use of sensors in structural engineering by pirpasha ujede

Pirpasha Ujede http://www.facebook.com

/pirpashaINTRODUCTION

Sensors and actuators are either embedded in or attached to the system to form an integral part of it.

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.


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


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OPTICAL FIBER SENSORS Fiber-optic sensors (also called optical fiber

sensors) are fiber-based devices for sensing some quantity, typically temperature or strain, but 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.


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ADVANTAGES They consist of electrically insulating

materials. They can be safely used in explosive

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 .


/pirpashaOPTICAL FIBER BRAGG GRATING SENSORS (FBGS) Optical fiber Bragg grating sensors are strain measuring

devices. 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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ADVANTAGES FBGS can measure very high strain. FBGS are small sized and light weight. 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.


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PIEZOELECTRIC SENSORS Piezoelectric sensors are used to detect crack in

structures. 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.


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HUMIDITY SENSORS Humidity sensor is base on the microwave

reflection of concrete at different moisture condition.

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


/pirpasha 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.

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.


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


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STRUCTURAL HEALTH MONITORING (SHM)

The process of implementing a damage detection 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.


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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.

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.


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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 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.


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THE BIGGEST ON-GOING BRIDGE MONITORING PROJECTS The Rio–Antirrio Bridge, Greece: has more than 100

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.


/pirpasha 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 Kong: approximately 900 sensors are monitoring the structure


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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 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.


/pirpasha 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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SEISMIC DAMAGE IDENTIFICATION A distributed fiber optic monitoring methodology based

on optic time domain reflectometry technology is developed for seismic damage identification of steel 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.


/pirpashaCONCLUSION

For the safety of a structure its strength and integrity, the sensors are used to provide real time information of the structure.

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


/pirpashaREFERENCES

www.opticsinfobase.org Dan Mateescu, Yong Han and Arun Misra “Dynamics of

Structures with Piezoelectric Sensors and Actuators for 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.

http://www.opticsinfobase.org/


/pirpasha 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: Academic Press. pp. Chapter 7.

use of sensors in structural engineering by pirpasha ujede

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