Bridge Structural Health
Monitoring
A Key to Structural Performance
Evaluation and Infrastructure Asset Management
Abstract
The failure of structures during
their construction and service life has seen an upward trend in the recent
days. Challenging designs, demanding timeframes and unmanageable hugeness of
projects are said to be primary causes of the often-premature structural
failures. Many a time, the construction stage monitoring had revealed that the behavior
of the structural members during their construction and /or erection was far
different to what had been envisaged by the designer. The causes for these variations
are many - some are technical and others are commercial.
On the other hand, the performance
of the structures during their service life, from the day of their
commissioning, reduces with time, at whatever rate it could be. Time dependent
factors, like creep, material deterioration due to atmospheric exposure
conditions, cyclic loading, fatigue etc. exacerbate the situation. Corrosion of
reinforcing steel in concrete reduces the load carrying capacity of the
structures. The combined effect of the above factors poses higher risk to the
public and necessitated a thorough assessment of structures continuously. This
can be achieved by using the Structural Health Monitoring System (SHM) that
would enable diagnostic and prognostic assessment of the structures either
discretely or on real time basis. SHM is about assessing the performance of
structures either during its construction or during its service life, using a
variety of measurement techniques leading to "smart" structures. It
is also a process of implementing damage control strategy for engineering
infrastructures.
What is structural health
monitoring (SHM)?
The Structural Health Monitoring
can be defined in different ways. SHM is about assessing the performance of
structures either during its construction or during its service life, using a
variety of measurement techniques leading to "smart" structures. It
is also a process of implementing damage control strategy for engineering
infrastructures.
Key benefits of SHM for bridges
The key benefits of Bridge
Monitoring Solution for operators, constructors and designers are:
Avoidance of hazardous situations
and even lifesaving, by controlling access to the asset under hazardous
conditions.
Elimination of major breakdowns
through advance monitoring systems, warning harsh environmental conditions and/
or onset of structural problems.
Repair cost savings through
elimination of unexpected failures.
Maintenance cost savings through
extended life of expensive components.
Personnel cost savings and
motivation improvements through reduction of unnecessary inspection and
maintenance work and consolidation of monitoring to a central location
Status elevation of the
asset/site/project, through presentations of real-time environmental
conditions, traffic and structural information to decision makers and general
public.
When to monitor the structures?
Monitoring of the structures can
be done at every stage from construction to service life to service life
enhancement.
1. Construction stage monitoring-helps
to ensure structural behavior of the members, during construction. To quote a
few examples - monitoring of settlement, verticality of members, stress levels
during launching and erection, cantilever deflections, mid-span deflections,
stresses on the stay cables in form traveler operation etc.
2. Commissioning stage
monitoring-involves, monitoring during static/dynamic load tests, to ensure the
performance of the structure prior to putting the structure on service.
3. In-service monitoring-is a long-term
monitoring either on a periodical basis or on a continuous basis. This helps to
monitor the long-term behavior of the structures under service loads/
conditions. This includes, creep, fatigue, corrosion etc.
4. Maintenance stage Monitoring-Generally
this is to monitor the behavior of the structure during shut down for
maintenance. For example, behavior of the bridge girders when they are lifted
for changing defunct or defective bridge bearings, uneven loading of spans
during maintenance etc.
5. Pre and post restoration
measurements or monitoring-helps to ensure that the structure has been rendered
to normalcy after an intervention is affected.
6. Monitoring the structural effect-of partial demolition of a structure on the residual portion or comparing the structural behavior before and after the demolition /de-construction is done.
shm TECHNOLOGY
Multidisciplinary system
SHM technology has been extensively used in aeronautical and aerospace engineering since long time. However, its application in civil engineering is a reasonably recent development. SHM is a multidisciplinary system, as it employs the knowledge of various branches of engineering and technology such as sensing technology, power electronics, communication engineering, signal processing and structural engineering. The understanding of inter and intra disciplines are very important for the successful implementation of SHM to get optimum results. This complexity calls for trained and experienced personnel in SHM activities. The various technologies employed in SHM are illustrated in the following chart.
Principle sections of SHM
Fig 1. SHM system - Simultaneous deployment of various disciplines
Regardless of their extent, all SHM systems can be designed based on a common array that will consist of the following major section to organize the activities:
Sensing modules placed on the
structure - These modules consist of various types of sensors depending on the
nature of the structure. This also includes a signal collection, conditioning
and digitization unit.
Portable and/or fixed data
acquisition systems - These modules execute pre-processing and local buffering
for sensors distributed in a limited geographical area.
Data communication system for the
transfer of the collected data to a remote computer.
Data Processing and Control System
with database application - These modules collect, store and process the sensor
data in real time, in order to provide an evaluation of the condition of the
structure.
User Interface.
mAINTENANCE TOOLS
Typical layout of SHM system
Sensory systems
The sensory system will include
the sensors and their corresponding interfacing units for input signals
gathered from various monitoring equipments and sensors such as anemometers,
temperature sensors, dynamic weigh-in-motion sensors, corrosion cells,
hygrometers, barometers, rainfall gauges, digital video cameras, weldable
strain gauges, displacement transducers, global positioning systems, fixed and
removable accelerometers, etc.
Data Acquisition System
Only a decade ago A/D converters
were still expensive and therefore typical monitoring system designs were based
on either moving a portable logger around or by making star configurations with
analogue cables connected to multiplexers at a central A/D converter at an
acquisition unit. In this configuration problem with analogue noise and too
slow sample rates for dynamic incidents often occurs.
Now that A/D converters have
become relative inexpensive there is a tendency to use these converters right
at the sensor points, forming so called distributed data acquisition networks.
The benefit is that digital data can either be stored locally at the A/D
converter or sent through a LAN without any quality loss.
Modern measurement systems for
bridges tend to grow rather large. This forces the designer to take into
account not only the measurement that is done, but also the topology of the
network that connects the different measurement points.
Topology of network
In a simple situation when
different sensors are close to each other, it is possible that independent
sensor is connected to the data logger by its own dedicated cable. This
solution is intuitive and easy to implement, but it becomes complex to manage
when the number of sensors exceeds to over 25 and the distance is more than 80
meters.
For larger networks some form of
serial topology is necessary. This means a structure where one single cable
runs through all the sensors and they share this cable connection. This
arrangement works painlessly up to structure sizes of say 1000 m. after that it
is easy to split the network into several sub networks connected to a long
fibre optic main cable. The fibre optical truck can also be structured as a
logical loop which means that even if the fibre is damaged at any point, still
all the sensors points are accessible, ensuring a high level of redundancy.
Fibre optic cables (FOC) are an
easy way to control several common problems with long wired networks. It
eliminates problems like short-circuits or energy surges and interference from
a nearby lightning strike or electrical or magnetic fields. The downside of FOC
is that the costs of connecting to fibre cables are still rather high. It is
better to connect the sensors that are close to each other in a wired sub
network and then to let the data signal jump to the FOC in an easy to control
and well screened position.
The design of the main fibre and
the small sub networks ensures high redundancy and makes it easy to isolate
problem areas for maintenance. When the number of measurement points goes
beyond 4000 the maintainability and ability to isolate problematic areas
becomes a major factor. The larger the system the more important it is to keep
things simple and easy to control.
The normal mix of measurements
often has fast and slow measurements close to each other. For example
accelerometers, strain gages, wind vanes and temperature sensors. These would
normally be measured with speeds of 100 Hz, 10 Hz, 1 Hz and at every 10 minutes
respectively. It is important to be able to collect the data from, for example
strain gages and accelerometers in synchronization with each other, so that it
is possible to cross-correlate the wind and structural vibrations and note what
kind of stresses it causes on the structure.
Data Communication
The Data Acquisition Units (DAU or
Processor) shall be installed in the bridge-deck or bridge towers and will be
used for the collection and pre-processing of signals from the sensory system.
The local cabling network system
refers to the cabling network (shielded instrumentation cables) connecting and transmitting
signals from sensory system to respective DAU’s.
The global cabling network system
refers to the fibre optic cabling networks installed in the bridge-deck or
tower-shaft that are used for the transmission of digitized data from DAU’s,
GPS reference stations, Weigh in Motion systems and digital video equipment to
the SCADA (Supervisory Control and Data Acquisition) Room.
 Data Processing and Control System
The master screen can be either a
desktop display with the ability to transfer between different functions
quickly and easily, or a large display wall which can preferably be divided
into three large screens, build up of standard display modules, with the
following functions:
Provide overall control of the
DAUs through the Data Acquisition System (DAS) backbone network, regarding data
acquisition and processing, data transmission and filing control, data
archiving and backup, and all display and operational control.
Post-processing and analysis of
the collected data from the DAS.
Generation of instant monitoring
reports regarding the monitoring of loading sources and bridge responses.
 User Interface (UI)
The man-machine-interface is made
use of, for displaying views from the SHM systems in understandable and
readable graphical screens called mimic panels. The SHMS operator stations
display of structural events related to the maintenance of the bridge, as well
as allows the provision of remote control of threshold values for each SHMS
sensor system.
For a large SHM projects, the
system will visualize in real time all collected information, in the most
suitable way for an immediate and efficient representation (graphs, tables,
videos), and will allow research, visualization and elaboration at user
specified periods.
The large display wall can preferable
be divided into three large screens, built up of standard display modules, with
the following functions:
Screen 2: Technical systems status
and events monitoring
Screen 3: Safety systems status
monitoring (Surveillance)
The screens can be freely used by
the operator as desired. The required reports, either on real time basis or
historical can be generated by the operator as and when needed. Generally web
based User Interfaces are used for remote monitoring. The typical screen of an
UI is illustrated in Fig.
 Instrumentation of the bridge
The instrumentation essentially comprise of installation of the sensors at pre-determined locations on the bridge structure connected with different components such and A/D convertors and data loggers in the wired or wireless networks. The selection of the type of sensors and their locations are decided based on the inputs obtained in the Finite Element Analysis (FEA) model of the bridge to be monitored. A typical example of a bridge monitoring instrumentation scheme is illustrated in Fig.
User interface display of a typical mimic panel of a bridge monitoring system and alarms
The bridge responses can be
recorded either discretely or continuously. The responses are recorded in a
data logger which in turn can be transferred to a computer hard disc or can be
accessed remotely using the web based user interface. These responses can be
compared with that obtained from FEM model analysis in order to estimate the
health of the bridge.
Typical example of sensor
locations in bridge instrumentation
Levels of structural health
monitoring
The levels of structural health
monitoring can be classified in to four categories from the basic level to most
sophisticated levels as detailed below:
LEVEL I: This basic level SHM
system- It is capable of detecting damage in a structure, but cannot provide any
information on the nature, location, or severity of the damage. It cannot
assess the safety of the structure.
LEVEL II: Slightly more
sophisticated than Level I SHM systems-Level II systems can detect the presence
of damage and can also provide information of its location.
LEVEL III: A Level III SHM
system-It can detect and pinpoint damage, and can provide some indication of
its severity.
LEVEL IV: This most sophisticated
level of SHM systems- It is capable of providing detailed information on the
presence, location, and severity of damage and it is able to use this
information to evaluate the safety of the structural system.
Closing remarks
 The foregoing discussions can be summed up in
the following points:
Structural health monitoring
systems can be a right tool to make the structures (bridges) 'smart'.
Due to its multi-disciplinary
nature, SHM should be provided by the personnel trained in various faculties of
SHM.
SHM can be used for diagnostic and
prognostic purposes and also for assessing the residual life of the structures.