TY - JOUR

T1 - Influence of system changes on closely spaced modes of a large-scale concrete tower for the application to structural health monitoring

AU - Jonscher, Clemens

AU - Liesecke, Leon

AU - Penner, Nikolai

AU - Hofmeister, Benedikt

AU - Grießmann, Tanja

AU - Rolfes, Raimund

N1 - Funding Information: We greatly acknowledge the financial support of the German Research Foundation (CRC 1463, subproject C02), the Federal Ministry for Economic Affairs and Energy of Germany (research projects Deutsche Forschungsplattform für Windenergie, FKZ 0325936E and HyTowering - Optimierung der Bemessung hybrider Türme und Entwicklung eines geeigneten Monitoringkonzepts zur Schadensdetektion und -quantifizierung, FKZ 0324221A) and the project partner Max Boegl that enabled this work.

PY - 2023/6

Y1 - 2023/6

N2 - Concrete steel towers are increasingly being used for onshore wind turbines. The lower part consists of separated segmented concrete rings connected with dry joints. Due to slight deviations from the axisymmetric cross-section, closely spaced modes occur. Therefore, the influences of small system changes on closely spaced modes, particularly the mode shapes, should be investigated to enable reliable vibration-based monitoring. In this context, the influence of imperfections due to the waviness of the dry joints requires attention. As no acceleration measurements on concrete towers considering small system changes have been performed so far, this has not yet been investigated. Therefore, an experiment is carried out using a large-scale laboratory model of a prestressed concrete segment tower. The system modifications are introduced by changing the preload. This changes the influence of imperfections of the surfaces of the horizontal dry joints, estimated by measuring strain and displacement at the lowest joint. An increasing preload causes the first two pairs of bending modes to move closer together. This enables to study the effect of the closeness of natural frequencies on the related mode shapes based on the same structure. Thus, the known effects of increasing uncertainty of the alignment and a rotation of the mode shape in the mode subspace with closer natural frequencies can be shown experimentally. In this work, the operational modal analysis (OMA) methods Bayesian-OMA (BAYOMA) and Stochastic Subspace Identification (SSI) are used. Local imperfections can significantly affect modal parameters, so these should be considered for vibration-based monitoring.

AB - Concrete steel towers are increasingly being used for onshore wind turbines. The lower part consists of separated segmented concrete rings connected with dry joints. Due to slight deviations from the axisymmetric cross-section, closely spaced modes occur. Therefore, the influences of small system changes on closely spaced modes, particularly the mode shapes, should be investigated to enable reliable vibration-based monitoring. In this context, the influence of imperfections due to the waviness of the dry joints requires attention. As no acceleration measurements on concrete towers considering small system changes have been performed so far, this has not yet been investigated. Therefore, an experiment is carried out using a large-scale laboratory model of a prestressed concrete segment tower. The system modifications are introduced by changing the preload. This changes the influence of imperfections of the surfaces of the horizontal dry joints, estimated by measuring strain and displacement at the lowest joint. An increasing preload causes the first two pairs of bending modes to move closer together. This enables to study the effect of the closeness of natural frequencies on the related mode shapes based on the same structure. Thus, the known effects of increasing uncertainty of the alignment and a rotation of the mode shape in the mode subspace with closer natural frequencies can be shown experimentally. In this work, the operational modal analysis (OMA) methods Bayesian-OMA (BAYOMA) and Stochastic Subspace Identification (SSI) are used. Local imperfections can significantly affect modal parameters, so these should be considered for vibration-based monitoring.

KW - Close modes

KW - Concrete steel tower

KW - Identification uncertainty

KW - OMA

UR - http://www.scopus.com/inward/record.url?scp=85152778731&partnerID=8YFLogxK

U2 - 10.1007/s13349-023-00693-6

DO - 10.1007/s13349-023-00693-6

M3 - Article

VL - 13

SP - 1043

EP - 1060

JO - Journal of Civil Structural Health Monitoring

JF - Journal of Civil Structural Health Monitoring

IS - 4-5

ER -