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Deformation monitoring of load-bearing reinforced concrete beams
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Available online www.sciencedirect.com Available online at at www.sciencedirect.com StructuralIntegrity Procedia 00 (2017) 000–000
ScienceDirect ScienceDirect
Procedia Structural (2017) 620–626 Structural IntegrityIntegrity Procedia500 (2016) 000–000
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
2nd International Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, Madeira, Portugal
Deformation monitoring of load-bearing beams XV Portuguese Conference on Fracture, PCF 2016, 10-12reinforced February 2016,concrete Paço de Arcos, Portugal a R. Tsvetkova, I. Shardakov , A.ofShestakov , G. Guseva,turbine V. Epina blade * Thermo-mechanical modeling a high apressure of an Institute of Continuous Media Mechanics Russian Academy of Science, engine 1,Koroleva str., Perm, 614013,Russia airplane gas turbine aa
Abstract
P. Brandãoa, V. Infanteb, A.M. Deusc*
a
of Mechanical Instituto Superior de Lisboa,structures Av. Rovisco 1, 1049-001 Lisboa,and AccurateDepartment and reliable estimation Engineering, of the durability and serviceTécnico, life of Universidade reinforced concrete is Pais, crucial for predicting Portugal extending their service life. Therefore, we propose a version of the automatic deformation monitoring system deployed to control b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, the reinforced concrete structure, which is an air bridge connecting two parts of the structure. The load-bearing elements of this Portugal c bridge are theDepartment reinforcedof concrete boundInstituto together and resting iron support columns. deformation monitoring CeFEMA, Mechanicalbeams Engineering, Superior Técnico,onUniversidade de Lisboa, Av. The Rovisco Pais, 1, 1049-001 Lisboa, Portugalchanges in the macroscopic structure parameters (vertical system allows making crack opening measurements and controlling displacement of beam transverse sections). Control of the vertical displacement of beam elements is implemented by means of hydrostatic level and taut string techniques. The system is provided with IP–cameras (sensors), which makes possible the Abstract photogrammetric evaluation of crack width and beam vertical displacements (deflections). Based on the estimates obtained from mathematical modeling, the crack opening values corresponding to pre-critical and critical fracture conditions and the appropriate During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, loads are obtained. The accumulated data are used as the basis for assessing the current deformation state of the structure in especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent relation to the obtained estimates and for predicting the evolution of damage and the residual service life of structures. The degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict proposed automatic monitoring been data successfully to control the state aircraft, of the real structurebyfora2commercial years. Analysis of the creep behaviour of HPTsystem blades.hasFlight records used (FDR) for a specific provided aviation thecompany, obtained were database indicates seasonal changes in the crack width and reveals the tendency of crack opening to increase with used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model time. needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were © obtained. 2017 The The Authors. Published by Elsevier data that wasbygathered was B.V. fed into the FEM model and different simulations were run, first with a simplified 3D © 2017 The Authors. Published Elsevier B.V. Peer-review under responsibility of to thebetter Scientific Committee of ICSI 2017.with the real 3D mesh obtained from the blade scrap. The rectangular block shape, in order establish the model, and then Peer-review under responsibility of the Scientific Committee of ICSI 2017 overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a Keywords:deformation monitoring, crack opening width, reinforced-concrete beam model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
© 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.
* Corresponding author. Tel.: +7-342-237-8330; fax: +7-342-237-8487. E-mail address: [email protected] 2452-3216© 2017 The Authors. Published by Elsevier B.V. Peer-review underauthor. responsibility the Scientific Committee of ICSI 2017. * Corresponding Tel.: +351of218419991. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017 10.1016/j.prostr.2017.07.028
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V. Epin et al. / Procedia Structural Integrity 5 (2017) 620–626 R. Tsvetkov/ StructuralIntegrity Procedia 00 (2017) 000–000
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1. Introduction Cracking, as well as other and other damages, may occur in reinforced concrete building and engineering structures and their structural components under operational loads. The most dangerous cracks are those that run in the load-bearing elements (beams, supports, pillars, and slabs) responsible for the integrity of an entire structure. Crack propagation can significantly reduce the longevity of the structure and even can lead to its complete failure. The safety factor of the main structural elements is currently assessed by analyzing the monitoring data. There are various methods to monitor the deformation behavior of structures. One of the methods includes local deformation measurements with strain sensors of different types: strain gauge, vibrating wire sensor and fiber optic sensor. However, reliable estimates of the stress-strain state of an entire structure can be obtained only with a great number of such local strain sensors. Another approach to the assessment of the deformation state of the structure is the analysis of its response to dynamic loading, which involves the estimate of natural frequencies, mode shapes, and damping ratios (Staszewski et al. (2007)). The occurrence of defects in the structure changes its frequency and phase characteristics, which should be controlled., This approach is particularly advantageous for monitoring simple structures subjected to dynamic loads. For the structures of complex cross-sectional shape, the detection of the region and degree of probable damages based on dynamic data is a much more complicated task. Different factors might influence the structure response, and not all of these can be associated with defect accumulation. Determination of the critical threshold for changes in the dynamic characteristics necessary to assess the longevity of this structure is also a challenge. Direct measurements of the geometric parameters of structures are also important for deformation monitoring. For this purpose, different methods can be used. For instance, control of the geometry of the entire structure can be performed by photogrammetry techniques, which provides the accuracy of measurements exceeding 0.1 pixel (Maas et al. (2006)). These data can be used as boundary conditions in calculating the stress-strain state of the structure. Crack dimensions can also serve as the damage parameters of concrete structure. There are many companies, which produce crack opening sensors for monitoring systems. The usage of LVDT crack-width sensors manufactured for monitoring bridge constructions is described in the paper (Khrahmalny et al. (2016)). The noncontact methods such as the image-based method for crack analysis (IMCA) can also be applied to measure crack parameters (Barazetti et al. (2009)). 2. The monitoring object A monitoring object represents a reinforced concrete air bridge, which connects two parts of the building. The dimensions of the structure are as follows: length 28 m and width 56 m. The load-bearing elements of this bridge are the reinforced concrete beams of complex cross-sectional shape, resting on iron columns. One of these beams is shown schematically in Fig. 1. The scheme of the load-bearing elements (top view) is given in Fig.2. As one can see, the longitudinal beams 1..9 are rigidly attached to the elements of the main structure. In the areas of location of iron supports, these beams are bound together by transverse beams, thereby forming a lattice structure.
Fig. 1. Schematic diagram of one of the transverse beams (side view).
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Under service loads, cracks appear in the stretch zones of the longitudinal beams. In Fig.1, crack localization areas are shown by a wave-like symbols. A deformation monitoring system was developed, which, on the one hand, registers the local deformation processes concentrated in the vicinity of the crack and, on the other hand, controls the changes in the macroscopic parameters of the structure (vertical displacements in a few transverse cross-sections along the beam length). 3. The deformation monitoring system The proposed automatic deformation monitoring system comprises sensors of our own design, data acquisition and storage system, data processing software, and web-based graph visualization system. The monitoring system can notify specialists by SMS about the probable dangerous situations, i.e. the structure conditions at which the strain criteria reach the value corresponding to the pre-critical stage of failure. The sensors of the monitoring system are based on various measurement principles and aimed at recording various-scale deformation processes: local deformations (crack opening) and changes in the macroscopic parameters of the structure (vertical displacement of beams, beam deflection). The process of measurement is implemented in a non-contact way by using IP cameras, which transfer images to the server by cable or wireless technologies. Interrogation of sensors is carried out in automatic mode at a given frequency. Decoding of the received images by photogrammetry techniques yields deformation parameters.
Fig. 2. Diagram of sensor location on the beams of the air bridge (top view).
3.1. Crack width measurements The monitoring system is based on the non-contact method of crack opening width measurements with the use of photogrammetry technique. The images of the crack- containing area are recorded by the IP camera. Control of the crack opening width is accomplished by measuring the distance between the markers – special marks placed on the opposite crack edges. The marker represents a square, which is separated by a diagonal into black and white-colored zones. The algorithm for searching markers in the images is based on comparing the contrast ratio of adjacent regions. Here, the known quantity is the mutual arrangement of two markers in the initial state. Its change can be used to fix a change in the distance between the crack edges and relative displacement of the crack edges in the plane of the crack. Resolution of such sensor depends on the resolution of the camera and the dimensions of the visible area with the introduced markers. For a single measurement the resolution is 30µm, for a series of 10 measurements it is equal to 15 µm. Sensors testing has shown that cameras with the diagonal field of view (angle of view) up to 45º allow measuring the crack opening up to 3 mm with an accuracy of ±50 µm until the effect of distortion appears and up to 6mm with an accuracy of ±100µm. Measurements can be made up to the crack opening of 12 mm, until the markers are in the field of view of the camera.
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The above monitoring system provided control of 12 cracks localized in potentially dangerous zones of the beams. In Fig. 2, the crack sensors are denoted with capital letter CS (crack sensor). 3.2. Measurement of vertical displacements of beams by the hydrostatic levelling technique The hydrostatic level is a system of communicating measuring vessels filled with a liquid. The liquid level in these vessels determines the plane relative to which quasi-static vertical displacements can be controlled. Such systems can be used to monitor vertical displacements of the structure elements at the points of mutual arrangement of hydrostatic level sensors. The operation capability of the hydrostatic levelling system at low temperatures is sustained by the use of 50% aqueous solution of glycerin, which freezes at temperatures lower than –25 C. A hydrostatic level sensor is a measuring graduated tube filled with a liquid. The liquid level in the tube is registered automatically by detecting the position of the meniscus relative to the location of markers. A detailed description of recognition process for the hydrostatic level sensor is given in work (Epin et al. (2016)). Resolution of the sensor is 90 µm for a single measurement and increases up to 50 µm for a series of measurements. The monitoring system controls the vertical displacements of 3 longitudinal beams (see Fig. 2, beams No. 3, 5, 7). Each beam is provided with 5 sensors: at the beam-to-building wall junctions, in the zones near the columns and in the middle of span of the beam. Thus, 15 sensors are located at the specified points (denoted with capital letters HS- hydrostatic sensors), as shown on the diagram in Fig.2. 3.3. Measurements of vertical displacements of beams by the taut string method Monitoring of the vertical displacements of structure elements are duplicated by registration of beam deflection relative to the position set by a taut metal string connecting the opposite ends of the beam (for example, Stanton et al. (2003)). The load of a constant mass provides permanent and stable stretching of the string, specifying the position of the base straight line. Measurements of vertical displacements with respect to this line are made in the following manner. A group of markers is fixed on the beam and string in one and the same plane and changes in their position are recorded with the IP-camera fixed to the beam. Processing of the obtained images with the above mentioned algorithm allows us to determine the distance between the groups of markers located on the beam and the string. The vertical displacement of the beam at the point of camera attachment caused by structure deformation is estimated by a change in the vertical component of the distance between the markers. Setting of several cameras on the beam makes it possible to register beam deflections at different points. This measuring technique is practically independent of fluctuations of the environment air temperature, since stretching of the string remains steady and small changes in its length produce negligible effect on its deflection Resolution of the sensors are 15 µm for a single measurement and 5 µm for a series of 10 measurements. One string is stretched along each of 9 beams and each is provided with 3 sensors denoted on the diagram in Fig. 2 with SS (string sensors). The string is fastened to the beam at a distance of 1m from the beam ends, which allows control of deflections almost along the entire length of the beam. In contrast to the hydrostatic level sensor, the SS executes control of deflection changes only within one longitudinal beam, which is quite sufficient for estimation of the deformation state of a separate beam not of the entire structure. 4. Deformation analysis The system of reinforced beams resting on iron columns is the main load-bearing element of the air bridge. A mathematical model has been developed to describe the deformation state of the system of beams of the air bridge taking into account its interaction with all elements of the structure. The strain-state of the reinforcement material was described in the framework of the elastoplasticity theory (Kachanov (1971)), and the concrete condition was assessed in the context of the theory of brittle fracture (William et al. (1974)). The finite-element method was used for the numerical implementation of the model, which took into account the location and state of cracks at the instant of crack sensor installation. The numerical experiments allowed us to reveal the specific features of the deformation interaction of the iron reinforcement with concrete in the presence and absence of cracks and taking onto account the probability of
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generation of new cracks. It was assumed that the cracks detected in the load-bearing beams at the instant of installation of first sensors appeared during the previous service life of the building. This made possible the estimation of the deformation level of all the structural elements before the installation of the monitoring system. The numerical analysis reveals the following features of the deformation state of the structure under consideration: - the regions of beam-column junctions are most critical; - the measured beam deflection values correlate with the resultant crack opening near the support columns. Based on the obtained results, the arrangement scheme of sensors was defined, and the developed mathematical model became a part of the analytical unit of the automatic deformation monitoring system. This allowed us to interpret and analyze the measured values of crack width and beam deflections. 5. Monitoring results The described automatic deformation monitoring system was installed in the above mentioned structure in the early 2015. The sensors for measuring crack opening were placed on the longitudinal beams of the air bridge. After the analysis of the deformation processes developing over a period of 2 years, the system was supplemented with sensors for measuring vertical displacements and beam deflections. At present, 54 sensors of different types are placed on 9 beams (see Fig.2). The results given below illustrate the operation of the proposed automatic deformation monitoring system. 5.1. Results of crack opening Changes in the crack width are shown in Fig.3: (a) recorded by two sensors of the monitoring system during 24 months and (b) during 2 weeks. The sensor CS2 is placed at the junction of one of the column and beam No.2, and the sensor CS7 is placed in a similar way on beam No.3. The corresponding ambient temperature changes are shown in Fig.4. Seasonal ambient temperature changes observed throughout the year were of the order of 50°C (Fig.4a), and diurnal ambient temperature changes – about 10–12°C (Fig. 4b).
Fig. 3. Changes in the crack width registered by the sensors CS2and CS7 during two-year monitoring (a) and by the sensor CS7 during two-week monitoring (b).
Fig. 4. Changes in the ambient air temperature after two years (a) and two weeks (b) of monitoring.
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The value of crack width recorded by a number of sensors correlates with seasonal and diurnal air ambient temperatures. The influence of diurnal temperature changes on the crack width is less pronounced because the beam is affected per day by comparable factors (e.g. loads caused by the number of pedestrians). With account of these temperature changes some sensors (CS2, CS7, CS11) showed the tendency of the crack to increase with time. This fact motivated the installation of additional sensors for measuring beam deflections. 5.2. Results of beam vertical displacements Fig. 5 presents the plots illustrating the changes in beam vertical displacements recorded by the hydrostatic level../../../../../../Documents and Settings/trans1/Local Settings/Temp/-970360710 sensors HS3 and HS4 placed on beam No.3 in the region near the support column and at the beam center relative to the sensor HS5 placed at the junction of beam No.3 and the wall of the main building. Fig.5a presents data obtained during two month monitoring, Fig.5b – during two-week monitoring. The hydrostatic level../../../../../../Documents and Settings/trans1/Local Settings/Temp/-970360710 sensors provide accurate measurements of seasonal and diurnal changes in the vertical displacements of the beams relative to the wall of the main building.
Fig. 5. Changes in the vertical displacements registered by the sensors HS3 an HS4 during two months of monitoring (a) and by the sensor HS4 during two weeks of monitoring (b).
The taut string sensors SS8 and SS9 (see Fig.2) placed on beam No. 3 at the beam center and near the support column reflect seasonal and diurnal changes in vertical displacements.
Fig. 6. Changes in the vertical displacements registered by the string sensors SS8, SS9 after two months of observations (a) and by the sensor SS9 after two weeks of recording (b).
The taut string and hydrostatic level sensors, independently of each other, execute control over the deformation process taking place in reinforced concrete beams. The results obtained by both types of sensors are in good qualitative and quantitative agreement. A slight quantitative discrepancy can be attributed to the fact that the external factors have different impacts on measurement subsystems. In the case of taut string sensors, the higher level of noise is associated with their higher sensitivity to dynamic factors. Both measurement subsystems are able to measure the vertical displacement of the beams with the accuracy quite sufficient for reliable registration of the deformation process in these beams at pre-critical and critical fracture stages.
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6. Conclusion The analysis of the measured results obtained for the crack width and beam deflections revealed a number of trends in the deformation behavior of the load-bearing elements of the air-bridge. Of most importance are the cyclic changes of vertical displacements, which correlate with seasonal and diurnal ambient air temperature changes. Due to variations in ambient temperature, the length of the metal support columns changes, causing thus the displacement of beam-column junctions, beam deflection and crack growth. Recurring of this process leads to a gradual increase in the damage degree of the structure and crack opening, which is the main cause of the deformation of the load-bearing elements of the air-bridge. The validity of the developed mathematical model was verified using the experimental measurements obtained in this investigation. Based on the monitoring data, the model was updated, which allowed us to evaluate the limiting values of measurements (i.e. the values, at which the critical state of the structure can be achieved). The safe service life of the structure is warranted at the values of measured parameters not exceeding 80% of the limiting values. Acknowledgements This research was performed under the grant of the Russian Science Foundation (grant № 14-29-00172-П). References Barazzetti, L., Scaioni, M., 2009. Crack measurement: Development, testing and applications of an automatic image-based algorithm. ISPRS journal of photogrammetry and remote sensing 64(3), 285-296. Epin, V.V, Tsvetkov, R.V., Shestakov, A.P., 2016. Application of feature recognition to hydrostatic leveling systems. Measurement Techniques 59, 405-409. Kachanov, L.M., 1971. Foundations of Theory of Plasticity, North-Holland, Amsterdam, pp. 496. Krakhmal'ny, Ɍ.Ⱥ., Evtushenko, S.I., Krakhmal'naya, M.P., 2016. New System of Monitoring of a Condition of Cracks of Small Reinforced Concrete Bridge Constructions. Procedia Engineering 150, 2369-2374. Maas, H.G., Hampel, U., 2006. Photogrammetric techniques in civil engineering material testing and structure monitoring. Photogrammetric engineering and remote sensing 72(1), 39-45. Stanton, J.F., Eberhard, M.O., Barr, P.J.,2003. A weighted-stretched wire system for monitoring deflections. Engineering Structures 25, 347-357. Staszewski, W.J., Robertson, A.N., 2007. Time–frequency and time–scale analyses for structural health monitoring. Philosophical Transactions of the Royal Society A 365, 449-477. William, K.J., Warnke, E.P., 1974. Constitutive Model for the Triaxial Behavior of Concrete. Proceedings of International Association of Bridge Structural Engineering 19, 1–30.
ScienceDirect
Available online www.sciencedirect.com Available online at at www.sciencedirect.com StructuralIntegrity Procedia 00 (2017) 000–000
ScienceDirect ScienceDirect
Procedia Structural (2017) 620–626 Structural IntegrityIntegrity Procedia500 (2016) 000–000
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
2nd International Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, Madeira, Portugal
Deformation monitoring of load-bearing beams XV Portuguese Conference on Fracture, PCF 2016, 10-12reinforced February 2016,concrete Paço de Arcos, Portugal a R. Tsvetkova, I. Shardakov , A.ofShestakov , G. Guseva,turbine V. Epina blade * Thermo-mechanical modeling a high apressure of an Institute of Continuous Media Mechanics Russian Academy of Science, engine 1,Koroleva str., Perm, 614013,Russia airplane gas turbine aa
Abstract
P. Brandãoa, V. Infanteb, A.M. Deusc*
a
of Mechanical Instituto Superior de Lisboa,structures Av. Rovisco 1, 1049-001 Lisboa,and AccurateDepartment and reliable estimation Engineering, of the durability and serviceTécnico, life of Universidade reinforced concrete is Pais, crucial for predicting Portugal extending their service life. Therefore, we propose a version of the automatic deformation monitoring system deployed to control b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, the reinforced concrete structure, which is an air bridge connecting two parts of the structure. The load-bearing elements of this Portugal c bridge are theDepartment reinforcedof concrete boundInstituto together and resting iron support columns. deformation monitoring CeFEMA, Mechanicalbeams Engineering, Superior Técnico,onUniversidade de Lisboa, Av. The Rovisco Pais, 1, 1049-001 Lisboa, Portugalchanges in the macroscopic structure parameters (vertical system allows making crack opening measurements and controlling displacement of beam transverse sections). Control of the vertical displacement of beam elements is implemented by means of hydrostatic level and taut string techniques. The system is provided with IP–cameras (sensors), which makes possible the Abstract photogrammetric evaluation of crack width and beam vertical displacements (deflections). Based on the estimates obtained from mathematical modeling, the crack opening values corresponding to pre-critical and critical fracture conditions and the appropriate During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, loads are obtained. The accumulated data are used as the basis for assessing the current deformation state of the structure in especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent relation to the obtained estimates and for predicting the evolution of damage and the residual service life of structures. The degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict proposed automatic monitoring been data successfully to control the state aircraft, of the real structurebyfora2commercial years. Analysis of the creep behaviour of HPTsystem blades.hasFlight records used (FDR) for a specific provided aviation thecompany, obtained were database indicates seasonal changes in the crack width and reveals the tendency of crack opening to increase with used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model time. needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were © obtained. 2017 The The Authors. Published by Elsevier data that wasbygathered was B.V. fed into the FEM model and different simulations were run, first with a simplified 3D © 2017 The Authors. Published Elsevier B.V. Peer-review under responsibility of to thebetter Scientific Committee of ICSI 2017.with the real 3D mesh obtained from the blade scrap. The rectangular block shape, in order establish the model, and then Peer-review under responsibility of the Scientific Committee of ICSI 2017 overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a Keywords:deformation monitoring, crack opening width, reinforced-concrete beam model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
© 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.
* Corresponding author. Tel.: +7-342-237-8330; fax: +7-342-237-8487. E-mail address: [email protected] 2452-3216© 2017 The Authors. Published by Elsevier B.V. Peer-review underauthor. responsibility the Scientific Committee of ICSI 2017. * Corresponding Tel.: +351of218419991. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017 10.1016/j.prostr.2017.07.028
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V. Epin et al. / Procedia Structural Integrity 5 (2017) 620–626 R. Tsvetkov/ StructuralIntegrity Procedia 00 (2017) 000–000
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1. Introduction Cracking, as well as other and other damages, may occur in reinforced concrete building and engineering structures and their structural components under operational loads. The most dangerous cracks are those that run in the load-bearing elements (beams, supports, pillars, and slabs) responsible for the integrity of an entire structure. Crack propagation can significantly reduce the longevity of the structure and even can lead to its complete failure. The safety factor of the main structural elements is currently assessed by analyzing the monitoring data. There are various methods to monitor the deformation behavior of structures. One of the methods includes local deformation measurements with strain sensors of different types: strain gauge, vibrating wire sensor and fiber optic sensor. However, reliable estimates of the stress-strain state of an entire structure can be obtained only with a great number of such local strain sensors. Another approach to the assessment of the deformation state of the structure is the analysis of its response to dynamic loading, which involves the estimate of natural frequencies, mode shapes, and damping ratios (Staszewski et al. (2007)). The occurrence of defects in the structure changes its frequency and phase characteristics, which should be controlled., This approach is particularly advantageous for monitoring simple structures subjected to dynamic loads. For the structures of complex cross-sectional shape, the detection of the region and degree of probable damages based on dynamic data is a much more complicated task. Different factors might influence the structure response, and not all of these can be associated with defect accumulation. Determination of the critical threshold for changes in the dynamic characteristics necessary to assess the longevity of this structure is also a challenge. Direct measurements of the geometric parameters of structures are also important for deformation monitoring. For this purpose, different methods can be used. For instance, control of the geometry of the entire structure can be performed by photogrammetry techniques, which provides the accuracy of measurements exceeding 0.1 pixel (Maas et al. (2006)). These data can be used as boundary conditions in calculating the stress-strain state of the structure. Crack dimensions can also serve as the damage parameters of concrete structure. There are many companies, which produce crack opening sensors for monitoring systems. The usage of LVDT crack-width sensors manufactured for monitoring bridge constructions is described in the paper (Khrahmalny et al. (2016)). The noncontact methods such as the image-based method for crack analysis (IMCA) can also be applied to measure crack parameters (Barazetti et al. (2009)). 2. The monitoring object A monitoring object represents a reinforced concrete air bridge, which connects two parts of the building. The dimensions of the structure are as follows: length 28 m and width 56 m. The load-bearing elements of this bridge are the reinforced concrete beams of complex cross-sectional shape, resting on iron columns. One of these beams is shown schematically in Fig. 1. The scheme of the load-bearing elements (top view) is given in Fig.2. As one can see, the longitudinal beams 1..9 are rigidly attached to the elements of the main structure. In the areas of location of iron supports, these beams are bound together by transverse beams, thereby forming a lattice structure.
Fig. 1. Schematic diagram of one of the transverse beams (side view).
V. Epin et al. / Procedia Structural Integrity 5 (2017) 620–626 R. Tsvetkov / Structural Integrity Procedia 00 (2017) 000–000
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Under service loads, cracks appear in the stretch zones of the longitudinal beams. In Fig.1, crack localization areas are shown by a wave-like symbols. A deformation monitoring system was developed, which, on the one hand, registers the local deformation processes concentrated in the vicinity of the crack and, on the other hand, controls the changes in the macroscopic parameters of the structure (vertical displacements in a few transverse cross-sections along the beam length). 3. The deformation monitoring system The proposed automatic deformation monitoring system comprises sensors of our own design, data acquisition and storage system, data processing software, and web-based graph visualization system. The monitoring system can notify specialists by SMS about the probable dangerous situations, i.e. the structure conditions at which the strain criteria reach the value corresponding to the pre-critical stage of failure. The sensors of the monitoring system are based on various measurement principles and aimed at recording various-scale deformation processes: local deformations (crack opening) and changes in the macroscopic parameters of the structure (vertical displacement of beams, beam deflection). The process of measurement is implemented in a non-contact way by using IP cameras, which transfer images to the server by cable or wireless technologies. Interrogation of sensors is carried out in automatic mode at a given frequency. Decoding of the received images by photogrammetry techniques yields deformation parameters.
Fig. 2. Diagram of sensor location on the beams of the air bridge (top view).
3.1. Crack width measurements The monitoring system is based on the non-contact method of crack opening width measurements with the use of photogrammetry technique. The images of the crack- containing area are recorded by the IP camera. Control of the crack opening width is accomplished by measuring the distance between the markers – special marks placed on the opposite crack edges. The marker represents a square, which is separated by a diagonal into black and white-colored zones. The algorithm for searching markers in the images is based on comparing the contrast ratio of adjacent regions. Here, the known quantity is the mutual arrangement of two markers in the initial state. Its change can be used to fix a change in the distance between the crack edges and relative displacement of the crack edges in the plane of the crack. Resolution of such sensor depends on the resolution of the camera and the dimensions of the visible area with the introduced markers. For a single measurement the resolution is 30µm, for a series of 10 measurements it is equal to 15 µm. Sensors testing has shown that cameras with the diagonal field of view (angle of view) up to 45º allow measuring the crack opening up to 3 mm with an accuracy of ±50 µm until the effect of distortion appears and up to 6mm with an accuracy of ±100µm. Measurements can be made up to the crack opening of 12 mm, until the markers are in the field of view of the camera.
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The above monitoring system provided control of 12 cracks localized in potentially dangerous zones of the beams. In Fig. 2, the crack sensors are denoted with capital letter CS (crack sensor). 3.2. Measurement of vertical displacements of beams by the hydrostatic levelling technique The hydrostatic level is a system of communicating measuring vessels filled with a liquid. The liquid level in these vessels determines the plane relative to which quasi-static vertical displacements can be controlled. Such systems can be used to monitor vertical displacements of the structure elements at the points of mutual arrangement of hydrostatic level sensors. The operation capability of the hydrostatic levelling system at low temperatures is sustained by the use of 50% aqueous solution of glycerin, which freezes at temperatures lower than –25 C. A hydrostatic level sensor is a measuring graduated tube filled with a liquid. The liquid level in the tube is registered automatically by detecting the position of the meniscus relative to the location of markers. A detailed description of recognition process for the hydrostatic level sensor is given in work (Epin et al. (2016)). Resolution of the sensor is 90 µm for a single measurement and increases up to 50 µm for a series of measurements. The monitoring system controls the vertical displacements of 3 longitudinal beams (see Fig. 2, beams No. 3, 5, 7). Each beam is provided with 5 sensors: at the beam-to-building wall junctions, in the zones near the columns and in the middle of span of the beam. Thus, 15 sensors are located at the specified points (denoted with capital letters HS- hydrostatic sensors), as shown on the diagram in Fig.2. 3.3. Measurements of vertical displacements of beams by the taut string method Monitoring of the vertical displacements of structure elements are duplicated by registration of beam deflection relative to the position set by a taut metal string connecting the opposite ends of the beam (for example, Stanton et al. (2003)). The load of a constant mass provides permanent and stable stretching of the string, specifying the position of the base straight line. Measurements of vertical displacements with respect to this line are made in the following manner. A group of markers is fixed on the beam and string in one and the same plane and changes in their position are recorded with the IP-camera fixed to the beam. Processing of the obtained images with the above mentioned algorithm allows us to determine the distance between the groups of markers located on the beam and the string. The vertical displacement of the beam at the point of camera attachment caused by structure deformation is estimated by a change in the vertical component of the distance between the markers. Setting of several cameras on the beam makes it possible to register beam deflections at different points. This measuring technique is practically independent of fluctuations of the environment air temperature, since stretching of the string remains steady and small changes in its length produce negligible effect on its deflection Resolution of the sensors are 15 µm for a single measurement and 5 µm for a series of 10 measurements. One string is stretched along each of 9 beams and each is provided with 3 sensors denoted on the diagram in Fig. 2 with SS (string sensors). The string is fastened to the beam at a distance of 1m from the beam ends, which allows control of deflections almost along the entire length of the beam. In contrast to the hydrostatic level sensor, the SS executes control of deflection changes only within one longitudinal beam, which is quite sufficient for estimation of the deformation state of a separate beam not of the entire structure. 4. Deformation analysis The system of reinforced beams resting on iron columns is the main load-bearing element of the air bridge. A mathematical model has been developed to describe the deformation state of the system of beams of the air bridge taking into account its interaction with all elements of the structure. The strain-state of the reinforcement material was described in the framework of the elastoplasticity theory (Kachanov (1971)), and the concrete condition was assessed in the context of the theory of brittle fracture (William et al. (1974)). The finite-element method was used for the numerical implementation of the model, which took into account the location and state of cracks at the instant of crack sensor installation. The numerical experiments allowed us to reveal the specific features of the deformation interaction of the iron reinforcement with concrete in the presence and absence of cracks and taking onto account the probability of
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generation of new cracks. It was assumed that the cracks detected in the load-bearing beams at the instant of installation of first sensors appeared during the previous service life of the building. This made possible the estimation of the deformation level of all the structural elements before the installation of the monitoring system. The numerical analysis reveals the following features of the deformation state of the structure under consideration: - the regions of beam-column junctions are most critical; - the measured beam deflection values correlate with the resultant crack opening near the support columns. Based on the obtained results, the arrangement scheme of sensors was defined, and the developed mathematical model became a part of the analytical unit of the automatic deformation monitoring system. This allowed us to interpret and analyze the measured values of crack width and beam deflections. 5. Monitoring results The described automatic deformation monitoring system was installed in the above mentioned structure in the early 2015. The sensors for measuring crack opening were placed on the longitudinal beams of the air bridge. After the analysis of the deformation processes developing over a period of 2 years, the system was supplemented with sensors for measuring vertical displacements and beam deflections. At present, 54 sensors of different types are placed on 9 beams (see Fig.2). The results given below illustrate the operation of the proposed automatic deformation monitoring system. 5.1. Results of crack opening Changes in the crack width are shown in Fig.3: (a) recorded by two sensors of the monitoring system during 24 months and (b) during 2 weeks. The sensor CS2 is placed at the junction of one of the column and beam No.2, and the sensor CS7 is placed in a similar way on beam No.3. The corresponding ambient temperature changes are shown in Fig.4. Seasonal ambient temperature changes observed throughout the year were of the order of 50°C (Fig.4a), and diurnal ambient temperature changes – about 10–12°C (Fig. 4b).
Fig. 3. Changes in the crack width registered by the sensors CS2and CS7 during two-year monitoring (a) and by the sensor CS7 during two-week monitoring (b).
Fig. 4. Changes in the ambient air temperature after two years (a) and two weeks (b) of monitoring.
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The value of crack width recorded by a number of sensors correlates with seasonal and diurnal air ambient temperatures. The influence of diurnal temperature changes on the crack width is less pronounced because the beam is affected per day by comparable factors (e.g. loads caused by the number of pedestrians). With account of these temperature changes some sensors (CS2, CS7, CS11) showed the tendency of the crack to increase with time. This fact motivated the installation of additional sensors for measuring beam deflections. 5.2. Results of beam vertical displacements Fig. 5 presents the plots illustrating the changes in beam vertical displacements recorded by the hydrostatic level../../../../../../Documents and Settings/trans1/Local Settings/Temp/-970360710 sensors HS3 and HS4 placed on beam No.3 in the region near the support column and at the beam center relative to the sensor HS5 placed at the junction of beam No.3 and the wall of the main building. Fig.5a presents data obtained during two month monitoring, Fig.5b – during two-week monitoring. The hydrostatic level../../../../../../Documents and Settings/trans1/Local Settings/Temp/-970360710 sensors provide accurate measurements of seasonal and diurnal changes in the vertical displacements of the beams relative to the wall of the main building.
Fig. 5. Changes in the vertical displacements registered by the sensors HS3 an HS4 during two months of monitoring (a) and by the sensor HS4 during two weeks of monitoring (b).
The taut string sensors SS8 and SS9 (see Fig.2) placed on beam No. 3 at the beam center and near the support column reflect seasonal and diurnal changes in vertical displacements.
Fig. 6. Changes in the vertical displacements registered by the string sensors SS8, SS9 after two months of observations (a) and by the sensor SS9 after two weeks of recording (b).
The taut string and hydrostatic level sensors, independently of each other, execute control over the deformation process taking place in reinforced concrete beams. The results obtained by both types of sensors are in good qualitative and quantitative agreement. A slight quantitative discrepancy can be attributed to the fact that the external factors have different impacts on measurement subsystems. In the case of taut string sensors, the higher level of noise is associated with their higher sensitivity to dynamic factors. Both measurement subsystems are able to measure the vertical displacement of the beams with the accuracy quite sufficient for reliable registration of the deformation process in these beams at pre-critical and critical fracture stages.
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6. Conclusion The analysis of the measured results obtained for the crack width and beam deflections revealed a number of trends in the deformation behavior of the load-bearing elements of the air-bridge. Of most importance are the cyclic changes of vertical displacements, which correlate with seasonal and diurnal ambient air temperature changes. Due to variations in ambient temperature, the length of the metal support columns changes, causing thus the displacement of beam-column junctions, beam deflection and crack growth. Recurring of this process leads to a gradual increase in the damage degree of the structure and crack opening, which is the main cause of the deformation of the load-bearing elements of the air-bridge. The validity of the developed mathematical model was verified using the experimental measurements obtained in this investigation. Based on the monitoring data, the model was updated, which allowed us to evaluate the limiting values of measurements (i.e. the values, at which the critical state of the structure can be achieved). The safe service life of the structure is warranted at the values of measured parameters not exceeding 80% of the limiting values. Acknowledgements This research was performed under the grant of the Russian Science Foundation (grant № 14-29-00172-П). References Barazzetti, L., Scaioni, M., 2009. Crack measurement: Development, testing and applications of an automatic image-based algorithm. ISPRS journal of photogrammetry and remote sensing 64(3), 285-296. Epin, V.V, Tsvetkov, R.V., Shestakov, A.P., 2016. Application of feature recognition to hydrostatic leveling systems. Measurement Techniques 59, 405-409. Kachanov, L.M., 1971. Foundations of Theory of Plasticity, North-Holland, Amsterdam, pp. 496. Krakhmal'ny, Ɍ.Ⱥ., Evtushenko, S.I., Krakhmal'naya, M.P., 2016. New System of Monitoring of a Condition of Cracks of Small Reinforced Concrete Bridge Constructions. Procedia Engineering 150, 2369-2374. Maas, H.G., Hampel, U., 2006. Photogrammetric techniques in civil engineering material testing and structure monitoring. Photogrammetric engineering and remote sensing 72(1), 39-45. Stanton, J.F., Eberhard, M.O., Barr, P.J.,2003. A weighted-stretched wire system for monitoring deflections. Engineering Structures 25, 347-357. Staszewski, W.J., Robertson, A.N., 2007. Time–frequency and time–scale analyses for structural health monitoring. Philosophical Transactions of the Royal Society A 365, 449-477. William, K.J., Warnke, E.P., 1974. Constitutive Model for the Triaxial Behavior of Concrete. Proceedings of International Association of Bridge Structural Engineering 19, 1–30.