Bayesian operational modal analysis of offshore rock lighthouses for SHM

During 2016 and 2017 a program of field vibration measurements was made on a set of Victorian era granite lighthouse towers around the British Isles. The field tests were designed for structural identification to enable condition assessment and identification of extreme wave loads through long term monitoring. The primary test method was forced vibration, and ambient vibration measurements was used as a backup. The best operational modal analysis (OMA) results were obtained using Bayesian OMA, which provide a clear picture of the directionality of the mode shapes which appeared at very close frequencies due to the symmetry of the towers. The paper describes measurements and sample analysis illustrating difficulties and achievements.