Experimental modal analysis of British rock lighthouses
Experimental modal analysis of British rock lighthouses
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DOI：
10.1016/j.marstruc.2018.07.001
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发表时间：
2018-11-01
影响因子：
3.9
通讯作者：
Dobson, Peter
中科院分区：
文献类型：
--
作者：
Brownjohn, James Mark William;Raby, Alison;Dobson, Peter
19世纪在不列颠群岛周围的近海礁石上建造的标志性灯塔继续在安全航行中发挥关键作用，但这些历史建筑的寿命受到极端天气的威胁。利用监测到的响应数据，进行了一系列的实验动力学研究，以支持对这些结构的极端冲击破碎波载荷的表征。本文描述了该项目的程序和结果，其中包括2016年6月至2017年10月期间对其中六座灯塔进行的模态测试。在测试的六座灯塔中，有五座(Les Hanois、Wolf Rock、Longship、Bishop Rock和Eddystone)在一座19世纪的砖石塔上安装了20世纪的金属螺旋桨，苏格兰灯塔(Dubh Artach)是一个例外，它提供了相对简单的塔的基线行为。所有的砖石塔在某种程度上都是不完美的轴对称，都给实验工作带来了后勤挑战，因为它们只能通过直升机飞行进入，受到恶劣天气和时间的限制。面对这样的挑战，有可能识别关键的模态参数，并突出由于对称性和直升机舵改造而产生的一些有趣的影响。值得注意的是，最重要的模态的频率范围在4赫兹到7赫兹之间，模态质量低至200t。研究了很少被研究的轴对称对强迫振动测试的影响，以及由于改造舵舵而引入的额外模式。最后，通过2017-2018年冬季风暴季在狼岩灯塔上记录的加速度数据表明，模态试验数据可以用来推断高达8KN的破碎波模态脉冲。
Iconic lighthouses constructed on offshore reefs around the British Isles in the 19th century continue to play a crucial role in safe navigation, but the longevity of these historical structures is threatened by extreme weather. A program of experimental dynamic investigations has been carried out to support characterisation of extreme impulsive breaking wave loads on these structures, using monitored response data. This paper describes the procedures and outcomes of this program, which included modal tests of a collection of six of these lighthouses between June 2016 and October 2017.Five of the six lighthouses tested (Les Hanois, Wolf Rock, Longships, Bishop Rock and Eddystone) feature a 20th century metal helideck atop a 19th century masonry tower, with a Scottish lighthouse (Dubh Artach) being the exception that provides baseline behaviour of a relatively simple tower. All the masonry towers are imperfectly axisymmetric to some degree and all present logistical challenges for experimental work as they can only be accessed by helicopter flights subject to severe weather and time constraints. Against such challenges it was possible to identify key modal parameters, and to highlight some interesting effects due to symmetry and helideck retrofit.Notable findings were that most important modes have frequencies ranging between 4 Hz and 7 Hz and modal masses as low as similar to 200 t. The rarely investigated effect of imperfect axisymmetry on forced vibration testing is studied, along with the introduction of additional modes due to retrofitted helideck. The implications of these effects on experimental modal analysis from forced vibration test data is illustrated.Finally, accelerations recorded on Wolf Rock Lighthouse during the 2017-2018 winter storm season show the modal test data can be used to infer breaking wave modal impulses up to 8 kNs.