PSI - Issue 5

V. Epin et al. / Procedia Structural Integrity 5 (2017) 620–626

623

R. Tsvetkov/ StructuralIntegrity Procedia 00 (2017) 000 – 000

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The above monitoring system provided control of 12 cracks localized in potentially dangerous zones of the beams. In Fig. 2, the crack sensors are denoted with capital letter CS (crack sensor).

3.2. Measurement of vertical displacements of beams by the hydrostatic levelling technique

The hydrostatic level is a system of communicating measuring vessels filled with a liquid. The liquid level in these vessels determines the plane relative to which quasi-static vertical displacements can be controlled. Such systems can be used to monitor vertical displacements of the structure elements at the points of mutual arrangement of hydrostatic level sensors. The operation capability of the hydrostatic levelling system at low temperatures is sustained by the use of 50% aqueous solution of glycerin, which freezes at temperatures lower than – 25 C. A hydrostatic level sensor is a measuring graduated tube filled with a liquid. The liquid level in the tube is registered automatically by detecting the position of the meniscus relative to the location of markers. A detailed description of recognition process for the hydrostatic level sensor is given in work (Epin et al. (2016)). Resolution of the sensor is 90 µm for a single measurement and increases up to 50 µm for a series of measurements. The monitoring system controls the vertical displacements of 3 longitudinal beams (see Fig. 2, beams No. 3, 5, 7). Each beam is provided with 5 sensors: at the beam-to-building wall junctions, in the zones near the columns and in the middle of span of the beam. Thus, 15 sensors are located at the specified points (denoted with capital letters HS- hydrostatic sensors), as shown on the diagram in Fig.2. Monitoring of the vertical displacements of structure elements are duplicated by registration of beam deflection relative to the position set by a taut metal string connecting the opposite ends of the beam (for example, Stanton et al. (2003)). The load of a constant mass provides permanent and stable stretching of the string, specifying the position of the base straight line. Measurements of vertical displacements with respect to this line are made in the following manner. A group of markers is fixed on the beam and string in one and the same plane and changes in their position are recorded with the IP-camera fixed to the beam. Processing of the obtained images with the above mentioned algorithm allows us to determine the distance between the groups of markers located on the beam and the string. The vertical displacement of the beam at the point of camera attachment caused by structure deformation is estimated by a change in the vertical component of the distance between the markers. Setting of several cameras on the beam makes it possible to register beam deflections at different points. This measuring technique is practically independent of fluctuations of the environment air temperature, since stretching of the string remains steady and small changes in its length produce negligible effect on its deflection Resolution of the sensors are 15 µm for a single measurement and 5 µm for a series of 10 measurements. One string is stretched along each of 9 beams and each is provided with 3 sensors denoted on the diagram in Fig. 2 with SS (string sensors). The string is fastened to the beam at a distance of 1m from the beam ends, which allows control of deflections almost along the entire length of the beam. In contrast to the hydrostatic level sensor, the SS executes control of deflection changes only within one longitudinal beam, which is quite sufficient for estimation of the deformation state of a separate beam not of the entire structure. The system of reinforced beams resting on iron columns is the main load-bearing element of the air bridge. A mathematical model has been developed to describe the deformation state of the system of beams of the air bridge taking into account its interaction with all elements of the structure. The strain-state of the reinforcement material was described in the framework of the elastoplasticity theory (Kachanov (1971)), and the concrete condition was assessed in the context of the theory of brittle fracture (William et al. (1974)). The finite-element method was used for the numerical implementation of the model, which took into account the location and state of cracks at the instant of crack sensor installation. The numerical experiments allowed us to reveal the specific features of the deformation interaction of the iron reinforcement with concrete in the presence and absence of cracks and taking onto account the probability of 3.3. Measurements of vertical displacements of beams by the taut string method 4. Deformation analysis