CC BY
13
УДК 528.482
Ashraf Abd El-Wanis Abd El-Mawla Beshr
Egypt
ACCURATE SURVEYING MEASUREMENTS FOR MONITORING THE STRUCTURAL DEFORMATION
Abstract
The safety concepts form the basis of modern structures design and assessment codes. The detailed information about the structural deformations can help to determine the health of these structures. This paper investigates an integrated monitoring system for estimation the deformation behavior of structural members. Two different surveying techniques (one and two total stations measurement techniques) are presented to evaluate the deformation behavior of structural members. The comparison study between the surveying and structural techniques for computation the structural deformation of reinforced concrete beams is introduced and discussed.
1. Introduction
In many civil structures like bridges, tunnels and dams, the deformations are the most relevant parameters to be monitored. So monitoring the structural deformation and dynamic response to the large variety of external loadings has a great importance for maintaining structures safety and economical design of man-made structures. Dial gauge, accelerometer, Tiltmeter, etc. are traditional tools and methods to measure structure displacement, rotation and together with temperature, wind speed and direction allow the comprehensive investigation of structure dynamics behaviors. These tools must be installed, maintained, and frequently recalibrated to produce reliable results. The collected data from these tools need to be interpreted to obtain direct geometric results which in many cases is very complicated procedure and out of the control of the general structural engineers. Hence, a flexible surveying technique is needed to overcome these obstacles, and make the process of measurements easier and more accurate.
2. Pre-analysis study of the used surveying techniques
Pre - analysis of the surveying measurements is the analysis of the component measurements before the project is actually undertaken. Main items to be considered in the pre-analysis study of a certain survey project are: Possible surveying techniques, and thus the corresponding mathematical model, and available instruments (cost, simplicity and the precision of a single measurement).
2.1 One total station technique
From figure (1), the X-axis is chosen arbitrary as a horizontal line in the direction of the base of the monitoring building, where the Y-axis is a horizontal line perpendicular to the building base direction and positive in the direction towards the monitoring object, and the Z- axis is a vertical line determined by the vertical axis of the instrument at occupied station. There is a known coordinate's
point (A), and these coordinates are (XA, YA, ZA). From this point, we can monitor the movements
of any point (B) in space in order to determine its local coordinates (XB, YB, XB) and its accuracy. This case has a unique solution, so the multivariate propagation technique will be used.
2.2 Two total station technique
The two total stations technique employees the intersection process in three dimensions to determine the spatial coordinates of a specific target. As shown in figure (1), a local three-dimensional rectangular coordinates system is needed to calculate the spatial coordinates of any target points. There are two known coordinates points (XA, YA, ZA) and (XC, YC, ZC). From these two known points (A and C), we can determine the coordinates of unknown point B.
L
Point (A,C) is the known coordinates points (occupied Stations ) Point ( B) is the monitored point (Sheet Prism )
Figure (1) The geometry of two total stations technique
There are three unknowns (XB, YB, ZB) and six observations. Then the least squares adjustment technique will be used to calculate the coordinates of point (B) and their accuracy. The observations equation technique will be used.
3. Structural Data Analysis
Structural analysis is required to determine whether significant movements are occurred between the monitoring campaigns. Geometric modeling is used to analyze spatial displacements. General movement trends are described using a sufficient number of discrete point displacements (dn): dn (Ax, Ay, Az) for n = point number
Point displacements are calculated by differencing the adjusted coordinates for the most recent survey campaign (f), from the coordinates obtained at reference time (i). Each movement vector has magnitude and direction expressed as point
displacement coordinate differences. These vectors describe the displacement field over a given time interval. Comparison of the magnitude of the calculated displacement and its associated accuracy indicates whether the reported movement is more likely due to observations error.
Where: |dn| is the magnitude of the displacement for point n. It can be calculated as:
And (en) is the maximum dimension of combined 95% confidence ellipse for point (n), it can be calculated as following:
Where: af is the standard error in position for the (final) or most recent survey, ai is the standard error in position for the (initial) or reference survey.
4. Monitoring of the vertical wall
The precision of the points that have been monitored using the discussed surveying techniques should be evaluated to study the effect of the used instrument position distances and the angle of observations on the monitoring point accuracy. To achieve that goal, the monitoring of the vertical wall is done. A mesh of twelve monitoring points on the (7.7m x 3.0m) wall is distributed for coordinating a building facade. A local three-dimensional rectangular coordinates system is needed to calculate the spatial coordinates of any target points on the mesh. Two points are selected near the wall as a reference points. In each case, the coordinates of all points and its standard error are calculated.
4.1 Observations and analysis of one total station technique results
To find the best position of the used instrument and the best locations of monitoring points, some test measurements are carried out in the wall zone:
1. Reliance of the accuracy on the distance from the instrument position to the wall.
2. Reliance of the accuracy on the angular of line of sight.
4.1.1 Dependence on the distance from the instrument position to the wall
In this case, the line of sight of the instrument is perpendicular to the wall but the instrument position distance (D) differs five times (D=L/2, D=3L/4, D=L, D=5L/4 and D=3L/2), Where, L is the wide of the wall. From the differences between the measurements, a statement about the accuracy is possible. For " oX", when the instrument position distance increases, the standard deviations of all points will decrease. For "oY", when the instrument position distance increases, the standard deviations of all points will increase. For " oZ", when the horizontal angle (a) increase, the standard deviations will decrease, and when the vertical angle (y) increases, the standard deviations will increase. The graphical representation of
dn|< (en)
(1)
(2)
(D/H) ratio opposite the standard deviations is done. It is obvious that there is no optimum distance minimizes the standard deviations in three dimensions.
4.1.2 Reliance on the angular of line of sight
In this case, the distance between the instrument position and the wall is constant. Four different stations are used. Then, it is obvious that the horizontal angle (a) has a great effect on the standard deviation in X and Y -directions but this effect is small on Z- direction. Hence, when the horizontal angle (a) close to zero, the accuracy will increase. To achieve the maximum accuracy, it is important to ensure a maximum symmetrical configuration of the monitoring points on the monitoring object.
4.2 Observations and analysis of two total stations technique results
To find the best position of the used two instruments and the best locations of the monitoring points for this technique, some test measurements are carried out in the wall zone. The distributed targets are observed from two occupied stations O1 and O2 as shown in figure (2).
Figure (2) Geometric layout of the two total stations Position relative to
the object plane of the wall
The test measurements are carried out in the wall zone for: 1. Reliance of the accuracy on the distance from the two instrument stations to the wall (D) at constant (B). 2. Reliance of the accuracy on the distance between two instruments (B) at constant (D).The graphical representation of standard deviations opposite to the distance between the two instruments (B) is done. The best distance (B0) between the two instruments can be graphically determined according to that R must be minimum; this value relative to the façade building has been determined as: B0 = 0.7545 L
The graphical representation of standard deviations opposite to the distance (D) between the two instruments and the wall is done. The best distance (D0) between the two instruments and the monitoring wall can be graphically determined; this value relative to the façade building has been determined as: D0 = 0.242 L
5. The beam deformations monitoring
The structural application consists of four reinforced concrete beams, to estimate the deformation of these beams subjected to specified loads. The four tested beams have the same section (225 cm*20 cm*12 cm), but differ in reinforcement. Two of them have 2012 and the others have 2016. The steel used is high mild steel. The beams also have 506/m7 as stirrups. High Strength Concrete (HSC) mix is used. Two total stations, sheet prisms of diameter 1cm and calibrated dial and strain gauges are used in the field measurements. The rate of loading is 0.35 ton beginning at unload case to reach the failure load at 4.20 ton.
5.1 Analysis of one total station observations
Figure (3) Geometric layout of the monitoring beam and monitoring points
This beam is tested by using the one total station technique. The beam face is divided into ten monitoring points. The spatial distribution of these points should provide complete coverage of the beam as shown in figure (3). These points are located by using sheet prisms of diameter (1 cm), which are arranged to be visible from the location of the used total station as shown in figure (4). The adjusted vertical displacements of ten monitoring points under all cases of loading are calculated. A
Figure (4) The tested beam with the concentrated load and prisms
Comparison between the deflection values from one total station technique and dial gauge readings is done. The resulted deflection values from the one total station analysis are very close to those obtained from dial gauge readings. The differences between the two techniques are too small. By using the same structural analysis technique, the adjusted displacements in X- direction can be calculated. The maximum displacement value at point 10 and load 4.2 ton, and this value is 6.93mm. The directions of point's displacements in X-direction at failure load can be graphically done. By using the same structural analysis technique, the adjusted displacements in Y- direction can be calculated. It is obvious that no movement in Y- direction occurs.
5.2 Analysis of two total stations observations
The other beam is tested by using the two total stations technique. The beam face is divided into five monitoring points; the spatial distribution of these points should provide complete coverage of the beam. The adjusted coordinates and its associated accuracy of each point in the monitoring network are calculated by using least squares adjustment technique. A comparison between the deflection values obtained from the two total stations technique and dial gauge readings is done. It is obvious that the deflection values from the two total stations technique are very close to dial gauge readings from p= 0.35 ton to load p=3.85 ton. After load p=3.85 ton, there is a clear difference because of the vibrations of dial gauge during loading especially the dial gauges are placed under the tested beam.
6. Conclusions
The results of experimental work lead to the fowolling conclusions:
1. The two used surveying techniques (one total station and two total stations) can provide valuable data on the deflection of the structural members and movement of buildings because the resulted deflection values from surveying techniques with the discussed adjustment techniques are very close to the values from dial gauge readings.
2. The accuracy of the monitoring target coordinates is improved if the two total stations are set in the site at their best locations instead of using one total station. The best parameters were determined graphically:
B0 = 0.7545 L D0 = 0.242 L
© Ashraf AbdEl-Wanis AbdEl-MawlaBeshr, 2007
