PSI - Issue 64

Pavel Ryjáček et al. / Procedia Structural Integrity 64 (2024) 228 – 237 Pavel Ryjacek / Structural Integrity Procedia 00 (2019) 000 – 000

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measurement is prevented in praxis by the measurement time in the order of hours, during which temperature changes occur, this influence can be eliminated only by measuring in very stable conditions. Other results are based on the analysis by CAMOSUC(j),x, the change of the corresponding diagonal members of the modal flexibility matrix [  ] and the change of the second derivative of the modal flexibility matrix  r. Based on the evaluation of the 24 sections on the footbridge according to the measurements made, it can be concluded that the manifestation of very extensive (80%) corrosion of the cables Alpha was about 3 times smaller than the manifestation of the stiffening of the end sections of the footbridge and about 1.8 times smaller than the inaccuracy in the temperature detection by 5°C. In the case of the combination of 80% corrosion of the cables Alfa and 100% corrosion of the cables Beta, the manifestation was about 3 times smaller than the manifestation of the stiffening of the end parts of the footbridge, and about 1.8 times smaller than the inaccuracy in the detection of temperature by 5°C. Based on the CAMOSUC(j),x evaluation for 24 sections, the manifestation of very extensive (80%) corrosion of the cables Alpha was approximately 5 times smaller than the manifestation of the stiffening of the end sections of the footbridge and comparable to the inaccuracy in the detection of the temperature by 5°C. Better results in damage detection could be obtained if the measurements on the footbridge were made in a denser grid of 86 sections. However, it is a prerequisite that an initial same measurement is made on an undamaged structure so that the necessary comparison can be obtained. However, this was not realistic in 1984, given the measurement possibilities and the practice at that time. 13. Conclusion On the basis of the above results, it can be concluded that the effect of the considered significant corrosion damage of the Alfa and Beta cables on the dynamic properties of the footbridge is relatively small and smaller than the observed effect of the uncertainty in the measurement of the temperature, the effect of the stiffening of the end parts of the footbridge in the bearing area and the uncertainty in the measurement. Thus, these effects overlap the manifestation of damage to the Alpha and Beta cables, merge with each other and make their detection and identification on such a complex and temperature-sensitive structure almost impossible. This is mainly due to the extreme temperature sensitivity of the bridge. In other cases, the above detection methods work. Therefore, for dynamic measurements used to monitor structural damage, we recommend: • Perform an analysis of the effect of damage on the dynamic characteristics, to determine the position of the sensors, the range and density of the sensor net, and the sensitivity to temperature. Measurements without knowledge of the behaviour of the structure on a numerical model may lead to unusable results. • It is necessary that the person requesting, for example, a load test (static or dynamic) is aware of the importance of the numerical model and requires the test to be complemented by an appropriate numerical analysis. • Do not evaluate primarily the natural frequencies and shapes, their change may not be the result of damage but of completely different influences. • On the other hand, use advanced detection techniques coupled with knowledge of the actual action of the bridge and the manifestations of the faults detected in the numerical model. However, their sensitivity depends on the number of sensors; small numbers significantly reduce their informative ability. • Perform very accurate temperature measurements, while eliminating the temperature influence by performing measurements under constant temperature conditions. Research was supported by project of Technology agency of the Czech Republic No. CK03000219 “ Advanced Modular Cloud Computing System for Bridge Infrastructure Monitoring Utilizing Fibre Optics ”. Authors also greatly acknowledge the support from the Technical Road Administration Prague. References Strasky, J., The stress-ribbon footbridge across the river Vltava in Prague, L'Industria Italiana del Cemento, 10, pp. 638-653. 1987 Strasky, J. et al, Design drawings of the bridge, Dopravní stavby Olomouc, 1983, not publicly available Renovation of the footbridge in Prague Troja, TOP CON Servis, 1998, not publicly available