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|Title:||Displacement and Strain Field Photogrammetric Measurements of a Reinforced Concrete Slab Submitted to an Earthquakes Loading|
|Citation:||Proceedings of the 3rd Workshop on Optical Measurement Techniques for Structures and Systems (OPTIMESS) p. 1-8|
|Publisher:||OPTIMESS Scientific Research Network supported by the Fund for Scientific Research, Flanders|
|Type:||Articles in periodicals and books|
|Abstract:||These measurements are part of an action aiming to develop and generalise the use of photogrammetry in the European Laboratory for Structural Assessment. ELSA belongs to the Joint Research Centre, a DG of the European Commission. Its experimental facility allows to test full-size buildings with respect to static and cyclic loading, and to simulate earthquake loading through the use of the so-called pseudo-dynamic method â?“ which magnifies the time scale. Structures are usually driven to ruin through series of tests, which implies that fractures opening and closing, and even material removal occur during each cycle of the experiment. The characteristic scales of the tested structures span 2 to 20 meter, and they are made of a wide range of construction materials such as brick masonry, reinforced concrete, steel or composite. The large variety of experimental conditions at ELSA provides a rich field of application for photogrammetric techniques. In the present experiment, the camera was obliquely aiming at the ceiling of the first floor of a building, near the connexion to a pillar, while the structure was under earthquakes loading. In the field of view, a pixel would be of the order of 1 mm. A network of 147 circular targets was disposed on a rectangular mesh (of step 10 cm) on the ceiling, and a series of 1250 photos were sampled at regular time steps in synchronisation with the various sensors and the control loop of the loading. The camera has a resolution of 1536x1024 pixel, with 12 bit sampling and Peltier cooling. This cooling stage provoked the formation of a condensation pattern on the sensor, a parasitic effect that could be fortunately removed through the use of a condensation mask computed by ad-hoc technique applied to the set of images. The camera was calibrated using the method of Bouguet. In first approximation, the ceiling was considered to move quasi-parallel to itself. The targets were monitored along time by various techniques, like fitting of an ellipse to their border, or optimising on the current photo the projection of the perimeter of the target, with respect to translation. The Heidenhain reference sensor of the control loop was placed on the external border of the slab, I meter far from the nearest target. However, the longitudinal displacement are in a very good agreement, showing a difference of 1 mm at maximum, while the amplitude of the movement was 50 mm. On both extremities of the signal, when the building is at rest, a noise of 0.04 mm amplitude could be observed on the difference between Heidenhain and optical method. At the end of the experiment, the difference between the most distant targets (along loading) shows an elongation of approximately 2 mm, corresponding to the remnant opening of the fractures. The quality of the measured displacement field, coupled with the fracture segmentation via morphological image analysis, permitted to compute longitudinal and lateral strain maps as a function of time, with the fracture pattern. A film will be shown, illustrating this quantitative analysis.|
|JRC Institute:||Institute for the Protection and Security of the Citizen|
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