Long-gauge Fiber Optic Sensors Research Articles

A two-level strategy to assess the bearing capacity of corroded structures via long-gauge fiber-optic sensors

Corrosion of steel in concrete is the most common deterioration mechanism for structures located in aggressive environments. This research proposes a two-level strategy to assess the residual bearing capacity of corroded structures by detecting and quantifying corrosion via a new kind of long-gauge fiber Bragg grating (FBG) sensors. Compared with the traditional point strain gauges, this new sensor has been developed for both local and global structural monitoring by measuring the averaged strain within a long gauge length. According to the mechanical relationship between the moving load and the long-gauge strain influence line (IL), an IL aera-based difference index is established to identify the presence of the damage and the corrosion is quantified in terms of reduction of sectional area of reinforcement (Level 1); and then an elaborate finite element (FE) model is built to perform failure analysis to assess the residual bearing capacity by modifying the material constitutive model according to the corrosion condition (Level 2). A laboratory experiment is conducted to verify the effectiveness and accuracy of the proposed method.

Condition assessment of concrete piers subjected to impact load using fiber optic sensing

A novel strategy for condition assessment of concrete piers subjected to impact load is presented in this article. The method of estimating residual displacement (lateral displacement and rotation angle) of concrete piers using long-gauge fiber optic sensing is firstly established. Theoretical derivation reveals that there exists linear relationship between the residual average strain and lateral displacement or rotation angle of the concrete piers. Then, a scaled concrete pier subjected to impact load was designed to verify the condition assessment strategy. The damages of the pier gradually increase by changing the impact velocity of the moving vehicle. Experimental results show that the residual average strain can well reflect the location and severity of random crack damages of the pier under impact load. Meanwhile, residual lateral displacement and rotation angel distribution along the specimen can be identified using the residual average strain measured by long-gauge fiber optic sensors. In addition, hammer excitation was also conducted on the pier after each impact in order to obtain structural dynamic properties. The extracted natural frequencies can reflect global performance (boundary condition and global stiffness) of the pier at each damage state. Therefore, the presented strategy could be a new alternative for condition assessment of concrete piers or columns in civil engineering.

Displacement shape measurement of continuous beam bridges based on long-gauge fiber optic sensing

Abstract Displacement shape is an important index for performance assessment of bridges. It is usually hard to find “reference point” for direct arrangement of displacement transducers on real bridges, and therefore indirect measurement methods using strain sensors attracts increasing attention. A reference-free method for monitoring displacement shape of continuous beam bridges using long-gauge fiber optic sensors is proposed in this article. Firstly, the method for calculating the displacement shape of continuous beam bridges using fiber optic strain sensors along the whole structure was theoretically established. Meanwhile, displacement shape determination of a certain span using fiber optic strain sensors along the corresponding span was also presented for the purpose of cost saving in practical engineering. The presented method is independent of external loads and could be applied to both static and dynamic loads. Results from numerical simulation of a three-span bridge show that both methods could obtain the displacement shape of each span with high precision. Furthermore, experimental results indicate that the presented method could perform online monitoring of time-displacement series at each cross section with high accuracy, and could achieve the displacement shape along the continuous beam bridge at a certain time. Therefore, it could be a convenient reference-free strategy for monitoring the displacement shape of continuous beam bridges in actual engineering.

Simultaneously identifying displacement and strain flexibility using long-gauge fiber optic sensors

Abstract The method of simultaneously identifying displacement and strain flexibility matrices of the structure is proposed by performing the multiple reference impact test and using long-gauge (LG) fiber optic sensors. This method has the following promising features: (1) The impact test is performed in which not only structure responses but also the input force are measured, which ensures the consistency between the identified Frequency Response Function (FRF) and the analytical one, thus it has the advantage of identifying structural flexibility. In contrast, the ambient vibration test cannot output FRF magnitudes. (2) The long-gauge fiber optic sensors are used to measure structure strain responses, which has the merit of reflecting both the macro and micro characteristics of a structure. Both strain and displacement flexibility are identified simultaneously by establishing the relation between LG strain and displacement through an improved conjugate beam method. (3) The subspace identification method in the time domain is developed for structural flexibility identification. It has the merit to deal with the test data with multi-reference multi-peak impacting forces which can better excite the dynamic characteristics of the structure, while traditional frequency domain methods generally are difficult to do that due to the leakage problem. Numerical and experimental examples have been studied to verify the validity and demonstrate the promising features of the proposed method.

Monitoring of long-term prestress losses in prestressed concrete structures using fiber optic sensors

This study presents a method for on-site assessment of prestress losses in prestressed concrete structures. The study is motivated by the increased use of prestressed concrete, the importance of prestressing force levels as a parameter, and the lack of formalized methods for its on-site assessment. The proposed method uses strain measurements from long-gauge fiber optic sensors to study strain changes at the centroid of stiffness (i.e. centroid of composite section) of the cross-sections. Its advantages include (1) robustness to operational load on the structure caused by seasonal and daily temperature variations, in addition to loading; (2) rigorous quantification of uncertainties associated with measurements and parameters; and (3) applicability to a wide range of beam-like structures. The application of the method is illustrated through application to measurements collected over a 7-year period from strain sensors embedded in Streicker Bridge, a post-tensioned concrete pedestrian bridge on the Princeton University campus. Application of the method indicates that prestress losses measured by sensors are of comparable magnitude to design estimates, which implies that estimates are not necessarily overly conservative.

Deflection distribution estimation of tied-arch bridges using long-gauge strain measurements

Deflection, as a critical indicator for structural performance evaluation, is difficult to be measured accurately for long-span bridges, though various deformation sensors and devices have been developed. In this paper, a new scheme using long-gauge fiber optic sensors for estimating the deflection distribution of tied-arch bridges is proposed. First, the complex strain state of the tied-arch bridge is investigated, in which a cubic function describing the axial strain distribution of the tie beam is obtained. Second, the bending strain is separated from the measured long-gauge strain which is a combination of axial and bending components by a sensor layout scheme. Finally, the separated bending strain is utilized to estimate the deflection distribution of the tied-arch bridge through an improved conjugate beam method. Numerical and experimental examples are studied to illustrate the effectiveness of the proposed method for deflection estimation of tied-arch bridges in static and dynamic loading cases.

Damage detection on output-only monitoring of dynamic curvature in composite decks

Installation of sensors networks for continuous in-service monitoring of structures and their efficiency conditions is a current research trend of paramount interest. On-line monitoring systems could be strategically useful for road infrastructures, which are expected to perform efficiently and be self-diagnostic, also in emergency scenarios. This work researches damage detection in composite concrete-steel structures that are typical for highway overpasses and bridges. The techniques herein proposed assume that typical damage in the deck occurs in form of delamination and cracking, and that it affects the peak power spectral density of dynamic curvature. The investigation is performed by combining results of measurements collected by long-gauge fiber optic strain sensors installed on monitored structure and a statistic approach. A finite element model has been also prepared and validated for deepening peculiar aspects of the investigation and the availability of the method. The proposed method for real time applications is able to detect a documented unusual behavior (e.g., damage or deterioration) through long-gauge fiber optic strain sensors measurements and a probabilistic study of the dynamic curvature power spectral density.

Vibration and Deformation Monitoring of a Long-Span Rigid-Frame Bridge with Distributed Long-Gauge Sensors

AbstractAmbient vibration tests and analyses of a long-span rigid-frame bridge were conducted using distributed long-gauge fiber-optic sensors. The concept of a long-gauge fiber-optic sensor and it...

Fiber Bragg grating-based long-gauge fiber optic sensor for monitoring of a 60 m full-scale prestressed concrete girder during lifting and loading

Fiber Bragg grating (FBG) sensors have many advantages in that they can be multiplexed and miniaturized, they can produce absolute measurements, and they are unaffected by electromagnetic interference. For these reasons, their application to civil engineering structures has been actively studied. In this study, FBG-based long-gauge fiber optic sensors (LGFOSs) were used to monitor a prestressed concrete (PSC) girder bridge. LGFOSs, which can measure the average strain over long lengths, were fastened to a PSC girder during the construction and loading processes. The test structure was a full-scale PSC girder with a 60m-span, and its behavior was monitored during construction processes, such as lifting and transporting the girder, after prestressing. Then, a four-point bending test assuming service loading conditions was conducted, and the strains were measured during the test. By comparing to results measured from conventional electrical resistance strain gauges, the results measured from the LGFOSs were investigated. The LGFOSs effectively measured the strain histories during the lifting and transporting processes. From the loading test, the LGFOSs could also measure the strains effectively after cracking, unlike conventional sensors. Moreover, using the measured data before and after cracking, the deflection of the girder was estimated and subsequently compared to the results of a linear variable differential transformer (LVDT).

Optimal layout of long-gauge sensors for deformation distribution identification

Structural deflection can be identified from measured strains from long gague sensors, but the sensor layout scheme greatly influences on the accuracy of identified resutls. To determine the optimal sensor layout scheme for accurate deflection identification of the tied arch bridge, the method of optimal layout of long-gauge fiber optic sensors is studied, in which the characteristic curve is first developed by using the bending macro-strain curve under multiple target load conditions, then optimal sensor layout scheme with different number of sensors are determined. A tied arch bridge is studied as an example to verify the effectiveness and robustness of the proposed method for static and dynamic deflection identification.

Fiber Segment Interferometry for Dynamic Strain Measurements

Using a novel range-resolved interferometric signal processing technique based on the sinusoidal optical frequency modulation of a cost-effective laser diode, a fiber sensing approach termed fiber segment interferometry (FSI) is described. In FSI, a chain of long-gauge length fiber optic strain sensors are separated by identical in-fiber partial reflectors. Targeted at dynamic strain analysis and ultrasound detection for structural health monitoring, this approach allows integrated strain measurements along fiber segments, removing the sensing gaps and sensitivity to inhomogeneities found with localized fiber sensors. In this paper, the multiplexing of six fiber segments, each of length 12.5 cm, is demonstrated. The sensor array can be interrogated at 98 kHz data rate, achieving dynamic strain noise levels ≤ 0.14 nϵ · Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-0.5</sup> . The reflector fabrication is discussed, an analysis of linearity and noise performance is carried out and results from an exemplar experiment to determine the speed-of-sound of a stainless steel rod are shown.

Change Localization of a Steel-Stringer Bridge through Long-Gauge Strain Measurements

AbstractDevelopment of the long-gauge fiber-optic sensor provides an excellent opportunity for structural-change localization, because it measures averaged strains over a long gauge length. A practical structural-change localization, procedure using long-gauge dynamic strains in the modal space is proposed to process the ambient vibration data of a steel-stringer bridge. The proposed procedure is easy to execute by general bridge engineers; meanwhile, it has a solid theoretical basis, thus guaranteeing its effectiveness. By theoretically deriving the magnitude ratios between the analytical strain frequency response functions (FRFs) and the estimated strain FRFs from ambient vibration data, it is proved that the proposed procedure is the same as using long-gauge strain mode shapes for change location; therefore, it is effective and much simpler for general bridge engineers to use. An ambient vibration test of a steel-stringer bridge using long-gauge fiber-optic sensors has been performed. The results of ap...

Structural identification and damage detection through long-gauge strain measurements

Strain measurements from traditional point-type gauges only reveal structural local information. This feature significantly limits the development and application of strain modal theory in civil engineering. The long-gauge fiber optic strain sensor emerging recently has the unique merit of reflecting both local and global information of a structure by measuring the averaged strain within a long gauge length (e.g., 1–2m). Based on this advantage, strain modal identification by processing the dynamic macro strain measurement is investigated. First, the macro strain frequency response function is derived based on the mapping relationship between the long gauge strain and displacement. Then, two methods are proposed for structural strain modal identification and their results are compared. Moreover, damage indexes based on identified macro strain modal shapes are investigated for structural damage detection. The example of a steel stringer bridge model successfully illustrates that the proposed methods accurately identify structural macro strain modal shapes, and the results also reveals that macro strain based indexes are much more robust than traditional modal parameter based indexes for structural damage detection.

Structural identification of short/middle span bridges by rapid impact testing: theory and verification

A structural strain flexibility identification method by processing the multiple-reference impact testing data is proposed. First, a kind of novel long-gauge fiber optic sensor is developed for structural macro-strain monitoring. Second, the multiple-reference impact testing technology is employed, during which both the impacting force and structural strain responses are measured. The impact testing technology has unique merit because it is able to extract exact structural frequency response functions (FRFs), while other test methods, for instance ambient tests, can only output the FRFs with scaled magnitudes. Most importantly, the originality of the article is that a method of identifying the structural strain flexibility characteristic from the impact test data has been proposed, which is useful for structural static strain prediction and capacity evaluation. Examples of a six meter simple supported beam and a multiple-span continuous beam bridge have successfully verified the effectiveness of the proposed method.

Strain flexibility identification of bridges from long-gauge strain measurements

Strain flexibility, defined as the strain response of a structure׳s element to a unit input force, is import for structural safety evaluation, but its identification is seldom investigated. A novel long-gauge fiber optic sensor has been developed to measure the averaged strain within a long gauge length. Its advantage of measuring both local and global information of the structure offers an excellent opportunity of developing the strain flexibility identification theory. In this article, the method to identify structural strain flexibility from long-gauge dynamic strain measurements is proposed. It includes the following main steps: (a) macro strain frequency response function (FRF) estimation from macro strain measurements and its feature characterization; (b) general strain modal parameter identification; (c) scaling factor calculation, and (d) strain flexibility identification. Numerical and experimental examples successfully verify the effectiveness of the proposed method.

Analysis of the status of pre-release cracks in prestressed concrete structures using long-gauge sensors

Prestressed structures experience limited tensile stresses in concrete, which limits or completely eliminates the occurrence of cracks. However, in some cases, large tensile stresses can develop during the early age of the concrete due to thermal gradients and shrinkage effects. Such stresses can cause early-age cracks, termed ‘pre-release cracks’, which occur prior to the transfer of the prestressing force. When the prestressing force is applied to the cross-section, it is assumed that partial or full closure of the cracks occurs by virtue of the force transfer through the cracked cross-section. Verification of the closure of the cracks after the application of the prestressing force is important as it can either confirm continued structural integrity or indicate and approximate reduced structural capacity. Structural health monitoring (SHM) can be used for this purpose. This paper researches an SHM method that can be applied to prestressed beam structures to assess the condition of pre-release cracks. The sensor network used in this method consists of parallel long-gauge fiber optic strain sensors embedded in the concrete cross-sections at various locations. The same network is used for damage detection, i.e. detection and characterization of the pre-release cracks, and for monitoring the prestress force transfer. The method is validated on a real structure, a curved continuous girder. Results from the analysis confirm the safety and integrity of the structure. The method and its application are presented in this paper.

Structural Identification by Processing Dynamic Macro Strain Measurements

In contrast to traditional modal identification methods processing structural accelerations for modal parameter identification, a multiple reference impact testing method using long-gauge fiber optical sensors and the related data processing approach are proposed. First, macro strains of key structural elements during the impact testing are measured through long-gauge fiber optic sensors, which subsequently are used to estimate the strain frequency response functions (FRFs) of the structure. Second, an approach is proposed to estimate strain mode shapes from the strain FRFs, and corresponding displacement mode shapes are calculated by applying an improved conjugate beam method. Finally, the strain flexibility character of the structure is identified by using the identified strain and displacement mode shapes with normalization. The advantage of long-gauge fiber optic sensors measuring averaged strains in a long length (e.g., 1 meter) provides the opportunity for strain modal analysis. The output of the proposed method, strain flexibility, is meaningful for static strain prediction and structural capacity evaluation. Examples investigated successfully verified the effectiveness of the proposed method for structural strain flexibility identification.

A method for the on-site determination of prestressing forces using long-gauge fiber optic strain sensors

Structural health monitoring (SHM) consists of the continuous or periodic measurement of structural parameters and their analysis with the aim of deducing information about the performance and health condition of a structure. The significant increase in the construction of prestressed concrete bridges motivated this research on an SHM method for the on-site determination of the distribution of prestressing forces along prestressed concrete beam structures. The estimation of the distribution of forces is important as it can give information regarding the overall performance and structural integrity of the bridge. An inadequate transfer of the designed prestressing forces to the concrete cross-section can lead to a reduced capacity of the bridge and consequently malfunction or failure at lower loads than predicted by design. This paper researches a universal method for the determination of the distribution of prestressing forces along concrete beam structures at the time of transfer of the prestressing force (e.g., at the time of prestressing or post-tensioning). The method is based on the use of long-gauge fiber optic sensors, and the sensor network is similar (practically identical) to the one used for damage identification. The method encompasses the determination of prestressing forces at both healthy and cracked cross-sections, and for the latter it can yield information about the condition of the cracks. The method is validated on-site by comparison to design forces through the application to two structures: (1) a deck-stiffened arch and (2) a curved continuous girder. The uncertainty in the determination of prestressing forces was calculated and the comparison with the design forces has shown very good agreement in most of the structures’ cross-sections, but also helped identify some unusual behaviors. The method and its validation are presented in this paper.

Testing and Monitoring for a Large Scale Truss Bridge Using Long-Gauge Fiber Optic Sensors

Fiber optic sensors become very popular for structural testing and monitoring in civil engineering nowadays, due to its advantage of high resolution and environment durability. In this paper, long-gauge fiber optic bragg grating sensors will be introduced. Structural damage detection stratagem using the micro-strain mode will be studied. Then its application to a structural testing and monitoring for a real long span truss bridge will be discussed in detail. In the testing, 23 long-gauge fiber optic bragg grating sensors were deployed on the mid span of the bridge. Testing were made under conditions either there is train on the bridge or no train on it. Corresponding dynamic characteristics were analyzed and discussed. Results of the testing show that long-gauge fiber optic sensors can work well for structural testing and also damage detection for truss bridges.

Ten-year monitoring of high-rise building columns using long-gauge fiber optic sensors

A large-scale lifetime building monitoring program was implemented in Singapore in 2001. The monitoring aims of this unique program were to increase safety, verify performance, control quality, increase knowledge, optimize maintenance costs, and evaluate the condition of the structures after a hazardous event. The first instrumented building, which has now been monitored for more than ten years, is presented in this paper. The long-gauge fiber optic strain sensors were embedded in fresh concrete of ground-level columns, thus the monitoring started at the birth of both the construction material and the structure. Measurement sessions were performed during construction, upon completion of each new story and the roof, and after the construction, i.e., in-service. Based on results it was possible to follow and evaluate long-term behavior of the building through every stage of its life. The results of monitoring were analyzed at a local (column) and global (building) level. Over-dimensioning of one column was identified. Differential settlement of foundations was detected, localized, and its magnitude estimated. Post-tremor analysis was performed. Real long-term behavior of concrete columns was assessed. Finally, the long-term performance of the monitoring system was evaluated. The researched monitoring method, monitoring system, rich results gathered over approximately ten years, data analysis algorithms, and the conclusions on the structural behavior and health condition of the building based on monitoring are presented in this paper.