PSI - Issue 64

Maciej Kulpa et al. / Procedia Structural Integrity 64 (2024) 1673–1680 Kulpa / Structural Integrity Procedia 00 (2019) 000 – 000

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Fibre optic sensors without a cover were employed to reduce interference in the laminate and ensure correct connection (Fig. 7). Reinforced yellow sections with a plug, called pigtails, were welded to the measurement fibres that secured the sensors in the crucial zone, where they exited the structure. Laser testers were used to verify the continuity of the sensors after installation. After the span installation at the construction site, all pigtails were hidden

in a box attached to the side of the span. 3. Proof load test of the footbridge 3.1. Static test

The static tests were performed by loading the spans with concrete slabs. The slabs were placed individually, in two layers. Under maximum load, 16 slabs with a total weight of 260 kN were placed on the span. This corresponded to 85% of the characteristic load for which the footbridge was designed (5 kN/m 2 ). After verifying the measurement system without loading, the subsequent slabs were placed on the span in one layer from one support zone to another. After positive verification of the footbridge behaviour under half load, the second layer of slabs was placed in the same way. After 30 minutes of maximum loading, the slabs were removed from the span in the opposite order. The same loading scheme was applied for each span (Fig. 8a).

Fig. 8. Proof load tests of the footbridge: static (on the left) and dynamic (on the right)

During static tests, strains in the relevant composite elements were measured with all installed sensors. An example of measurements in the form of an increase in longitudinal strains in the upper laminate as a result of loading with subsequent slabs (P01-P16) is shown in Fig. 9. As can be seen, there are local fluctuations on the plot, but they are visible from the very beginning of the test, i.e., they do not result from the local damage caused by the increasing load. Such fluctuations in the structure, probably induced by infusion, are a natural phenomenon related to manufacturing imperfections due to high infusion pressure. The maximum measured strains were 500 × 10 -6 , which for a material with a Young modulus of 33 GPa corresponds to approximately 16.5 MPa of stresses, well below the compression strength of the material, that is, 145 MPa. This confirms that the ultimate limit state is usually not decisive for this type of structure. Due to the use of distributed sensors that run along the entire length of the single span, it was possible not only to determine the strains (measured directly) but also to calculate the vertical displacements of the span. This calculation procedure for that DFOS monitoring system was tested under laboratory conditions by Kulpa et al. (2021). To verify this calculation performed on the basis of the DFOS measurement, displacements were also measured using high precision geodetic techniques with an accuracy of 0.1 mm as a reference system. The comparison of vertical displacements under the maximum static load along the length of the span determined by the DFOS system and the reference system, as well as the design values (FEM) is shown in Fig. 10. As can be seen, all three results were almost identical (93-98%). This confirms the correct and accurate operation of the DFOS monitoring system.