber Optic Bragg Grating Sensors forBridge Monitoring. Cement and Concrete Composites, 19(1), 1997, pp. 21–33.

14. Inaudi, D.: (1999) Long-gage fibre optic sensors for structur-al monitoring. In P. K. Rastogi (ed.), Optical MeasurementTechniques and Applications. Artech House, 1997, pp.255–275.

15. Fernando, G. F., et al.: Structural Integrity Monitoring ofConcrete Structures via Optical Fiber Sensors: Sensor Pro-tection Systems. Journal of Structural Health Monitoring, 2(2), 2003, pp. 123–135.

16. Rodrigues, C., Félix, C., Figueiras, J.: “Fiber Optic StrainTransducer – Conception, analysis and calibration.” InternalReport, LABEST, Faculty of Engineering, University ofPorto, 2007 (in Portuguese).

17. Figueiras, J., et al.: Construction of the Tagus Crossing inCarregado Subsection A1/Benavente, of A10 Highway Buce-las/Carregado/IC3; Structural and Durability MonitoringExecutive Project G – Operations Manual. LABEST, Facultyof Engineering, University of Porto, 2007 (in Portuguese).

18. Figueiras, J., et al.: Construction of the Tagus Crossing inCarregado Subsection A1/Benavente, of A10 Highway Buce-las/Carregado/IC3; Structural and Durability MonitoringExecutive Project B – Contract Drawings. LABEST, Facultyof Engineering, University of Porto, 2007 (in Portuguese).

19. Figueiras, J., et al.: Construction of the Tagus Crossing inCarregado Subsection A1/Benavente, of A10 Highway Buce-las/Carregado/IC3; Structural and Durability MonitoringExecutive Project C – Specifications and Procedures.LABEST, Faculty of Engineering, University of Porto, 2007(in Portuguese).

20. Rudaz, S., et al.: AC-23-DH Downhole Accelerometer – In-stallation Instructions – Version 1 online.. GeoSIG Ldt.2006. Available from: http://www.geosig.com/downloads/manuals/AC-23-DH_Installation_Instructions.pdf [accessed16 September 2008]

Joaquim FigueirasFull Professor, LABESTFaculty of EngineeringUniversity of Porto

João BentoCivil Engineering,Executive Director,BRISA – Auto-Estradasde Portugal S.A.

Carlos FélixCivil Engineering, LABESTSchool of EngineeringPolytechnic Institute of Porto

Helder Sousa (corresponding author)Civil Engineering, LABESTFaculty of EngineeringUniversity of [email protected]

design and implementation of a monitoring system applied to a long-span prestressed concrete bridge

Documents