Cepstral Analysis Applied to Identification of Longitudinal Vibration in Timber Piles

A method for field measurement of timber pile length in bridge foundations is possible by determining the natural frequency spacing between consecutive resonant peaks in the longitudinal frequency response function (FRF) as well as experimental determination of the longitudinal wave speed. The measurement procedure relies on vibration modes excited through the attachment of small actuators driven with random input and the recorded response of attached accelerometers. The identification of natural frequencies in timber piles is complicated by the inhomogeneous nature of wood and the location of measurement points at possible longitudinal nodes along the pile. Missing peaks and corrupted spectrums are common in the experimental data collected during the testing of timber piles. A form of cepstral analysis using the FRF shows promise towards overcoming some of the inherent difficulties in identifying the natural frequency spacing between resonant peaks in the spectrum. The details of the signal processing and natural frequency spacing identification are presented and demonstrated on a laboratory aluminum bar and typical timber pile data.