PSI - Issue 64

Paul Winkler et al. / Procedia Structural Integrity 64 (2024) 1264–1270 Winkler et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction There are over 5,800 motorway bridges in Germany that are currently in need of renovation and others will reach their planned service life in the next few years (BMDV 2022). This raises the question of whether these structures can continue to be used. In some cases, the load-bearing capacity can no longer be calculated in accordance with current regulations, even though the condition of the structures is good and there is no major damage, or this has already been rectified by a strengthening measure. Often there is also a lack of information about the structure, which then leads to conservative assumptions for a recalculation. In order to ensure safe utilisation of the structure and to be able to assess its condition, structural health monitoring (SHM) methods are used. The recorded data is interpreted and a condition assessment is carried out using numerical structural modelling. As part of the German Research Foundation's Priority Programme 2388 (SPP "100+"), the DIVING research project is working on the "Digital linking of multiscale analyses in modelling and monitoring". In this way, the full potential of the structure can be utilised without risk, thus extending its service life. This can make a significant contribution to the safety of the infrastructure and minimise the use of limited resources. 2. Component test 2.1. Test setup A component test was used to develop methods that will later be applied to existing structures. Based on a section of a box girder of a reinforced concrete bridge superstructure, a reinforced prefabricated T-angle support element (Fig. 1a) was selected for this purpose. As part of the experimental investigations, the structural element is damaged in a controlled manner by a gradually increasing quasi static point load. The force is applied to the vertical part of the test specimen via a centred load plate. The load was applied in 8 load steps of 20 kN each from 0 to 160 kN. The levels were selected in such a way that no, minor and major damage occurred until shortly before the failure state of the plate. The lower and upper edges of the vertical plate were held in place by crossbeams during the loading phases. To check the position and deformations of the test specimen, the horizontal and vertical displacements of the component were recorded at 6 and 3 points respectively. At the same time, optical methods were also used to record deformations and identify cracks. These investigations were carried out in co-operation with the Institutes for Flight Guidance and for Geodesy and Photogrammetry at the TU Braunschweig, which are also involved in the SPP 100+. The focus of this contribution is on the identification of changes in the dynamic structural behaviour from vibration tests with 24 accelerometers arranged in a grid on the vertical plate and the evaluation of structure-borne sound signals recorded during the loading phases and simultaneous crack formation with two acoustic emission sensors. During the load increase, the deformations and rigid body displacements of the test body were measured with inductive displacement transducers in addition to structure borne acoustic signals. After reaching each load level, the deformation was kept constant by the impact (displacement-controlled test) and recorded using photogrammetric images with a high-resolution digital camera and stereoscopic images with a stripe light scanner. The static load was then reduced and the structure unloaded. In order to ensure consistent boundary conditions, the crossbeams required as abutments for static loading were removed, now the vibration tests were carried out to record the dynamic behaviour of the structure. In the vibration tests, the component was struck several times with an impulse hammer at a defined point in the upper edge area. Two series of tests were carried out for each damage state, then the crossbeams were reassembled to go on with the next load step. The cracks visible on the surface from the 3rd load level (60 kN) were measured with a crack width ruler, labelled and documented in the course of the load increase. From the 6th load level (120 kN), the uppermost crack formed in the end area of the staggered reinforcement, which opened to over two millimetres at the 8th load level (160 kN) and signalled a break in the slab, whereupon a further load level was dispensed with (Fig. 1b). 2.2. Test implementation