Conference Paper

FRF-based modal testing of horizontally swaying structures using OCXO synchronized wireless accelerometers for simultaneous force and vibration responses measurements

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Abstract

This paper describes an experimental validation of a novel FRF-based modal testing system designed to measure experimentally sway modes of tall timber and other building structures. The test uses a set of synchronised electrodynamic shakers and oven-controlled crystal oscillator (OCXO) high-precision synchronised wireless accelerometers for simultaneous force and response measurements. This modal testing system makes no use of cables or radio-waves to connect all accelerometers simultaneously with the multi-channel data acquisition system but provides a perfect synchronised measurement of a practically unlimited number of force and response channels. The system will be used to to estimate experimentally sway modes of as-built tall timber buildings. Therefore, the aim of this paper is to demonstrate the feasibility of the OCXO-based system in high-rise building FRF measurements. Two nominally identical FRF-based modal testing exercises were carried out on a 15-tonne laboratory-based test floor structure supported by 4 columns in order to measure its horizontal 'swaying' modes of vibration using: (1) a well-established and quality assured 'wired' system based on a 20-channel spectrum analyser, and (2) OCXO-based wireless system. It was shown that the two methodologies produce almost identical modal testing results and that the OCXO-based method is robust and reliable for field utilisation on high-rise buildings.

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... In the input-output testing, both the excitation force and the corresponding dynamic response are measured, allowing estimation of the frequency response functions (FRFs). The AVT-based estimated modal properties are less reliable than their FRF-based counterparts, [12], yet FRFs have very rarely been used for constructed civil engineering systems. One of the reasons is practical difficulties related to forced excitation of a building or a bridge during its operation. ...
... One of the reasons is practical difficulties related to forced excitation of a building or a bridge during its operation. Another reason is a logistical complication in measuring responses simultaneously throughout the structure (which can be solved by using synchronized wireless accelerometers [12]). ...
... The surrogate model also allows for fast computation of global sensitivities [33] and statistics of modal properties [31]. The experimental modal data used for model updating were obtained from the input-output testing, where both the excitation force and the corresponding dynamic response were measured, see [12]. FRF-based modal identification was applied to get experimental estimates of the modal properties of the considered tall CLT building. ...
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A framework for the probabilistic finite element model updating based on measured modal data is presented. The described framework is applied to a seven-storey building made of cross-laminated timber panels. The experimental estimates based on the forced vibration test are used in the process of model updating. First, a generalized Polynomial Chaos surrogate model is derived representing the map from the model parameters to the eigenfrequencies and the eigenvectors. To overcome the difficulties caused by mode switching, we propose a novel approach to mode tracking based on partitioning an extended and low-rank representation of the k mode shapes resulting from different setups of the finite element model into k clusters by the k-means clustering algorithm. Second, the surrogate model derived with the help of mode pairing is used to efficiently perform sensitivity analysis and uncertainty quantification of the first five frequencies and the corresponding mode shapes. Finally, the surrogate-based Bayesian update of the model parameters is efficiently performed, providing engineers not only with a finite element model that gives a good fit to the experimental modal data, but also a stochastic model that represents the uncertainties originating from the initial model and the uncertainties of measuring modal properties.
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