Quality analysis and acquisition of information may open up new possibilities for quality control and surface modeling in tension structures.
By René Sander
The rapid development of laser scanners, paired with suitable evaluation software, has produced new applications of quality analysis and acquisition of information in the building industry and architecture. This paper demonstrates this concept through the examples of one membrane structure and two bridges. The concept may be used for dual functions of design development and the construction of membrane roof structures. During the building phase, different conditions can be examined and information obtained about the influence of the individual correcting variables. From these data the deviation between on-site layout and the target requirement (as shown on drawings) can be assessed and where possible, construction adjusted. This article illustrates the incorporation of the field-scanned data into the existing computational models.
Trends in contact-less measurement
By using terrestrial laser scanners as measurement tools, many possible new applications can be opened. The scanning of the surface of objects with a contact-less measuring system opens new ways in quality control and surface modeling. This way, digitized surfaces can be generated and evaluated within a very short time. This potential for use by the building industry and architecture requires only interdisciplinary communication as illustrated by the examples in this article of the Expo 2000 “worm” and two bridges. At Expo 2000 (World’s Fair) in Hannover, students of the University of Applied Sciences Anhalt (Fachhochschule) designed and constructed a pavilion using a membrane structure. The building (in the form of a worm-like undulating structure) was used primarily for different cultural meetings during the summer months of the World Fair and as a meeting place.
From a course on membrane structures given at Anhalt by Professor Dr.-Ing. Robert Off and the acquisition of a laser scanner, HDS 3000 of Leica Geosystem, the idea ensued to extend the scanner surveying to field measurement for creating digital models. The method was first applied to the modeling of an existing membrane structure, i.e., the “worm.”
New technology, new application
Laser scanners today can survey between 1,000 and 250,000 points per second. This is a faster and more efficient way to measure positions of discrete points, which represent the building in its entirety. Rasters or selected grid points can be used for approximate, arbitrary width measurements on surfaces. Thus, not only can the method of the actual data production change but also the possibilities of the evaluation. In fig. 1, the shaded measuring points are shown which illustrate the structure’s surface in a 2cm x 2cm raster. The gaps within the cover range can develop due to technical limitations of the equipment used and site conditions during data collection. They can be closed by curvature-dependent intelligent software or eliminated by slight geometrical adjustments with minor inaccuracy.
Producing the measuring data for inside and outside surfaces of a membrane structure is possible in one day or less, depending on the scanner used. Since not all measuring points can be captured from a single point of view or reference point, individual reference points must be registered with one another. That can be made by fixed points, those already used for the execution of construction or by means of reference surfaces. If two points of view overlap by approximately 30%, both proceedings are valid. The client that wants only a comparison of nominal and actual values can do so without fixed points. If an assessment is done that includes the surrounding field, (e.g., for spacer control or constructional supervision, into the foreground), it should not be done without fixed points.
Surface modeling/reverse engineering
Within the range of mechanical engineering/automotive manufacture there are different providers, from software to surface fitting. This report used a test version software from Geomagic that showed itself to be intuitively easy to learn and innovative and/or versatile in its evaluations. From the measuring points, a first meshing was counted (shown in fig. 2.) This first simple surface can be exported as an .igs file or to a .stl file and is thereby (at the user’s disposal) available for two different applications. For the comparison of nominal and actual values it is necessary to have cross-sectional data. These are available in the form of a base grid (fig. 3), which is also used for NURBS (Non-Uniform Rational B-Spline) computations.
Nominal vs. actual
The overlay of the surface from the measuring points and the NURBS from the target can take place in different ways. On the one hand, an absolute comparison of nominal and actual values is possible, if points of reference are used. In addition, it is possible to make a relative comparison of nominal and actual values, if both models are set by means of a “best-fit” transformation to the same scale of approximation. The information gain can be of a different nature, which will be demonstrated later in this paper with bridges. On the one hand, deviations from planning can be determined for constructional supervision, (fig. 4); where the color olive shows the actual and the grey is deviating runs. And on the other hand, relative deviations (fig. 5) can be represented on the plan, surface by surface, to show how they adjust themselves during the construction.
Here also, the increase in value of the new technology becomes obvious. Models in scientific research can be examined and accompanied, measured variables become in their effect quantifiable and improved models of the cross-sectional data can be constructed. With a small additional expenditure, data can be obtained both for construction supervision and research and development purposes. The genuine 3D-comparison of the data can take place (depending upon the cost of the preparation) directly following the measurement. Thus, within the shortest timeframe and with restrictions, on-line also would be possible.
Here the use of the scanner survey in the execution and the supervision of construction is demonstrated. By the example of the “Nasebachtal-bridge” of the BAB A17, it can be demonstrated how the information gained by the new technology can be used effectively.
Traditionally, one has to measure with Tachymeter the coordinates of a roadway using a 1.50m to 5m raster or grid points, in order to determine the planarism of the cover (i.e., digital map of the surface), but now the same can be obtained more accurately by using substantially more survey points within a short time using the scanner survey in figs. 6 and 7.
Since certain tolerances apply to the planarism of the building, it can be argued that only the largest deviations from an average level are of interest. These points can be located comfortably and handed over. Somewhat differently, an absolute connection must be interpreted over the fixed-point field of the building, (fig. 8.)
The building is within the usual construction tolerances. The roadway inclines, which promotes stormwater run-off. Even if the two bridge ends are apparently too high and/or too deep, then the connection to the road is important for smooth driving, which due to the construction activities on site could not be included. The planarism can be determined better with a relative comparison.
The range of scanners makes it possible to also control the round arch after production. In fig. 9, details of a round arch are to be seen, which was taken looking up from a point of view. The evaluation resulted in the normal construction tolerances, while because of the building size, influences of sun exposure and temperature straight deviations of the model are expected within the range of the resulting pylon connection.
A new measuring technology alone is not a means to an end. The needs of the associated disciplines have opened up new fields of deployment of this technology. In discussion with all those involved, new ideas and tasks are being developed to solve the challenges and from using this new measuring device the information gained will benefit all concerned. It is seen that the production of digital mapping offers great advantages over the traditional mapping methods and the information gained can be stored for long term use.
René Sander was a scientific assistant in surveying at the University of the Applied Sciences (Fachhochschule) Anhalt, Dessau, Germany. This paper is the result of the cooperation between Anhalt and the Institute of Membrane Structures (IMS e.V) and is part of his requirement for a Master in Membrane Engineering.
Stegner, Prof. G., “Experimenteller PavilionExpo Wurm”, VDV Magazine: Der Vermessungsingenieur, 2/2001.
Weber, Thomas, “Olympia-Schwimmhalle–München-Geometriebestimmung deinneren Dachhaut,” TerrestrischesLaserscanning (TLS). Ein geodätischesMessverfahren mit Zunknft” Schriftenreihe de DVW, Band 48/2005, Wießner–Verlag, Augsburg. ISBN 3-89639-511-4,ISSN 0940-4260, pp. 205–217.