ENGINEERING SURVEYING 2. Lecture Notes for the ... - Hírek

1 ENGINEERING SURVEYING 2. Lecture Notes for the BSc course BMEEOAFASI1 Szabolcs R zsa Budapest, 2009 1. The importance of geodetic control in the construction process The purpose of construction guidance observations Geodetic observations are quite often required during the construction of the ENGINEERING structures. The purpose of these observations is either the geometric guidance of the construction or the control of the geometrical quality of the built structures. In both cases surveyors are required to compare the as built status of the structure with the geometric position and dimensions of the structures on the plans. construction guidance observations are a part of the construction activities. These observations are carried out continuously during the construction process.

2 The purpose of these observations is to quantify the discrepancies between the built structures and the planned positions and dimensions, thus these discrepancies can be corrected for by applying construction /assembly corrections . Continuous guidance observations are required for example in the case of the construction of reinforced concrete structures using sliding formworks. Let s imagine that the chimney is being constructed, that has a circular cross section. The purpose of the construction guidance observations is to determine the radius and the center of the cross section of the built part. The discrepancies between the constructed cross sections and the planned cross sections provide the geometrical correction for the placement of the sliding formwork.

3 These corrections show not only the correction of the center line, but also the correction of the radius of the structure. Thus both the position of the sliding formwork and the dimension of the formwork can be corrected based on the construction guidance observations. In many cases the purpose of construction guidance observations is to adjust the position and the orientation of structural elements (for example: pillars). These adjustments are done with an iterative procedure. Firstly the structural element is placed approximately to the correct place and it is oriented approximately in the correct direction. Afterwards surveyors measure the exact position and orientation of the element and compute the required corrections to meet the planned position and orientation values.

4 Based on these corrections, construction workers can adjust the placement and the orientation of the structural element. Unfortunately mainly due to the large size and weight of these structures, this adjustment can not be done in a single step. Thus the new discrepancies in position and orientation must be quantified by the surveyor again, and new correction values are computed. Usually the correction values have a decreasing trend, thus the position and the orientation of the structure should converge to its planned position and orientation. This iterative process is done until the computed correction values are below the given tolerance. construction control observations are done after the construction process is finished. In this case the purpose of the observations is the determination of the geometric error of the construction .

5 The final approval of the construction usually depends on the results of these construction control observations. In some cases, when the construction process is separated into different individual steps, the results of the construction control observations are taken into consideration during the planning of the next construction step. Such an example can be the construction of the nuclear power plant in Paks, when the small geometric error of the construction of the buildings was taken into consideration in the planning of the technological facilities. In some cases the documentation of geometric construction error helps to determine the cause of malfunctioning structures. It is also important to mention that the positioning observations of the entire structures are usually distinguished from the positioning and dimensional observations of the structural elements.

6 This is because of the fact that in most cases a lower accuracy is required for the positioning and the orientation of the entire structure compared to the positioning and the orientation of the structural elements. Imagine that a large hall can be misplaced by several centimeters without any serious consequences, but the misplacement of pillars inside the building by the same amount would lead to the failure of the structure. The accuracy requirements of the construction guidance and control observations The accuracy requirements of the construction guidance and control observations are usually defined separately for the structural and positional guidance and controls. Structural requirements define the guidance and control accuracies for the positioning and orientation of structural elements, while positioning requirements define the accuracies applied for the positioning and orientation of the entire structure/building.

7 The requirements of the structural guidance and control observations are usually defined by the chosen construction method and the type of the structure. The numerical definition of the accuracies of the geodetic observations is usually based on the largest acceptable discrepancy, which can be defined as a function of the construction tolerance. In exceptional cases the construction tolerance or the mean error can be used as accuracy requirements. During the definition of the accuracy requirements one must bear in mind that these observations are carried out on built structures. Thus the first task is the definition of the observed (sample) points on the structure. On some of the structural elements it is easy to localize points, which can be undoubtedly observed by geodetic methods.

8 For example the head of the screws on steel structures can be used as precise marking for the points. In other cases the marking of the observational points is not so unambiguous, thus the marking of the points might increase the error of the observations. Such an example can be the control of the horizontality of reinforced concrete walls. The accuracy requirements of the construction guidance and control observations are usually computed using the acceptable construction discrepancy: ctzt =, where t is the accuracy requirement, tc is the largest acceptable construction discrepancy and z is the scale factor between the two aforementioned quantities. According to the Hungarian regulations z= , when the observed points are marked precisely.

9 A usual values of z= In case of non precise marking z= , with a usual value of The Hungarian Guide for ENGINEERING SURVEYING defines the accuracy of construction control and guidance observations are a function of the structural discrepancy and the distance between the control points and the observed structure (Table 1.) Acceptable structural observation discrepancy Distance [m] 10 20 40 80 100 5 mm 2 3 6 12 15 5 10 mm 4 6 11 18 22 10 20 mm 5 8 14 24 30 20 mm 8 12 19 30 40 Table 1-1. The accuracy of construction control as a function of acceptable structural observation discrepancy and distance to the control points 2. The preparation of construction guidance and control observations The reconnaissance The purpose of reconnaissance is to get acquainted with the construction site, to understand the requirements related to surveys and to discuss the schedule of construction activities with the client(s).

10 Office preparations The office preparations can be classified into three steps, which affect each other: a) scheduling the observations; b) localization of observational points; c) planning the observational technology. Scheduling the observation The scheduling of the observations highly depends on the scheduling of the construction activities. This can be checked in the project plan. However a continuous negotiation and contact is necessary with the client. Based on the applied construction technology guidance observations are needed for some activities (for example when pillars are erected and set to a vertical position) or continuously during the whole construction (application of sliding formworks). The scheduling of as built surveys depends on the scheduling of the forthcoming construction activities as well as the further application of the SURVEYING data.