A field and laboratory study on the dynamic response of the Eddystone lighthouse to wave loading
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Because little was known about how the masonry lighthouses constructed during the 19th century at exposed locations around the British Isles were responding to wave action, the dynamic response of the Eddystone lighthouse under wave impacts was investigated. Like other so called 'rock lighthouses', the Eddystone lighthouse was built on top of a steep reef at a site that is fully submerged at most states of the tide. Consequently, the structure is exposed to loading by unbroken, breaking and broken waves. When the breaking occurs, wave loading leads to complex phenomena that cannot be described theoretically due to the unknown mixture of air and water involved during the wave-structure interaction. In addition, breaking waves are generally distinguished from unbroken and broken wave due to the fact that they cause impulsive loads. As a consequence, the load effects on the structural response require a dynamic analysis. In this investigation the dynamic response of the Eddystone lighthouse is investigated both in the field and by means of a small-scale model mounted in a laboratory wave channel. In particular, field data obtained by the use of geophones, cameras and a wave buoy are presented together with wave loading information obtained during the laboratory tests under controlled conditions. More than 3000 structural events were recorded during the exceptional sequence of winter storms that hit the South-West of England in 2013/2014. The geophone signals, which provide the structural response in terms of velocity data, are differentiated and integrated in order to obtain accelerations and displacements respectively. Dynamic responses show different behaviours and higher structural frequencies, which are related to more impulsive loads, tend to exhibit a predominant sharp peak in velocity time histories. As a consequence, the structural responses have been classified into four types depending on differences of ratio peaks in the time histories and spectra. Field video images indicate that higher structural frequencies are usually associated with loads caused by plunging waves that break on or just in front of the structure. However, higher structural velocities and accelerations do not necessarily lead to the largest displacements of around a tenth of mm. Thus, while the impulsive nature of the structural response depends on the type of wave impact, the magnitude of the structural deflections is strongly affected by both elevation of the wave force on the structure and impact duration, as suggested by structural numerical simulations and laboratory tests respectively. The latter demonstrate how the limited water depth strongly affects the wave loading. In particular, only small plunging waves are able to break on or near the structure and larger waves that break further away can impose a greater overall impulse due to the longer duration of the load. As a consequence of the depth limited conditions, broken waves can generate significant deflections in the case of the Eddystone lighthouse. However, maximum accelerations of about 0.1g are related to larger plunging waves that are still able to hit the lighthouse with a plunging jet. When compared to the Iribarren number, the dimensionless irregular momentum flux proposed by Hughes is found to be a better indicator concerning the occurrence of the structural response types. This is explained by the fact that the Iribarren number does not to take into account the effects of the wide tidal range at the Eddystone reef, which has a strong influence on the location of the breaking point with respect to the lighthouse. Finally, maximum run up were not able to rise up to the top of the lighthouse model during the laboratory tests, despite this having been observed in the field. As a consequence, the particular configuration of the Eddystone reef and the wind could have a considerable bearing and exceptional values of the run up, greater than 40 m, cannot be excluded in the field.
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