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- PublicationA bespoke signal processing algorithm for operational modal testing of post-tensioned steel and concrete beamsThe extraction of modal properties, specifically natural frequency, damping ratio and mode shape is a difficult task, especially when output-only data is measured. The accuracy of the estimation these modal properties is compromised by noisy signals, and signal filtering is required to suppress unwanted frequency content. Care is required however to avoid over-filtering of the output data, which can eliminate valid structural frequency content if required care is not exercised. This paper describes the development of a bespoke signal processing algorithm to extract the modal properties of both simply supported post-tensioned steel and concrete sections. Dynamic impact testing was conducted on a series of different post-tensioned steel rectangular hollow sections, and 9 different post-tensioned concrete beams, each with differing straight profiled post-tensioning strand eccentricities. Acceleration time-history data was recorded for each of the steel and concrete beams via an accelerometer. This data was subsequently processed, first centring the acceleration-time history using a moving average filter, and subsequently removing any zero drift in the accelerometer via a second order low pass Butterworth filter. Electrical noise was then removed via a notch filter. The accelerometer data was then smoothed in the time domain. The Fast Fourier Transform (FFT) was applied to the signal to convert into the frequency domain and finally a bespoke peak-picking algorithm was invoked to extract the natural frequencies of the beams. A comparison is subsequently made between the accuracy of the estimation of the modal properties of the steel and concrete beams for filtered and unfiltered data, and a sensitivity analysis of the filtering and peak picking parameters is conducted to determine the effect that this has on the accuracy of the estimation of the modal parameters. The results show the effectiveness of the bespoke signal processing algorithm in increasing the accuracy of the estimation of the modal properties as opposed to the raw unprocessed signals.
- PublicationDynamic impact testing on post-tensioned steel rectangular hollow sections; An investigation into the "compression-softening" effectThis paper describes the results of dynamic impact testing on externally axially loaded steel rectangular hollow sections (RHSs) and compares the response to that of externally post-tensioned steel RHSs. Both the fundamental natural bending frequency of the beam sections and the corresponding damping ratios have been calculated from the measured dynamic response of the beam to a series of impact hammer strikes. The validity of the "compression-softening" effect for post-tensioned sections is tested. The implications of the research are vast, as currently, there is significant disagreement among researchers about the effect of pre- and post-tensioning loads on the dynamic characteristics of structures. The fundamental bending frequencies have been calculated and corresponding damping ratio have been calculated from dynamic test results for each axial load level. The bending frequencies have been calculated repeatedly while changing the axial load level and the subsequent changes in both frequency and damping ratio, with increasing axial load level have been analysed to determine if the results are statistically significant. It has been determined that "compression softening" theory is not valid for pre- or post-tensioned sections.
- PublicationThe effect of prestress force magnitude and eccentricity on the natural bending frequencies of prestressed concrete structuresThis paper describes the outcome of static 3-point bending testing and output-only experimental modal analysis on 9 post-tensioned concrete beams. Static 3-point bending testing and dynamic impact testing were conducted on each of the 9 beams at different levels of post-tensioning force. The Fast Fourier Transform (FFT) was implemented on the dynamic accelerometer impact data, and the fundamental frequencies of the simply supported post-tensioned concrete beams were determined by a peak-picking algorithm at each post-tensioning load level. The tests were repeated 10 times at each impact location to ensure repeatability of the experiment. There were 3 impact locations per post-tensioning load level, and there were 11 post-tensioning load levels at which the beams were tested. A first-order linear regression model was then applied to the measured fundamental bending frequencies with increasing post-tensioning load. Statistical significance tests were subsequently conducted on the recorded data to determine if any statistically significant changes in fundamental bending frequency with increasing post-tensioning load was observed, for both static and dynamic results. The results obtained for the static 3-point bending tests were then compared and contrasted with the results obtained from dynamic testing. No statistically significant relationship between natural frequency and post-tensioning load level was found for these uncracked concrete beams.
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