Virtual Sensing for Fatigue Assessment of the Rautasjokk Bridge

Sammanfattning: This thesis treats virtual sensing for fatigue assessment of steel bridges. The purpose is to develop avirtual sensing method to use in the fatigue assessment process. The aim for the virtual sensing method is to only depend on strain measurements located on the bridge structure. The service life of bridges is often limited by fatigue and amending bridge design to improve fatigue resistance was developed in the 1970s. There are several bridges in Sweden, Europe and other countries that have exceeded their theoretical service life with regard to fatigue, and the need to replace them isboth a environmental and economical issue. The bridge over Rautasjokk north of Kiruna, Sweden is a specific example where the theoretical service life is limited by fatigue. Uncertainties in the theoretical fatigue assessment of bridges can be reduced by measuring strains atthe fatigue critical details, and therefore lead to a longer theoretical service life. Monitoring is, however,an expensive method and the procedure of installation and administration requires working time, and monitoring can only provide information at the gauge location. Hence, it is of great interest to optimizethe monitoring system. Virtual sensing is a method that could provide an alternative to conventionalmeasuring techniques. Virtual sensing combine measurement data with information from a model. Virtual sensing for fatigue assessment of the Rautasjokk Bridge was evaluated developing two methods. Both methods uses a finite element model of the bridge combined with strain measurements. The measurements were obtained on February 14 2018 and included a time signal and strain variations at six different locations of the structure. The accuracy of the virtual sensing methods were evaluated by comparing the fatigue damage of virtual sensing with the fatigue damage calculated using measured strains. The fatigue calculations were based on methods presented in the Eurocode EN-1993-1-9. The first method was based on the idea to find a relation between groups of stress ranges for two gauge locations on the bridge. The stress ranges were established by loading influence lines obtained from the finite element model with a fictitious train and the difference between two gauges was stored in a vector, the correlation vector. The correlation vector was applied on the measured stress ranges of the first gauge to estimate the actual stress ranges of the second gauge. No relation between groups of stress ranges for different loading cases was found and the correlation vector method for virtual sensing is not a sufficiently accurate method to apply in the fatigue assessment of the Rautasjokk Bridge. The second method was based on finding a relation between each stress range instead of a group of stress ranges. Influence lines from the finite element model were used to find a relation between each stress range of two gauges. Their relation was stored in a matrix, the correlation matrix. The matrix was applied on the measured stress ranges of the first gauge to estimate the actual stress ranges of the second gauge. The correlation matrix method for virtual sensing estimate the fatigue damage sufficiently accurate at the bridge locations where local stress ranges have the greatest impact on the fatigue damage. Results obtained through virtual sensing only include the same parameters that were used as inputs in the method. A credible virtual sensing method is crucial in order to achieve reliable results. In general, a virtual sensing method requires an extent amount of input data to validate its reliability. Further studies are required to investigate how the uncertainties of the correlation matrix affect the fatigue assessment.