Bayesian operational modal analysis of offshore rock lighthouses: Close modes, alignment, symmetry and uncertainty
Bayesian operational modal analysis of offshore rock lighthouses: Close modes, alignment, symmetry and uncertainty
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DOI：
10.1016/j.ymssp.2019.106306
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发表时间：
2019-11
影响因子：
8.4
通讯作者：
J. Brownjohn;A. Raby;S. Au;Zuo Zhu;Xinrui Wang;A. Antonini;A. Pappas;D. D’Ayala
中科院分区：
文献类型：
--
作者：
J. Brownjohn;A. Raby;S. Au;Zuo Zhu;Xinrui Wang;A. Antonini;A. Pappas;D. D’Ayala
尽管使用GPS，灯塔仍然是保护海员和海上贸易安全的关键基础设施，最引人注目的例子可能是维多利亚时代的砖石塔，位于不列颠群岛周围的偏远近海珊瑚礁上，暴露在极端天气条件下。由于它们的年龄和可能增加的未来负载，动态现场调查进行了条件assessment.The现场调查的样本7灯塔已集中在实验模态分析（EMA）的振动器的力量和加速度响应数据，以确定组的模态参数（MP），特别是包括模态质量，这是有用的连接负载和响应。然而，EMA遗漏了重要的有用信息，这些信息可以从现场测量期间记录的额外环境振动数据的操作模态分析（OMA）以及随后对Wolf Rock灯塔的长期监测中恢复。由于准轴对称结构形式，塔的水平振动模态表现为形状相似且固有频率接近的模态对，并且最低频率对对于识别是最重要的，因为它们对破碎波冲击载荷的响应贡献最大。可靠地识别接近的自然频率和相应的模态振型取向是不可能的EMA。贝叶斯OMA（BAYOMA）提供了对模态行为的最深入的了解，同时也提供了对识别接近模态的基本限制的深入了解。本文描述的OMA的具体结论是：·由于“凹椭圆截头体”灯塔形状的不同程度的不对称性，发现一对中的模态频率相差0.75%和3.8%。·与EMA不同，OMA能够识别（或估计）与非常接近的固有频率对应的振型对的水平方向。·视觉上明显的结构对称性可能与振型方向没有很强的联系。·模态频率随时间的变化可能会超过MP的计算识别不确定度，但不计入其中。·在模态形状方向不确定性和闭合模态对中频率的接近度之间存在折衷。
Despite use of GPS, lighthouses remain critical infrastructure for preserving safety of mariners and maritime trade, and the most dramatic examples are probably the Victorian era masonry towers located on remote offshore reefs around the British Isles and exposed to extreme weather conditions. Due to their age and likely increasing future loading, dynamic field investigations were undertaken for condition assessment.The field investigations of a sample of seven lighthouses had focused on experimental modal analysis (EMA) of shaker force and acceleration response data in order to identify sets of modal parameters (MPs) specifically including modal mass, which is useful for linking loading and response. However, the EMA missed significant useful information, which could be recovered from operational modal analysis (OMA) of additional ambient vibration data recorded during the field measurements, as well as from subsequent long-term monitoring of Wolf Rock lighthouse.Horizontal vibration modes of the towers appear as pairs of modes of similar shape and with close natural frequency due to the quasi-axisymmetric structural form(s), and the lowest frequency pairs are most important to identify since they contribute most to response to breaking wave impact loads. Reliably identifying both the close natural frequencies and the corresponding mode shape orientations was impossible with EMA. Bayesian OMA (BAYOMA) provided the most insight into the modal behaviour, while at the same time providing insight into the fundamental limitations for identifying close modes.Specific conclusions from the OMA described in this paper are:•Due to varying degree of asymmetry in the ‘concave elliptic frustum’ lighthouse shapes, mode frequencies in a pair were found to differ by between 0.75% and 3.8%.•Unlike EMA, OMA was able to identify (or estimate) the horizontal directions of the mode pairs corresponding to the very close natural frequencies.•Visually apparent structural symmetry may not be strongly linked to mode shape orientations.•Mode frequency variation over time may exceed -but is not accounted for in- the calculated identification uncertainty of MPs.•There is a trade-off between mode shape orientation uncertainty and closeness of frequencies in a close-mode pair.