Spacing for Accuracy in Ultrasonic Testing of Bridge Timber Piles

Nondestructive ultrasonic testing is a more accurate alternative for assessing the strength of timber piles than the conventional practice of visual inspection. However, because the ultrasonic method is relatively new, there is a need to develop guidelines that can be used to define the spacing between test points required for a desired level of accuracy in testing. Analyses of data obtained from timber piles covering a range of compressive strengths were used to develop spacing guidelines. The data base consisted of nine treated southern yellow pine timber piles from four bridges in Maryland. Semivariogram modeling was used as the statistical procedure for characterizing the stochastic properties of the ultrasonic wave velocity measurements. Kriging is used to estimate the expected wave velocity for points between measured values. The results provide a relationship between the relative accuracy and the relative spacing of point measurements. Confidence intervals can be applied to assess the expected error variation between point measurements. The methodology presented in the paper can also be utilized in the nondestructive evaluation of other structural components.