Evaluation of Structure Deformation Using Geodetic Methods

Transcription

1 Evaluation of Structure Deformation Using Geodetic Methods SORIN HERBAN, CARMEN GRECEA, GEORGIANA RUSU, ADRIAN ALIONESCU Overland Communication Ways, Foundations and Cadastral Survey Politehnica University of Timisoara, Timisoara, Traian Lalescu, no. 2 ROMANIA adrian.alionescu@upt.ro Abstract: - The development of measuring techniques and technologies has permitted the possibility to observe and emphasize the behaviour of the buildings and the way these modify their position in time under their own weight and under the influence of external dynamic factors. It is known that long term movements of structures and constructions can be monitored using geodetic instruments [1]. Measurements can be performed during a period of time, which can be minutes, hours, weeks, months or years to a number of targets in order to measure settlement, displacement or long term permanent or specific deformation, depending on the size and direction of displacement. At Politehnica University of Timisoara, one of the Land Measurements and Cadastre collective s research area is concentrated on the dynamic deformation of structures. Monitoring equipment includes precise and modern instruments like precise levels, total stations, GPS. The aim of this paper is a study regarding high precision geodetic measurements and the results for an industrial building situated in Timis County. The analysis of vertical displacements and deformations is essential for the study of constructions behaviour. In practice, these values are obtained by measuring of control points positioned on buildings. A series of cyclic measurements are conducted in order to determine the elevation of control points with respect to several fixed points of the local reference network [2]. Key words: displacement, movement, monitoring, dynamic deformations, high precision leveling, deflection. 1 Introduction There are loads of classification criteria s methods of research and observation of constructions and structures. Analyzing the in-situ behaviour of buildings, the data which describe the vertical and horizontal deformations and displacements are essential. Following a mathematical convention, settlements are assigned negative values, while uplifts have positive values [2]. Depending on the field of measurement and the precision that aims to be reached by the observations, modern measurement techniques in the field of engineering geodesy have seen an unprecedented development and a higher degree of automation together with the development through known technological aspect in the past two decades [3]. Taking this into consideration, there have been developed criteria s made by types of deformations, types of equipments and the position of the equipments during the observations. By the place where equipments are located during the observation process, there are two possibilities to determine the movements and the deformations: Physical methods: with the equipment located inside the building; in this case the equipments move at the same time with the building, so relative movements and deformations can be evaluated. Geometric methods: in this case, the equipments are placed outside the building or outside its influential area, the measurements will be linked to a network of fixed points situated outside the deformation area, protected against the instability factors that can affect the building and the foundation ground that it is situated on. Through this process absolute values of the horizontal or vertical movement will be established. The topographic-geodetic methods belong to this category of determinations of movements and deformations [4]. Monitoring the dynamic behaviour of large structures has been always a topic of great relevance, due to the impact that these structures on the landscape where they were built [5]. ISBN:

2 Many instruments and surveying methods have been used in order to support the control of these structures. However, the main aim in most of the developed plans has been to ensure the possibility of measuring displacements in a singular number of points. The difficulty in the measurement of these displacements is to find a spatial measurement technique that responds to numerous properties, such as precision, reliability, low cost and easiness to use. Some of these requirements may be accomplished using several methods, but it is really hard to find a method to meet all of them. In the next paragraphs some of the approaches developed in this context are reported (starting with the classical techniques and finishing with the new technologies). 2 Multicriteria approach to determining building deformations Over the years specialists in measurements developed techniques and technologies in order to estimate correct, accurate and precise structure movements. These techniques are presented as follows: Classical topographic methods based on angles, distances and height variation measurements are very popular in the quantitative surveying field. The equipment used consists of accurate and appropriate levels, theodolites or total stations. When the point that has to be determined is inaccessible, indirect methods are used, for example: precise leveling traversing, single or multiple intersections, etc. Furthermore, contact sensors can complete these measurements, such as: an inclinometer, a pendulum, dial gauges or extensometers. However, this contact nature prevents them from use at the final stages of destructive load testing and they can only acquire observations in one dimension. The Global Positioning System (GPS) has been used in structural monitoring of large structures with considerable range of displacements, as well as combined with other sensors. In spite of this, GPS has two significant limitations. Firstly, as signals are received from satellites, coordinates cannot be measured indoors or through above obstacles. The second limitation is that the current precision of GPS technology is limited to +/- 1cm horizontally and +/- 2cm vertically. Digital close-range photogrammetry has been used as an alternative and it provides high accuracy. It also offers a quick, remote, three-dimensional data acquisition with images that provide a permanent visual recording of the test, but the compulsory use of targets might be disadvantageous in some circumstances, especially when the access to the object is risky or when it is inaccessible to operators. Due to the lack of scale definition in the photogrammetric process, measurements must be taken by using additional instrumentation. Terrestrial Laser Scanning has become a new alternative to the monitoring of structures incorporating novelty approaches and computer methods. Although the approaches noted above present an accurate modeling strategy and have demonstrated their reliability for structural monitoring, none of them has been tested yet over complex structures such as large bridges and high constructions. The reported analysis focuses on two main problems: the first one is the accuracy and the stability of georeferencing, which is fundamental to make comparisons between different multi-temporal scans; the second one is the computation of deformation based on the acquired point-clouds. Particularly, a comparison is performed using different surfaces types, such as: resample point cloud, mesh and polynomial surface [6]. Determination of the movements and irregularities of an object in the primary analysis requires a close relation to time and space. Current development of constructions imposes the need to monitor changes and possible displacements. In this context, the behaviour of large structures such as bridges, dams, towers, buildings with a high pitch, has become critical to analyze, not only in terms of phenomenology, but also as a result of processing and the inclusion of cases in these models of analysis. The geometric leveling traversing that was used for this case study is achieved in compliance with the requirements of the technical standard imposed by the national geometric leveling of the order 0, I or II. The geometric leveling system consists of polygons which incorporate the fixed points (benchmarks) of the reference network, the control points and the intermediate points. At the same time, the station mid-points for geometric leveling are set out in such a way that their position remains unchanged for each measurement cycle. The selection of the topographic instruments and of the geometrical leveling methods is made in order to ensure the accuracy required by each specific construction. Depending on the size and importance of the construction, a compensation of the leveling and/or networks is performed using rigorous or semi-rigorous methods combined with the method of the least squares [2]. ISBN:

3 Phenomenon During Phenomenon type Characteristics Evaluation methods 0.01 s 1 s 10 s 24 h 10 yars 100 years OSCILLATIONS SHORT TIME MOTIONS LONG TIME MOTIONS Oscillation from own machines Oscillation from own construction Strains under the demands dynamic short Strains under the demands dynamic long Tectonic movements Movements of the earth's crust Continuous measurement Continuous measurement and specific geodethic methods Geodethic methods, fotogrametric methods, satellite methods Geodethic methods, fotogrametric methods, satellite methods Fig.1 Schematic representation of deformations and the methods used to determine movements [6] During the determination of a deformation, we will always find a cause that through a transmitting function will lead to the effect. The transmitting function is expressed through mathematical relations, being part of the statistics category. We cannot be sure which are the causes that led to certain phenomena, as well as the dimensions and the direction of the deformations. We can only find a relation with a certain probability. The effect is always the deformation, defined as a spatial modification [1]. The role of geodetic tracking networks is to observe and determine whether or not movements or deformations occurred at a certain building [4]. Usually, there is an initial information regarding the movements of interest or movements critical to an object. The precision and integrity of the apriorical knowledge on the behaviour of the building are differentiated, but the basic parameters for a deformation model that must be taken into consideration are: representative points; the delimitation of the information domain; the evolution in time of the movements; the direction of the studied movements. 3 Problem Solution, high precision leveling traversing Digital levels have become standard for leveling because they confer high accuracy and fast measurements in an automated measuring process [7]. A shortcoming is that they give less accurate measurements under some circumstances which are not always obvious. The activity of monitoring the in situ behaviour of constructions, which implies a corresponding endowment with specialized equipment and software, is regulated in Romania by Law No. 10 / 1995, regarding quality in constructions, amended by Law No. 123 / 2007, which specifies that tracking the behaviour of constructions in operation is made throughout their living duration". It is also regulated by the P Norm regarding behaviour in time of constructions and GE Norm regarding execution of maintenance and repairs to buildings and special constructions [8]. The geometrical leveling measurements are performed using high accuracy levels equipped with optical micrometers and with the aid of high precision (invar) measuring rods [2]. The use of electronic and digital levels provides a standard deviation of 1 mm for a level difference of double km leveling lines, respectively mm, using invar rods (bar code) [9]. Of the several methods used to determine the level differences in the field, the authors chose the most widely accepted one, in which level differences are measured with one level and readings performed on both scales of the invar rod use two different horizons of the instrument. This procedure is strongly related to the need to ensure the proper ISBN:

4 accuracy required to measure the level differences in the field [2]. The study revealed in this paper work refers to an industrial construction situated in Timis County. The hall that was the subject of monitoring was technically expertised by an expert in civil engineering, because cracks and degradation could be observed on the upper part of some of the beams supporting the roof, which could have a profound character that could affect the safety in exploitation of the construction. Fig.3 Materialization of leveling benchmarks on the construction beams The geometrical leveling was achieved with the LEICA DNA03 automatic electronic level and with an invar kit, having an accuracy of 0.3 mm/km, working radius between 1.8 meters and 110 meters and a minimum focusing distance of 0.6 meters. Leveling instruments used were technically checked before the execution of the measurements and found that they are in good condition. Fig.4 Digital levels and invar road Fig.2 The degradation of the beams The technical expert indicated the need to achieve some deflection determinations of the beams, both at rest and in charged dynamic condition, simulating the weight of snow during winther. The method used for determining vertical displacements and deformations was the precise geometric leveling method. It is the technique that provides the highest accuracy in measuring vertical displacements of the constructions. Finding that not all roof beams were affected by degradations, it was decided the placement of leveling benchmarks, both on the beams affected by degradations and on several beams showing no visible cracks, as seen in the next picture: The processing of the measurements was performed using the software solution which is part of the package that also contains equipment used in the acquisition of observations on the field. Compensation of field measurements was carried out with Leica Geo Office Combined software. Fig.5 Leveling data processing ISBN:

5 Topographic measurements for determining the vertical variation of an element of a structure is always achieved in the form of observation cycles. For the study and the determination of displacement of the observation marks, two cycles of topographic measurements were performed. The first cycle of measurements is considered the initial measurement, consisting of measurements of the leveling control benchmarks placed on the construction beams in condition of repose, without any loading. The second cycle of measurements was performed on leveling benchmarks after loading the beams of the building with a weight equal to the maximum load that can occur during winter due to the snow. The simulation was carried out using people placed uniformly along the construction beam. The values of the deflection differ from a beam to another, but it is not directly related to the degradation found at the surface of the beams. Were monitored a total amount of twenty-four beams that had deformation deflection values between 0.5 mm and 1.4 mm. These values enclosed the tolerances, complying with the standards and regulations in force in Romania. The measurement result represents the estimated value for the real value of its size determined from the measured values [10]. Fig.8 Example of graphic representation of construction beam deflection Fig.6 Leveling in condition of load Thus, the deflection of the deformation was determined as the difference between the vertical position of the control points, both in condition of repose and under tension or load. The processing and analyzing of the observations collected during the two cycles were performed for determining the deflection of the deformation of each construction beam which had benchmarks mounted on. Fig.7 Construction beams under observation 4 Conclusion The difficulty in the measurement of building displacements is to find a spatial measurement technique that meet the requirements, such as, precision, reliability, low cost and user friendly. Some of these advantages can be obtained by using topographic-geodetic methods, but main challenge is to find one to satisfy all the conditions. The geodesist s role is very important as he has the responsibility to choose the most convenient technologies and techniques in order to meet de requirements in a situation given and, just as important, to ensure the proper application. The case study presented in this paper is a good example of the importance of the collaboration between experts from different domains in order to provide complete reports and safe solutions. Using equipments that are composed of both precision measuring instruments and specialized processing software lead to results that provide a high degree of confidence. Even if only some of the beams were affected by the phenomenon of degradation, additional ISBN:

6 measurements were carried out on the beams that didn t present signs of degradations. This made it possible to achieve a comprehensive and conclusive study of the situation. By comparing the values obtained in the determination of the deflections, it was concluded that there was no significant difference between the deformations that occurred in the construction beams where could be observed degradations and the ones occurring in the beams unaffected by degradations. From the values obtained in the two cycles of measurements, it can be concluded that the variation of displacements is within the tolerances admitted by the Romanian legislation for this category of buildings, ensuring the safety in exploitation. The conclusion was that the degradation does not have a depth character and does not influence the behaviour of the roof structure when loaded with a weight that exceeds the values reached during the cold season snow. However, the degradations have to be fixed, in order to exclude the possibility to increase and to influence the behaviour of the whole ensemble of beams from the roof structure. 5 Acknowledgement This work was partially supported by the strategic grant POSDRU/159/1.5/S/ (2014) of the Ministry of National Education, Romania, cofinanced by the European Social Fund Investing in People, within the Sectoral Operational Programme Human Resources Development References: [1] I. S. Herban, C. C. Musat, Determinate Dynamic Deformation of Constructions Using Integrated System Leica 1200, RevCad Journal of Geodesy and Cadastre, No. 10, 2010, pp [2] Gh. Nistor, I. Nistor, Direct Algorithm for the Calculation of Vertical Displacements and Deformations of Constructions Using High- Precision Geometric Leveling, RevCad Journal of Geodesy and Cadastre, No. 7, 2007, pp [3] T. S. Clinci, P. I. Dragomir, Perspectives for development of new positioning systems, Recent Advances in Geodesy and Geomatics Engineering - Proceedings of the 1st European Conference of Geodesy & Geomatics Engineering (GENG '13), Antalya, Turkey, October 8-10, 2013, pp [4] S. Herban, C. Muşat, Developing Models in the Study and the Traking of Mouvement of Buildings, RevCad Journal of Geodesy and Cadastre, No. 7, 2007, pp [5] González-Aguilera, Diego, Javier Gómez- Lahoz, and José Sánchez, A new approach for structural monitoring of large dams with a three-dimensional laser scanner, Sensors , pp [6] S. Herban, Measuring and determinate the dynamic deformation of constructions using modern theologies and techniques, RevCad Journal of Geodesy and Cadastre, No. 9, 2009, pp [7] G. L. Gassner, R. E. Ruland, Investigations of Leveling Equipment for High Precision Measurements, SLAC, Stanford, CA 94025, USA, January [8] C. Onu, Current Trends on Monitoring the Deformations of Studied Constructions, RevCad Journal of Geodesy and Cadastre, No. 11, 2011, pp [9] P. D. Dumitru, C. Cosarca, A. Calin, Modelling the quasigeoid for the Dobrogea and Seaside area, Romania, Recent Advances in Geodesy and Geomatics Engineering - Proceedings of the 1st European Conference of Geodesy & Geomatics Engineering (GENG '13), Antalya, Turkey, October 8-10, 2013, pp [10] C. Cosarca, Considerations on the tolerances and precisions in engineering measurements, Recent Advances in Geodesy and Geomatics Engineering - Proceedings of the 1st European Conference of Geodesy & Geomatics Engineering (GENG '13), Antalya, Turkey, October 8-10, 2013, pp ISBN: