Use of sensors in Structural Engineering

1. INTRODUCTION
 Sensors and actuators are either embedded in or attached
to the system to form an integral part of it.
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 The structures which contain sensors, actuators can
respond to counteract loads, reduces vibration, change
shape and prevent their own degradation.
 The use of these sensors will improve the performance
of structures, safety, durability, reliability, decreased life
cycle costs and reduction of physical dimensions and
weight.

2. TYPES OF SENSORS
 Optical fiber sensors .
 Optical fiber Bragg grating sensors (FBGS).
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 Piezoelectric Sensors.
 Humidity Sensors.

3. OPTICAL FIBER SENSORS
 Fiber-optic sensors (also called optical fiber
sensors) are fiber-based devices for sensing some
quantity, typically temperature or strain, but
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sometimes also displacements, vibrations,
pressure, acceleration, rotation. measurement of
deformations, penetration of chemicals, and so
on.
 The general principle of such devices is that light
from a laser (often a single frequency fiber laser)
or from a superluminescent source is sent
through an optical fiber experiences subtle
changes of its parameters in the fiber and then
reaches a detector arrangement which measures
these changes.

4. ADVANTAGES
 They consist of electrically insulating materials.
 They can be safely used in explosive
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environments, because there is no risk of
electrical sparks.
 Their materials can be chemically passive, i.e., do
not contaminate their surroundings and are not
subject to corrosion.
 They have a very wide operating temperature
range .

5. OPTICAL FIBER BRAGG GRATING
SENSORS (FBGS)
 Optical fiber Bragg grating sensors are strain measuring
devices.
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 A principle operation of a fiber Bragg grating is
wavelength-dependent reflector formed by introducing a
periodic refractive index structure, with spacing on the
order of a wavelength of light, within the core of an
optical fiber.
 Optical Fiber Bragg grating sensors have been widely
used for monitoring of civil structures like highways,
bridges, buildings, dams, etc., remote sensing of oil
wells, power cables, pipelines, space stations, etc.

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7. ADVANTAGES
 FBGS can measure very high strain.
 FBGS are small sized and light weight.
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 FBGS are immune to electromagnetic
interference.
 FBGS signals are not distance dependent( up to
>50 km connection length is possible ).
 Long term stability is very high.
 Good corrosion resistance.
 Very low magnetic field interactions.
 Easy to install.

8. PIEZOELECTRIC SENSORS
 Piezoelectric sensors are used to detect crack in
structures.
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 Strain changes caused by cracks in the structure
are relatively small, and the resulting variations
of the voltage output of the piezoelectric sensors
(which are proportional to the strain changes) are
difficult to be measured.
 For this reason, in this detection strategy two
piezoelectric sensors are used.
 By conveniently measuring the voltage difference
between the two piezoelectric strip sensors, the
presence of cracks can be predicted.

9. HUMIDITY SENSORS
 Humidity sensor is base on the microwave
reflection of concrete at different moisture
condition.
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 The Humidity Sensor measures two parameters
for the evaluation of the humidity content of
concrete: electrical resistance and temperature.
 Those two measurements are performed at
different depths, typically placed between the
concrete surface and the reinforcement bars
depth.
 This allows the evaluation of water content ,
across the concrete depth.

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Installation of moisture sensor

11.  The Corrosion sensor is composed by 4 stainless steel
bars that are anchored to a stainless steel support.
 The 4 bars are placed in the concrete at 4 different
depths. The data logger measures the resistivity
between pairs of bars to determine the concrete
resistivity across the depth.
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 A low resistivity indicates higher humidity and vice
versa.
 The concrete temperature also influences the
resistivity and is therefore measured at two depths.
 This sensor is designed for installation in new
structures or for repair work, when the concrete cover
is replaced

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Corrosion sensor.

13. APPLICATIONS OF SENSORS
 Structural Health Monitoring (SHM).
 Inspections for Fatigue Cracks.
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 Seismic Damage Identification.

14. STRUCTURAL HEALTH MONITORING
(SHM)
 The process of implementing a damage detection
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and characterization strategy for engineering
structures is referred to as Structural Health
Monitoring (SHM).
 Here damage is defined as changes to the material
and/or geometric properties of a structural system,
which adversely affect the system’s performance.
 SHM aims to provide, in near real time, reliable
information regarding the integrity of the
structure.

15.  SHM technologies detect and identify the invisible
damage in the components before complete failure.
 The popular techniques for real-time SHM are
ultrasonic sensing (electronic) and fiber optic sensing.
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 Ultrasonic sensing offers a wealth of information and
straightforward damage detection.
 However, ultrasonic sensors and actuators require
separate electrical wiring and perform poorly in harsh
environments. Fiber optic sensors, on the other hand,
offer stability in harsh environments.

16.  These sensors provide real time monitoring of various
structural changes like stress and strain.
 In the case of civil engineering structures, the data
provided by the sensors is usually transmitted to a
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remote data acquisition centre's.
 With the aid of modern technology, real time control of
structures (Active Structural Control) based on the
information of sensors is possible.

17. THE BIGGEST ON-GOING BRIDGE
MONITORING PROJECTS
 The Rio–Antirrio Bridge, Greece: has more than 100
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sensors monitoring the structure and the traffic in real
time.
 Millau Viaduc, France: has one of the largest systems
with fiber optics in the world which is considered state
of the art.
 The Huey P Long Bridge, USA: has over 800 static and
dynamic strain gauges designed to measure axial and
bending load effects.

18.  The Fatih Sultan Mehmet Bridge, Turkey: also known
as the Second Bosphorus Bridge. It has been monitored
using an innovative wireless sensor network with
normal traffic condition.
 Tsing Ma, Ting Kau, and Kap Shui Mun bridges, Hong
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Kong: approximately 900 sensors are monitoring the
structure

19. INSPECTIONS FOR FATIGUE CRACKS
 The ultrasonic interrogation of components for the
detection and sizing of defects has advantages over other
techniques in that surface as well as subsurface defects
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can be probed.
 The experimental configuration of our ultrasonic
measurements is such that a fatigue crack is illuminated
by a longitudinal wave incident perpendicular to the
crack face and focussed in the plane of the crack.
 The longitudinal wave is partially transmitted by the
closure zone and picked up by another focused receiver
transducer.

20.  By changing the angular orientation of this receiver, one
can monitor the frequency, spatial, and angular
dependence of the crack transmissivity.
 The Eddy current system can be used for inspections of
fatigue crack.
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21. SEISMIC DAMAGE IDENTIFICATION
 A distributed fiber optic monitoring methodology based
on optic time domain reflectometry technology is
developed for seismic damage identification of steel
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structures.
 Sensing the local deformation of the structure, the epoxy
modulates the signal change within the optic fiber in
response to the damage state of the structure.
 Damage can be identified by the optic sensors, and its
maximum local deformation can be recorded by the
sensing system; moreover, the damage evolution can also
be identified.

22. CONCLUSION
 For the safety of a structure its strength and integrity, the
sensors are used to provide real time information of the
structure.
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 Sensors can be used to study the dynamic measurement,
in the structural elements and structures buildings,
bridges, dams and tanks.
 Because of use of these sensors we are able to get real
time information of the structure and further decisions
can be made with available information regarding
structure strength and its life.
 The development and the use of these sensors are to be
encouraged for the safety of the structures and life

23. REFERENCES
 www.opticsinfobase.org
 Dan Mateescu, Yong Han and Arun Misra “Dynamics of
Structures with Piezoelectric Sensors and Actuators for
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Structural Health Monitoring” Key Engineering Materials Vol.
347 (2007) pp 493-498.
 Encyclopedia of Laser Physics and Technology - fiber-optic
sensors.
 Manfred Kreuzer “Strain measurement with fiber Bragg grating
sensors”.
 Liu, Y. (2001). Advanced fiber gratings and their application.
Ph.D. Thesis, Aston University.

24.  C.R. Farrar.; S. W. Doebling and D. A. Nix (2001).
"Vibration-Based Structural Damage Identification".
Philosophical Transactions of the Royal Society
 M. Raymond. (2001). Structural Monitoring with Fiber
Optic Technology. San Diego, California, USA:
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Academic Press. pp. Chapter 7.