In risk based inspection planning the number of planned inspections directly depends on the uncertainty on the load history. In this paper it is shown how this uncertainty can be reduced by modal based fatigue monitoring. The technique is based on continuously measuring of the accelerations in few points of the structure. The accelerations are integrated to obtain the displacement and a modal decomposition of the obtained displacements is performed. The relation between the modal coordinates and the stress history in an arbitrary point of the structure is established by a calibrated finite element model. The technique has been applied on a wind loaded lattice pylon structure and compared with strain gauges measurements, showing that the uncertainty on the presented technique is quite small. It is shown that if the technique is applied to an offshore steel structure, then the number of required inspections may be reduced with more than 50%.

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