The effect of prestress force magnitude and eccentricity on the natural bending frequencies of prestressed concrete structures

This 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.