Measurement of Structural Deformation using Terrestrial Laser Scanners

Abstract

SUMMARY Three deformation measurement experiments have been undertaken where, in each experiment, a structure was subject to controlled loading. Terrestrial laser scanners (TLSs) were used to make measurements at critical intervals during each load test. TLSs, which are a relatively recent innovation, are capable of rapidly capturing thousands of three dimensional points over the surface of an object. They are largely untried in the area of structural deformation monitoring. The purpose of these tests is to assess the sensitivity of TLSs for the measurement of vertical deformation of loaded structures and to investigate their potential for metrology tasks where remote observations (completely non-contact and targetless at distances greater than 5m) are desirable. A Riegl LMS-Z210 laser scanner was used for all three experiments and a Cyra Cyrax 2500 was available for the first. The point cloud data were represented with analytical models representing the vertical deflection of each beam. The development of these models required knowledge of the dimensions of the beam, spatial position of the load point(s) and support points. Functional models (low-order polynomials) were derived based on generalised coefficients of the analytical models. These generalised coefficients were solved as unknown parameters in a least-squares estimation process. Analysis of the accuracy of the TLSs involved computing differences between benchmark photogrammetry and laser scanner vertical deflections. In the first experiment, the RMS of differences (for eight load cases) between the Cyrax 2500 and photogrammetry was ±0.29mm and ±3.6mm for the LMS-Z210 (using 104 samples). The RMS of differences for the LMSZ210 in the second experiment was ±2.4mm for 12 load cases (using 144 samples). The RMS of differences for the final experiment, where measurements were conducted on four horizontal beams, ranged between ±4.2mm to ±9.5mm. The greatest improvement of accuracy was up to 21 times the single-point precision of the TLS. These results highlight the potential application of TLSs for precision metrology, given that their accuracy can be greatly improved by exploiting the 3D point clouds with simple modelling techniques.