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Article published in Structural Concrete, Journal of the fib
(post-print version)
Design and implementation of a monitoring system
applied to a long-span prestressed concrete bridge
Helder Sousa, Carlos Félix, João Bento, Joaquim Figueiras
By contractual commitment, during the construction phase a set of periodic
observation reports were delivered to the bridge owner twice a week. Those reports
included (1) drawings with the positioning of all installed sensors; (2) the main events
organized as a schedule; (3) time series charts of the recordings and (4) summary
tables with the main statistical results. Figure 11 illustrates the information included
in the periodic observation reports delivered twice a month during the bridge
construction. This task allowed for a closer checking of the monitoring system during
its installation and has revealed to be useful in evaluating the structural response
during the construction process, which is one of the most important stages of the
structure’s life. Since opening to traffic, an observation report including all the
sensors’ records and main statistical information is delivered every semester to be
analyzed and accounted for.
Although the bridge maintenance includes comprehensive visual inspections
every 6 years, the sensors readings are extra knowledge to help in the interpretation of
damages identified in the visual inspections. Moreover, the monitoring system is
permanently informing about the bridge performance, reason why if abnormal values
are read at any time between those 6 years campaigns, the owner is able to proceed to
an extraordinary visual inspection, to facilitate interpretation of the situation.
Likewise, if during a regular visual inspection any given pathology is noticed, the
owner can resort to the model and to the history of measured data, as to promote a
better interpretation of the situation at hand.
6. Conclusions
The present paper described in detail the procedures related to the design and
installation of a concrete bridge monitoring system spanning from construction to life
cycle surveillance. The project complexity and its scale were thoroughly illustrated
adopting a hands-on approach and reflecting an implementation perception. Several
hierarchical stages had to be crossed to turn this system into a physical and
manageable reality, with emphasis in three fundamental phases:
(1) A conceptual design based in a set of structured documents. Due to the
system complexity, these documents were crucial for the following work stages. The
definition of intermediate objectives was an efficient strategy, with a full detail of all
work steps involved since the preparatory works until the desired measurements in
format of graphs and tables. It is fundamental to have a full pre-vision of the system
that integrates different systems (static, dynamic and optical systems) and components
to anticipate potential difficulties and/or problems in the implementation stage.
(2) Installation works that were performed during the bridge construction. The
document “C-Specifications and Procedures” [18], elaborated in the previous phase,
was an important guide for the installation works. Namely for a better mutual
understanding between the constructor and the monitoring team, and provide all the
necessary conditions for the implementation of the system. The monitoring
requirement during the bridge construction led to the installation team to explore
capabilities such as dynamism, flexibility, adaptability and integration to follow the
rhythm imposed by the construction works (often 24h/day). After the installation,
several tests were needed to consider the system ready and operational in full mode.
In a monitoring system like that of Lezíria bridge, it is fundamental to waterproof and
seal all the connection boxes and sensors, to maximize the system robustness and
durability in a long-term management process.
(3) Data acquisition and treatment was conceived to deliver to the
management authority the desire graphs and statistical tables. The reading procedures
for normal and alarm modes were established according to the project requirements,
and the collected measurements are stored in a remote database linked to the field
system via optical cable. The fact that the monitoring system has been operating since
the installation of the first sensors has the advantage of a closer checking of the
construction process, as well as the evaluation of the structural response from the
beginning of construction. Since the opening to traffic, the monitoring system has
been working in full mode, and periodical reports are delivered to the owner. The
possibility of crossing information with the visual inspections can certainly benefit the
surveillance and management of the bridge.
Acknowledgements
As usual for in-situ works, there are many relevant if not decisive personal contributions. It would be
impossible and displaced to mention them exhaustively here. Nevertheless, the authors wish to thank to
all those who contributed to the success of the implementation of this system, including the LABEST
team, the NewMENSUS, the contractor TACE, and the bridge owner, BRISA SA. The first author also
expresses thankfulness to the Portuguese Foundation for Science and Technology for the funding of his
doctoral studies.
Table 1 – Executive Project organization (Figueiras et al., 2007a). System installation
Document Objective Before During After
Presentation
Document
Project Executive organization; objectives description of each
document.
����
Project brief Monitoring system definition and specifications, namely:
sensors; acquisition systems; communication network; data
treatment and management software.
����
Contract drawings Plans and sections drawings of the monitoring system
implementation, namely: the instrumented sections; sensors;
acquisition nodes; cables path.
����
Specifications and
Procedures
Definition, sequence and description of a set of tasks to consider
during the monitoring system installation.
����
Observation
reports - Bridge
construction
Biweekly reports with the records obtained during the bridge
construction through time series graphs and summary tables
with the mainly statistical results.
����
Final report Verification of compliance of the monitoring system 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 parameters 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 description of the monitoring system
with: alert levels defined by the designers; operational mode in
terms of service; maintenance plan; recommendations to the
good practice; procedures to detect and correct possible fails.
����
Observation
reports - Service
life
Semestral reports with the records obtained during the service
life of the bridge through time series graphs and summary tables
with the mainly statistical results.
����
Table 2 – Characteristics and location of measured parameters. Instrumented zones Threshold
Parameter Acquisition
system
Measuring
Frequency Objective Soil Piles Pillars Bearings Deck
Surveillan
ce *a) Alert *b)
Strain Electric /
Optic
Static /
Dynamic
Concrete deformation ���� ���� ���� n/d
275mm 350mm
*c)
520mm 675mm
*d)
315mm 380mm
Relative
horizontal
displacement
Electric Static Relative
displacement
between pillar and
deck and in
expansion joints
����
*e)
Rotation Electric Static Rotation of structural
elements
���� n/d
Temperature Electric
Optic
Static Environment and
concrete
temperatures
���� ���� ���� n/d
Relative
Humidity
Electric Static Environment relative
humidity
���� n/d
-9.8m -13.5m Scour Electric Static Scouring ����
*f)
Durability Electric Static Corrosion potential in
reinforcing steel near
the concrete surface
���� ���� * g)
Acceleration Electric Dynamic Accelerations in three
orthogonal directions
of soil and structure
���� ���� ���� 0.05g 0.10g
50mm 100mm Vertical
displacement
Optic Static Vertical
displacements of the
main bridge
����
*h)
* 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 allowed for the joint expansion of the North approach Viaduct.
* d) Maximum values allowed for the joint expansion of the main Bridge.
* e) Maximum values allowed for the joint expansion for the South approach Viaduct.
* f) Maximum values, considering as 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 reinforcements will occur in half of the remaining lifetime of
the structure, with a minimum of 10 years.
* h) Maximum value allowed for the longest spans of the main bridge.
Figure 1 – Lezíria Bridge: a) north approach viaduct, b) main bridge, c) south
approach viaduct.
Figure 2 – Components of the monitoring system for the Lezíria bridge.
Figure 3 – Architecture of sensorial component.
Figure 4 – Constituents of sensorial component.
Figure 5 – Communication network integrating the various AN's in the CAN.
Figure 6 – Laboratory preparation works.
Figure 7 – First installation step of the monitoring system.
Figure 8 – Second installation step of the monitoring system.
Figure 9 – Peculiar tasks of the monitoring system installation.
Figure 10 – Image manual, waterproofing and sealing.
Figure 11 – Information included in the periodic observation reports during the bridge
construction.
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