In this paper it is shown how the accumulated fatigue in steel structures can be estimated with high accuracy by continuously measuring the accelerations in a few points of the structure. First step is to obtain a good estimate of the mode shapes by performing a natural input modal analysis. The so obtained mode shapes are then used to calibrate a Finite Element model of the structure and to obtain the modal coordinates of the active modes by inverting the mode shape matrix. If the number of active modes is smaller than the number of measurement points, then the problem is solved by regression. If the number of active modes is larger then the number of measurement points, then the number of active modes is reduced by band pass filtering. Once the modal coordinates are identified and thus the complete response is established in the modal domain, then the information is mapped to the physical domain by applying the mode shapes of the calibrated Finite Element model and strains are obtained using the shape functions for the actual elements. The technique has been applied on a model frame structure in the laboratory and on a wind loaded lattice pylon structure. In both cases the estimated stresses has been compared with direct strain gauge measurements and it appears that the difference between the strains measured by strain gauges and the strains estimated by the presented technique is quite small. Looking at the fatigue of the lattice pylon it appears that the estimated damage is significantly smaller than the damage forecasted by commonly accepted design rules. This points to possibilities of significantly increasing the fatigue lives of structures by performing continuous monitoring.
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