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James M W Ownjoh - One of the best experts on this subject based on the ideXlab platform.
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bayesian operational modal analysis of offshore rock Lighthouses close modes alignment symmetry and uncertainty
Mechanical Systems and Signal Processing, 2019Co-Authors: James M W Ownjoh, Aliso Raby, Alessandro Antonini, Athanasios Pappas, Zuo Zhu, Xinrui Wang, Dina DayalaAbstract:Abstract 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.
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bayesian oma of offshore rock Lighthouses surprises with close modes symmetry and alignment
2019Co-Authors: James M W Ownjoh, Aliso Raby, Zuo Zhu, Xinrui Wang, Alessandro AntoniniAbstract:A set of seven rock Lighthouses around the British Isles was studied by a combination of forced and ambient vibration tests executed with some extreme logistical constraints. Forced vibration testing of the circular section masonry towers combined with experimental modal analysis identified modes with alignment assumed the same as the shaker as well as some interesting effects of helideck retrofit, whereas operational modal analysis revealed the considerable degree of uncertainty in mode shape alignment. Hence Bayesian operational modal analysis was used to characterise the uncertainty and find the best representation of mode shape direction. While perfectly axisymmetric towers would show a single frequency omnidirectional mode, OMA reveals the split modes and allows un unbiased view of directionality. The variability and uncertainty of these mode shape directions are further revealed using Bayesian OMA.
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experimental modal analysis of british rock Lighthouses
Marine Structures, 2018Co-Authors: James M W Ownjoh, Aliso Raby, James Assi, Emma Hudso, Alessandro Antonini, Pete DobsoAbstract:Abstract 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 ∼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.
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modal testing of offshore rock Lighthouses around the british isles
Procedia Engineering, 2017Co-Authors: James M W Ownjoh, Aliso Raby, James Assi, Emma Hudso, Alessandro AntoniniAbstract:Abstract Given that 95 per cent of the UK’s international trade is transported by sea, and as a vital complement to fallible virtual navigational aids such as GPS, rock-mounted Lighthouses constructed in the 19th century have a crucial role to play in safe navigation. However the longevity of these historical structures is threatened by extreme weather so in the UK, the General Lighthouse Authorities comprising Trinity House, the Northern Lighthouse Board and the Commissioners of Irish Lights are supporting three British universities in a program of linked experimental and analytical investigations of full-scale performance under extreme wave loading. The aim is to use structural models calibrated by modal testing to deduce wave loading from response recorded by long-term monitoring. The paper describes the procedures for modal testing, taking into account the constraints on access, logistics, unfamiliar layout and time. The test program sequentially covered Les Hanois, Wolf Rock, Longships and Bishops Rock Lighthouses over summer 2016 followed by Fastnet Rock in December 2016. Some conventional techniques of forced and ambient vibration testing were used along with some unusual excitation methods. Results from the measurements and observations on the particular challenges associated with testing two of these iconic structures are presented.
Alessandro Antonini - One of the best experts on this subject based on the ideXlab platform.
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bayesian operational modal analysis of offshore rock Lighthouses close modes alignment symmetry and uncertainty
Mechanical Systems and Signal Processing, 2019Co-Authors: James M W Ownjoh, Aliso Raby, Alessandro Antonini, Athanasios Pappas, Zuo Zhu, Xinrui Wang, Dina DayalaAbstract:Abstract 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.
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bayesian oma of offshore rock Lighthouses surprises with close modes symmetry and alignment
2019Co-Authors: James M W Ownjoh, Aliso Raby, Zuo Zhu, Xinrui Wang, Alessandro AntoniniAbstract:A set of seven rock Lighthouses around the British Isles was studied by a combination of forced and ambient vibration tests executed with some extreme logistical constraints. Forced vibration testing of the circular section masonry towers combined with experimental modal analysis identified modes with alignment assumed the same as the shaker as well as some interesting effects of helideck retrofit, whereas operational modal analysis revealed the considerable degree of uncertainty in mode shape alignment. Hence Bayesian operational modal analysis was used to characterise the uncertainty and find the best representation of mode shape direction. While perfectly axisymmetric towers would show a single frequency omnidirectional mode, OMA reveals the split modes and allows un unbiased view of directionality. The variability and uncertainty of these mode shape directions are further revealed using Bayesian OMA.
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experimental modal analysis of british rock Lighthouses
Marine Structures, 2018Co-Authors: James M W Ownjoh, Aliso Raby, James Assi, Emma Hudso, Alessandro Antonini, Pete DobsoAbstract:Abstract 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 ∼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.
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invited rock mounted iconic Lighthouses under extreme wave impacts limit analysis and discrete element method
THE 9TH INTERNATIONAL CONFERENCE ON COMPUTATIONAL METHODS (ICCM2018), 2018Co-Authors: Athanasios Pappas, Alessandro Antonini, Dina Dayala, Aliso RabyAbstract:This paper deals with the resilience of rock mounted Lighthouses impacted by extreme waves. The investigated lighthouse of Wolf Rock is built with large and meticulously dovetailed granite blocks. For this structural typology, uplift of blocks and separation of joints is possible under intense wave impacts. Sliding between subsequent courses of stones is limited by the existence of vertical keys between the blocks. The lateral forces that can trigger uplift and rocking, a highly non-linear behaviour, are calculated with the use of Limit Analysis method through an iterative procedure written in Python programming language. Three different overturning mechanisms, which correspond to different hypotheses about the working section of the lighthouse, are considered for the uplift. The most conservative limits, i.e. with only a portion of the circular section taking the load, are representative for small and medium waves which can cause a small uplift and very local opening of a joint. The mechanism that considers the whole section, i.e. less conservative, is more representative for very strong wave impacts. The sliding failure mechanism is also considered. The Limit Analysis thresholds are validated with Discrete Element (DE) time-history analyses using the commercial software 3DEC. Intense uplift and rocking, especially present when the less conservative Limit Analysis thresholds are exceeded. The study also revealed the beneficial role of the vertical keys, without which the lighthouse would fail due to intense sliding before the overturning mechanisms are activated.
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modal testing of offshore rock Lighthouses around the british isles
Procedia Engineering, 2017Co-Authors: James M W Ownjoh, Aliso Raby, James Assi, Emma Hudso, Alessandro AntoniniAbstract:Abstract Given that 95 per cent of the UK’s international trade is transported by sea, and as a vital complement to fallible virtual navigational aids such as GPS, rock-mounted Lighthouses constructed in the 19th century have a crucial role to play in safe navigation. However the longevity of these historical structures is threatened by extreme weather so in the UK, the General Lighthouse Authorities comprising Trinity House, the Northern Lighthouse Board and the Commissioners of Irish Lights are supporting three British universities in a program of linked experimental and analytical investigations of full-scale performance under extreme wave loading. The aim is to use structural models calibrated by modal testing to deduce wave loading from response recorded by long-term monitoring. The paper describes the procedures for modal testing, taking into account the constraints on access, logistics, unfamiliar layout and time. The test program sequentially covered Les Hanois, Wolf Rock, Longships and Bishops Rock Lighthouses over summer 2016 followed by Fastnet Rock in December 2016. Some conventional techniques of forced and ambient vibration testing were used along with some unusual excitation methods. Results from the measurements and observations on the particular challenges associated with testing two of these iconic structures are presented.
F Quere - One of the best experts on this subject based on the ideXlab platform.
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attosecond Lighthouses in gases a theoretical and numerical study
Physical Review A, 2016Co-Authors: T Auguste, O Gobe, T Rucho, F QuereAbstract:We present an extensive theoretical and numerical study of the attosecond lighthouse effect in gases. We study how this scheme impacts the spatiotemporal structure of the driving laser field all along the generation medium, and show that this can modify the phase matching relation governing high-harmonic generation (HHG) in gases. We then present a set of numerical simulations performed to test the robustness of the effect against variations of HHG parameters, and to identify possible solutions for relaxing the constraint on the driving laser pulse duration. We thus demonstrate that the lighthouse effect can actually be achieved with laser pulses consisting of up to ∼8 optical periods available from current lasers without postcompression, for instance by using an appropriate combination of 800− and 1600-nm wavelength fields.
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attosecond Lighthouses from plasma mirrors
Nature Photonics, 2012Co-Authors: Jonatha Wheele, Henri Vincenti, Sylvai Monchoce, Antoni Oro, Arnaud Malvache, R Lopezmartens, Aurelie Ricci, F QuereAbstract:The nonlinear interaction of an intense femtosecond laser pulse with matter can lead to the emission of a train of sub-laser-cycle--attosecond--bursts of short-wavelength radiation1, 2. Much effort has been devoted to producing isolated attosecond pulses, as these are better suited to real-time imaging of fundamental electronic processes3, 4, 5, 6. Successful methods developed so far rely on confining the nonlinear interaction to a single sub-cycle event7, 8, 9. Here, we demonstrate for the first time a simpler and more universal approach to this problem10, applied to nonlinear laser-plasma interactions. By rotating the instantaneous wavefront direction of an intense few-cycle laser field11, 12 as it interacts with a solid-density plasma, we separate the nonlinearly generated attosecond pulse train into multiple beams of isolated attosecond pulses propagating in different and controlled directions away from the plasma surface. This unique method produces a manifold of isolated attosecond pulses, ideally synchronized for initiating and probing ultrafast electron motion in matter.
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Attosecond Lighthouses from plasma mirrors
2012 Conference on Lasers and Electro-Optics (CLEO), 2012Co-Authors: Jonathan Wheeler, Henri Vincenti, Sylvai Monchoce, Arnaud Malvache, Antonin Borot, Aurélien Ricci, Aurélie Jullien, Rodrigo Lopez-martens, F QuereAbstract:We demonstrate for the first time experimentally a universal method - attosecond lighthouse effect - for producing temporally synchronized and spatially isolated attosecond light pulses, applied to a few-cycle laser-driven.
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attosecond Lighthouses
Conference on Lasers and Electro-Optics, 2012Co-Authors: Henri Vincenti, Jonatha Wheele, Sylvai Monchoce, Antoni Oro, Arnaud Malvache, R Lopezmartens, F QuereAbstract:We show how to use spatio-temporally coupled light fields to generate isolated attosecond pulses. This general effect provides a very convenient scheme for attosecond pump-probe experiments, and constitutes a powerful new tool for ultrafast metrology.
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attosecond Lighthouses a new tool for ultrafast science and metrology
High Intensity Lasers and High Field Phenomena, 2012Co-Authors: F Quere, Henri VincentiAbstract:The attosecond lighthouse effect provides an unprecedentedly simple way of generating isolated attosecond pulses, and provides new opportunities for ultrafast measurements. It is analyzed theoretically, and first experimental evidence of this effect is presented.
Aliso Raby - One of the best experts on this subject based on the ideXlab platform.
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bayesian operational modal analysis of offshore rock Lighthouses close modes alignment symmetry and uncertainty
Mechanical Systems and Signal Processing, 2019Co-Authors: James M W Ownjoh, Aliso Raby, Alessandro Antonini, Athanasios Pappas, Zuo Zhu, Xinrui Wang, Dina DayalaAbstract:Abstract 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.
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bayesian oma of offshore rock Lighthouses surprises with close modes symmetry and alignment
2019Co-Authors: James M W Ownjoh, Aliso Raby, Zuo Zhu, Xinrui Wang, Alessandro AntoniniAbstract:A set of seven rock Lighthouses around the British Isles was studied by a combination of forced and ambient vibration tests executed with some extreme logistical constraints. Forced vibration testing of the circular section masonry towers combined with experimental modal analysis identified modes with alignment assumed the same as the shaker as well as some interesting effects of helideck retrofit, whereas operational modal analysis revealed the considerable degree of uncertainty in mode shape alignment. Hence Bayesian operational modal analysis was used to characterise the uncertainty and find the best representation of mode shape direction. While perfectly axisymmetric towers would show a single frequency omnidirectional mode, OMA reveals the split modes and allows un unbiased view of directionality. The variability and uncertainty of these mode shape directions are further revealed using Bayesian OMA.
-
experimental modal analysis of british rock Lighthouses
Marine Structures, 2018Co-Authors: James M W Ownjoh, Aliso Raby, James Assi, Emma Hudso, Alessandro Antonini, Pete DobsoAbstract:Abstract 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 ∼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.
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invited rock mounted iconic Lighthouses under extreme wave impacts limit analysis and discrete element method
THE 9TH INTERNATIONAL CONFERENCE ON COMPUTATIONAL METHODS (ICCM2018), 2018Co-Authors: Athanasios Pappas, Alessandro Antonini, Dina Dayala, Aliso RabyAbstract:This paper deals with the resilience of rock mounted Lighthouses impacted by extreme waves. The investigated lighthouse of Wolf Rock is built with large and meticulously dovetailed granite blocks. For this structural typology, uplift of blocks and separation of joints is possible under intense wave impacts. Sliding between subsequent courses of stones is limited by the existence of vertical keys between the blocks. The lateral forces that can trigger uplift and rocking, a highly non-linear behaviour, are calculated with the use of Limit Analysis method through an iterative procedure written in Python programming language. Three different overturning mechanisms, which correspond to different hypotheses about the working section of the lighthouse, are considered for the uplift. The most conservative limits, i.e. with only a portion of the circular section taking the load, are representative for small and medium waves which can cause a small uplift and very local opening of a joint. The mechanism that considers the whole section, i.e. less conservative, is more representative for very strong wave impacts. The sliding failure mechanism is also considered. The Limit Analysis thresholds are validated with Discrete Element (DE) time-history analyses using the commercial software 3DEC. Intense uplift and rocking, especially present when the less conservative Limit Analysis thresholds are exceeded. The study also revealed the beneficial role of the vertical keys, without which the lighthouse would fail due to intense sliding before the overturning mechanisms are activated.
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modal testing of offshore rock Lighthouses around the british isles
Procedia Engineering, 2017Co-Authors: James M W Ownjoh, Aliso Raby, James Assi, Emma Hudso, Alessandro AntoniniAbstract:Abstract Given that 95 per cent of the UK’s international trade is transported by sea, and as a vital complement to fallible virtual navigational aids such as GPS, rock-mounted Lighthouses constructed in the 19th century have a crucial role to play in safe navigation. However the longevity of these historical structures is threatened by extreme weather so in the UK, the General Lighthouse Authorities comprising Trinity House, the Northern Lighthouse Board and the Commissioners of Irish Lights are supporting three British universities in a program of linked experimental and analytical investigations of full-scale performance under extreme wave loading. The aim is to use structural models calibrated by modal testing to deduce wave loading from response recorded by long-term monitoring. The paper describes the procedures for modal testing, taking into account the constraints on access, logistics, unfamiliar layout and time. The test program sequentially covered Les Hanois, Wolf Rock, Longships and Bishops Rock Lighthouses over summer 2016 followed by Fastnet Rock in December 2016. Some conventional techniques of forced and ambient vibration testing were used along with some unusual excitation methods. Results from the measurements and observations on the particular challenges associated with testing two of these iconic structures are presented.
Dina Dayala - One of the best experts on this subject based on the ideXlab platform.
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bayesian operational modal analysis of offshore rock Lighthouses close modes alignment symmetry and uncertainty
Mechanical Systems and Signal Processing, 2019Co-Authors: James M W Ownjoh, Aliso Raby, Alessandro Antonini, Athanasios Pappas, Zuo Zhu, Xinrui Wang, Dina DayalaAbstract:Abstract 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.
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invited rock mounted iconic Lighthouses under extreme wave impacts limit analysis and discrete element method
THE 9TH INTERNATIONAL CONFERENCE ON COMPUTATIONAL METHODS (ICCM2018), 2018Co-Authors: Athanasios Pappas, Alessandro Antonini, Dina Dayala, Aliso RabyAbstract:This paper deals with the resilience of rock mounted Lighthouses impacted by extreme waves. The investigated lighthouse of Wolf Rock is built with large and meticulously dovetailed granite blocks. For this structural typology, uplift of blocks and separation of joints is possible under intense wave impacts. Sliding between subsequent courses of stones is limited by the existence of vertical keys between the blocks. The lateral forces that can trigger uplift and rocking, a highly non-linear behaviour, are calculated with the use of Limit Analysis method through an iterative procedure written in Python programming language. Three different overturning mechanisms, which correspond to different hypotheses about the working section of the lighthouse, are considered for the uplift. The most conservative limits, i.e. with only a portion of the circular section taking the load, are representative for small and medium waves which can cause a small uplift and very local opening of a joint. The mechanism that considers the whole section, i.e. less conservative, is more representative for very strong wave impacts. The sliding failure mechanism is also considered. The Limit Analysis thresholds are validated with Discrete Element (DE) time-history analyses using the commercial software 3DEC. Intense uplift and rocking, especially present when the less conservative Limit Analysis thresholds are exceeded. The study also revealed the beneficial role of the vertical keys, without which the lighthouse would fail due to intense sliding before the overturning mechanisms are activated.