The Horizontal Deformation Analysis of High-rise Buildings

© 2017 Žymantas Gražulis, Boleslovas Krikštaponis, Algirdas Neseckas, Darius Popovas, Raimundas Putrimas, Dominykas Šlikas, Evelina Zigmantienė. Published by VGTU Press. This is an open-access article distributed under the terms of the Creative Commons Attribution (CC BY-NC 4.0) License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. The Horizontal Deformation Analysis of High-rise Buildings


Introduction
Deformation measurements of large structures, especially building tall buildings (chimneys, cooling towers, television towers and poles, water towers, etc.) are one of the most important issues of surveying engineering. A chimney is a necessary part of a heat-producing plant, a refinery and many other types of industrial buildings. It poses potential dangers for human safety and property because if it collapses, it may cause immeasurable damage to human lives and wealth. It is essential to monitor and measure the chimney deformation on a regular basis. The main task of the deformation measurement is to measure inclination of the centerline of a chimney from the vertical, while it only can be determined indirectly (Zheng et al. 2012). Measurement devices and techniques in this field are mainly based on the monitoring of discrete points (Kregar et al. 2015). Despite of the development of the innovative monitoring engineering techniques, in practice, traditional measurement methods are still widely used. Besides classical techniques, for the recording of surfaces nowadays laser scanners are used, which acquire dense point clouds in very short time (Uchański, Soerensen 2010). Small-scale deformation monitoring using terrestrial laser scanning is gaining considerable attention mainly due to the high spatial resolution of the acquired data (Tsakiri et al. 2006). As highlighted in numerous previous research works, TLS can provide surveyors with the means to conduct far more complete (dense) measurements in relatively short time. This would lead to more reliable dimensional control results, despite the fact that single point precision is mostly not sufficient (Girardeau-Montaut et al. 2005;Park et al. 2007). It appears that precision potential in these applications is not limited by the single point accuracy of laser scanners, since surface elements derived from a large number of points are used for deformation analysis (Tang et al. 2010;Bosche, Haas 2008;Schneider 2006).
In article, we present a comparison of TLS and classical deformation measurement of two chimneys. In TLS approach the point cloud was cut into thin layers and projected onto 2D planes in different height levels. Then a cylinder-fit algorithm was applied to these cross section point clouds, where the center points of the cylinders have a much higher precision than single laser scanner points (Akca 2004). The connection of center points of these cylinders at different heights represents the centerline (axis) of the chimney. The comparison with the results from total station measurements also presented. Gražulis, Ž.; Krikštaponis, B.;Neseckas, A.;Popovas, D.;Putrimas, R.;Šlikas, D.;Zigmantienė, E. 2017.
The horizontal deformation analysis of high-rise buildings 2

Measurement methods and research object
The research object is two chimneys of joint-stock company "Vilniaus energija" in small town Trakų Vokė. The height of both chimneys are about 20 m and the diameters at the bottom are 0,74 m and 0,94 m respectively. Chimneys are part of heat producing facility located in Trakų Vokė.
To establish the geodetic control network for chimneys deformation monitoring the benchmarks were measured with GNSS receivers using LITPOS continuously operating GNSS stations network data as reference stations. For classical horizontal deformation measurements electronic total station Leica TCRP1200+, with angular accuracy of 1ʺ was used, and for TLS measurements laser scanner Leica Scanstation C10 with angular accuracy of 12ʺ was tested.
In most cases, deformation monitoring is performed by using electronic total stations. First of all the exact coordinates and heights of the monitoring benchmarks are derived and adjusted using geodetic methods. From these benchmarks the cross-sections at selected heights are measured. It is important that measurements from all stations is performed bisecting the same height (cross section) of the chimney. Chimney deformation is reflected by center displacement of each chimney cross-section. Measurements were carried from two stations at 6 cross-sections evenly distributed along the chimney (Fig. 1).
Terrestrial laser scanning is considered faster and more progressive geodetic measurement method. The terrestrial laser scanner position and orientation was set using three high reflective targets. Position of these targets was measured with electronic total station from the same geodetic control network points used in classical approach. The targets were positioned in a way that they will be visible from both measuring stations, and provided good geometric distribution for scanner position determination. Observations with the terrestrial laser scanner covered whole chimneys, not just chosen cross-sections ( Fig. 2). Processing of measurement data.
The observed chimneys have cylindrical shape, therefore cross-sections has a circular form. In order to determine deviations from the vertical it is necessary to calculate the centers of the circles of each cross-section from measured points. The circle center coordinates are calculated according to the following formulas. Circle arc points and the coordinates of the center can be linked with the radius by the equation: Here ‫ݔ‬ and ‫ݕ‬ -coordinates of chimney surface points in the corresponding cross-section, ‫ݔ‬ ir ‫ݕ‬ -the coordinates of a center of the cross-section, R -radius of the chimney cross-section.
After performing some mathematical operations: Applying correction equations for each measured point and using the least squares approach to find unknowns ߬_݅ we can estimate radius and center coordinates of the circle: Using the formulas above, cross-section center coordinates of both chimneys was calculated and chimneys deviations from the vertical were estimated. Deviations were estimated in each cross-section with respect to the lowest cross-section.
In each cross-section 8 surface points of the chimney were measured using total station. Estimated vertical deviations of the chimneys from total station measurements presented in tables 1 and 2, and figure 3 and 4.   Measurements with terrestrial laser scanner were performed with density 1point/1mm 2 across all surface of the chimney. From the point cloud 8 thin layers in different height levels (the same heights as total station measurements) was cut and center coordinates of these point cloud cross-sections were estimated. Center coordinates of each crosssection were computed from around one thousand points.
Estimated vertical deviations of the chimneys from TLS measurements presented in tables 3 and 4, and figures 5 and 6.  In the tables above, the center coordinates of the cross-sections expressed in LKS94 coordinate system, normal heights in LAS07 height system and the relative heights of the chimney expressed in relation with the bottom of the chimney. There are coordinate differences (chimney deviations from the vertical) in x and y components and linear deviation presented as well.

Comparison of the results
We compared estimated chimney deviations of the vertical from different measurement methods.
Deviation determination was done using the two methods and assuming that they have more less same accuracy, chimney deviation accuracy assessment was carried out by double measurements differences.
where: ߝ -differences of double measurements, n -number of differences.
It is also possible to calculate the deviations from the averages from the two methods, then the mean squared error of the deviation is: Deviation accuracy assessments presented in tables 5 and 6.