Geodetic Monitoring of Digitally Fabricated Structures Early After Construction

Abstract

The construction industry is currently in a process of increased automation and digitization which has an impact on all phases of a building's lifetime, including design, fabrication, assembly, and maintenance. The goal of this innovation is to primarily boost productivity and enhance sustainability. Achieving these goals requires geometrical information about the fabricated objects, first to check for discrepancies between the as-built state and the design one, and second to assess changes over time. Point-cloud based methods, augmented with traditional geodetic metrology are well suited to provide the necessary information. In the current phase of the development, monitoring is particularly relevant because it is necessary to assess the readiness of the innovative fabrication techniques to be adopted by the industry and to assure safety to the users despite the limited experience with the construction processes, designs, and materials used. Herein we present the geodetic monitoring of three digitally fabricated structures during the early phase after construction, along with the specific challenges and the chosen solutions. The structures are parts of a real building designed and built within the National Centre of Competence in Research Digital Fabrication (NCCR dfab) at ETH Zürich. Concretely, they comprise (i) a curved wall realized by robotically spraying concrete onto a wire mesh previously built in-situ by a robot; (ii) an ultra-thin curved ceiling created using 3D printed formworks, and (iii) a robotically cut and assembled timber beam construction. Terrestrial laser scanners (TLSs) and a laser tracker (LT) with a hand-guided triangulation scanner were used. The presented deformation analysis consists of point-wise coordinate comparisons within a network, of a cloud-to-mesh (C2M) comparison of pairs of point clouds, and of a hybrid approach denoted as virtual monitoring points (VMP) approach herein, i.e. point-wise analysis derived from the independent registration of small patches of point clouds. The latter is particularly useful for monitoring of structures with salient surface structures where high accuracy is needed but no markers may be applied. The analyses showed that the structures produced using the 3D printed formwork agree with the plan to within 1.5 cm (maximum C2M deviations). The concrete and the timber structures showed maximum changes of a few mm over the time span of seven months. Using the point-wise LT measurements we found an agreement of deformation patterns with temperature for the curved ceiling.