Monitoring Of Large Structures Research Articles

Measurement and Analysis of Wind-Induced Response of Tall Building Based on GPS Technology

The Global Positioning System (GPS) has been used in the health monitoring of large structures in recent years since it is weather independent, capable of autonomous operation, and does not require a line-of-sight between target points. In this paper, a structural health monitoring system based on the GPS is devised for a high-rise building. The field data, such as wind speed, wind direction and displacement responses were simultaneously and continuously measured under strong wind conditions. The relation between velocity components of the atmospheric wind and pressure fluctuations on the roof of the building are analyzed using the wavelet scalogram and wavelet coherence method, and a statistical significance level estimation method based on Monte Carlo simulation is presented to isolate the meaningful coherence from the spurious noise map. Finally, the responses of the high-rise building are investigated using the Periodogram method to conclude that the identified results agree well with the results computed by the Finite Element Method (FEM).

Identification of changes in the stiffness and damping matrices of linear structures through ambient vibrations

The practical difficulties presented by forced vibration testing of large structures, such as tall buildings, dams or bridges, led to a vast interest in structural monitoring through ambient vibrations, which usually allows the proper identification of modal properties, natural frequencies, damping and modes of vibration. Changes in these modal properties constitute an indication of structural damage, which may then be assessed on the basis of experimental evidence. The authors describe an approach to determine the so-called damage damping and stiffness matrices, which are essential to identify the location and intensity of damage. No restrictions are introduced on the damping matrix of the system. The approach requires ambient vibration data records of all relevant co-ordinates used in the structural model, which are processed employing the SSI-Cov method in conjunction with a modal analysis procedure. The identification method is illustrated with examples using experimental measurements on a reinforced concrete beam and a reduced scale multi-storey plane frame structure, also laboratory tested, as well as the theoretical simulated vibration response of a multi-storey concrete frame subjected to seismic excitation. Additional difficulties presented by field monitoring of large structures are briefly examined at the end of the paper. Copyright © 2007 John Wiley & Sons, Ltd.

Wireless Transmission for Health Monitoring of Large Structures

In order to monitor the health of large structures such as highways, bridges, buildings, and dams, the outputs from various kinds of sensors, which are positioned at different places of structures, should be collected simultaneously and/or sequentially. However, the placements of sensors and wire connections are time consuming and cost expensive. Furthermore, the wires are easy to be damaged during the placement operation or in service. In this paper, the configurations of the wireless health-monitoring systems for structures are discussed. A novel wireless signal retrieval system, which is suitable for transmitting wirelessly the response signals of large structures, is proposed and developed. Special attention is paid to the low-frequency characteristics of the wireless transmitter/receiver pairs due to the following two reasons: The response frequencies of large structures are very low; on the other hand, the commercially available transmitter/receiver pairs have poor characteristics at low frequencies. The principles, feasibility, and configuration of the wireless-transmission system for the health monitoring of large structures are investigated. Also, experiments for testing the system performances are performed in the laboratory. The results show that the proposed wireless-transmission method and the developed system are suitable for the in situ and in-service wireless health monitoring of large structures.

A carbon nanotube strain sensor for structural health monitoring

A carbon nanotube polymer material was used to form a piezoresistive strain sensor forstructural health monitoring applications. The polymer improves the interfacial bondingbetween the nanotubes. Previous single walled carbon nanotube buckypaper sensorsproduced distorted strain measurements because the van der Waals attraction force allowedaxial slipping of the smooth surfaces of the nanotubes. The polymer sensor uses largermulti-walled carbon nanotubes which improve the strain transfer, repeatability andlinearity of the sensor. An electrical model of the nanotube strain sensor was derived basedon electrochemical impedance spectroscopy and strain testing. The model is useful fordesigning nanotube sensor systems. A biomimetic artificial neuron was developedby extending the length of the sensor. The neuron is a long continuous strainsensor that has a low cost, is simple to install and is lightweight. The neuron has alow bandwidth and adequate strain sensitivity. The neuron sensor is particularlyuseful for detecting large strains and cracking, and can reduce the number ofchannels of data acquisition needed for the health monitoring of large structures.

Photogrammetric Techniques in Civil Engineering Material Testing and Structure Monitoring

Civil engineering material testing includes a wide range of applications requiring the determination of the threedimensional shape of an object and changes thereof. Large structure monitoring will often include the necessity of determining object deformations at a large number of points. Photogrammetric techniques offer a large potential for the solution of a wide range of measurement tasks in this field. A modular toolbox consisting of digital cameras, computer interfaces, illumination systems, calibration devices, combined with subpixel accuracy image measurement operators, multi-image matching techniques, and self-calibrating bundle adjustment in a suitable user interface, depicts a very powerful tool for tailoring custom-made solutions for material testing labs. Due to the wide range and the repetitive nature of measurements tasks in civil engineering, these applications could depict a significant future market for photogrammetry. This paper will briefly discuss the major hardware and software modules of a toolbox for civil engineering material testing and large structure monitoring. Based on several sample applications covering object dimensions from 10 cm to 500 meters, the potential of photogrammetric deformation measurement techniques will be shown. The major advantage of photogrammetric techniques can often be seen in the fact that they allow for highly automated measurements at a large number of points simultaneously. In many cases, object deformations can be determined at a precision in the order of 1:100,000 of the object dimension, based on off-theshelf hardware components.

Radiation-tolerant Raman distributed temperature monitoring system for large nuclear infrastructures

Raman Distributed Temperature Sensors (RDTS) are attractive for the monitoring of large structures in nuclear power plants such as containment structures and coolant loop systems. We demonstrate the high radiation tolerance of a Raman distributed fiber optic temperature sensor, up to total gamma doses in excess of 300 kGy, using a double-ended configuration and commercially-available optical fibers.

Spatial-frequency multiplexed fiber-optic Fizeau strain sensor system with optical amplification

A novel method for multiplexing fiber-optic Fizeau strain sensors with optical amplification is proposed and demonstrated. This method overcomes the two intrinsic disadvantages of fiber-optic Fabry–Perot (F–P) strain sensors, i.e. weak signal and difficult multiplexing. The amplified spontaneous emission (ASE) and optical amplification are used simultaneously to enhance the interferometric signal considerably. A Fizeau interferometer formed by two fiber ends with a quite different reflectivity is used to replace the F–P cavity in sensor head design. Such a Fizeau cavity can enlarge the cavity length by at least an order of magnitude and allows more than 10 sensors to be multiplexed simultaneously by using spatial-frequency multiplexing. The operating principle of the sensor system is discussed and an experiment is carried out to verify the concept of the method proposed. It is anticipated that such a sensor system could find important applications for health monitoring of large structures.

Subspace-based modal identification and monitoring of large structures: a scilab toolbox

Stochastic subspace-based structural identification and damage detection and localization methods are discussed, together with their implementation within the free INRIA software Scilab. Particular emphasis is put on the handling of high order models for large structures.

Application of Point and Distributed Optical Fiber Sensors to Health Monitoring of Smart Structures

Point optical fiber sensors are useful in the monitoring of localized structural damage, but a large number of the sensors must be multiplexed for large structure monitoring. On the other hand, distributed optical fiber sensors can obtain a continuous distribution of strain or temperature with one sensing fiber, and they are suitable for the large structure monitoring due to their measurement range reaching tens of kilometers. However, the distributed sensors have the spatial resolution of tens of centimeters to several meters, and they measure averaged strain or temperature. In this paper, the application results of transmission-type extrinsic Fabry-Perot interferometric (TEFPI) optical fiber sensors and Brillouin distributed optical fiber sensors to structural monitoring are presented. The TEFPI optical fiber sensors and Brillouin distributed sensors were applied to the fatigue damage monitoring of an aluminum plate patched with CFRP composite and the deflection monitoring of an alumimum-bending beam, respectively.

Structural monitoring ofa bending beam using Brillouin distributed optical fiber sensors

Due to the large measurement range, distributedoptical fiber sensors are very effective in the monitoring oflarge structures. Brillouin distributed optical fiber sensors,which measure strain by using Brillouin frequency shift, showsome measurement error for non-uniformly distributed strain because of their spatial resolution characteristics. Theyrequire compensation of measured values and positions in theend-parts of the measurement range because measured values arenot distributed over all the positions of the spatialresolution range. In this paper, we present the strain measurementcharacteristics of Brillouin distributed optical fiber sensors and the compensation method in the end-parts of the measurementrange. A deflection measurement method for a beamsubjected to a bending load using Brillouin distributedoptical fiber sensors is also presented. This is verified by athree-point bending experiment employing an 8 m aluminum beam. Theexperimental results showed that the measured value agrees with the one expected by Brillouin measurement theory.

Fibre optic sensor network for spacecraft health monitoring

Close meshed instrumentation or sensor networks applyingconventional sensors for temperature and strain monitoring may result inexcessive penalties in terms of weight constraints, reliability andsensitivity to environmental conditions, and complex interfaces. Thefibre-optic sensor network described in this paper is a multiplexed system offibre Bragg grating (FBG) strain and temperature sensors and was designed anddeveloped for a demanding space environment, but it can also be emphasized asa promising sensor technology with high potential for non-space applications.The FBG sensor network measures both strain and temperature at the measuringconditions of the structural core of the X-38 spacecraft, by means ofwavelength shifts due to tensile stress on a Bragg grating. Dependent on thefixation of the fibre, either isolated from or mechanically coupled to thestructure, local thermal or mechanical loads can be determined in thetemperature range from -40 to +190 °C, and in the strainrange from -0.1% to +0.3%. Short-term resolution and repeatabilityof the strain measurement amount to 5 µε and25 µε, respectively.The FBG sensor network is very suitable for structural health monitoring oflarge structures, i.e. to determine thermal and mechanical load profilesduring operation, to assess residual strength of structural elements orto detect irregular working conditions. In comparison to conventional sensorslike thermocouples and strain gauges, an FBG sensor network significantlyreduces the amount of required front end electronics (FEE) and harness.

Ranging of reflective markers in optical fiber sensors by double-pass phase modulation

A novel demodulation scheme for the ranging of reflective markers is presented. This method is useful for implementing in-line mul- tiplexing of displacement and temperature sensors based on low- coherence interferometry. It allows the discrimination of reflective mark- ers such as air-gap connectors or other partial reflectors spaced by as little as 0.1 m and the measurement of deformations occurring between two successive reflectors with a resolution of 2 mm. The multiplexing of two displacement sensors is demonstrated experimentally. This type of sensor is especially useful for the long-time geometrical monitoring of large structures. © 1997 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(97)01409-8)

Coherence multiplexing of in-line displacement and temperature sensors

A novel in-line multiplexing scheme for optical displacement and temperature sensors based on low-coherence interferometry is presented. It allows the measurement of deformations with a resolution of 10 μm and temperatures with a resolution of 1°C m. The multiplexing of two displacement sensors and one temperature sensor along the same fiber line is demonstrated experimentally, and a theoretical calculation for a system with 10 sensors is presented. This type of sensor would be especially useful for the long-time monitoring of large structures.

Low-coherence deformation sensors for the monitoring of civil-engineering structures

An optical-fiber deformation sensor with a resolution of 10 μm and an operational range of 60 mm has been realized. The system is based on low-coherence interferometry in standard single-mode telecommunication fibers. It allows the monitoring of large structures over several months without noticeable drift. No continuous measurement is needed and the system is insensitive to variations of the fiber losses. This technique has been applied to the monitoring of a 20 m × 5 m × 0.5 m, 120 ton concrete slab over six months. It is possible to measure the shrinkage of concrete and its elastic coefficient during pre-straining, giving reproducible results in good agreement with theoretical calculations and measurements performed on small concrete samples. This paper describes the optical arrangement and the procedures used to install optical fibers in concrete.

Interaction of Rayleigh waves with a rib attached to a plate

An analysis of the interaction of a pulse of Rayleigh waves on a plate with a rectangular flange or rib attached in the far field of its acoustic emission (AE) source is presented. A finite difference (FD) model is used to give numerical visualizations of the development of the complete wave-field propagating through the structure and also to give displacement data for selected points. An ‘energy partition model’, for Rayleigh wave propagation across the rib has been developed and is described. Ray theory wave arrival time calculations are given for the propagation of various mode-converted components transmitted across the flange. A series of experimental measurements of AE wave propagation across a T-butt welded rib on a plate were made and these are compared with the model predictions. The practical implications of these findings for acoustic emission monitoring of large structures of complex shape are discussed.