Nondestructive Testing of Joists By a Vibrational Technique

The main purpose of this study was to determine the effectiveness and accuracy of a vibrational nondestructive test in segregating joists into different strength levels. The joists used in this study had the nominal cross-sectional dimension of 2 by 6 inches, and were 12 feet long. Approximately half of the (109) joists were jack pine and the other half (110) were white spruce. The lumber from which they were prepared was bought locally in the rough, green state. Each joist was supported on edge at its two nodal points and vibrated transversely in the vertical plane to determine its fundamental resonant frequency. The driving force used was developed earlier for vibrating cross-arms, and employs pulses of compressed air. Experimental data from the vibrational measurements, weighing, deflection tests, and destructive tests were processed by computer to obtain dynamic, modulus of elasticity (MOE) on edge, static MOE on the flat for each face, static MOE on edge, and modulus of rupture (MOR). The results show a high correlation between MOR of the joists and their dynamic MOE on edge. The accuracy of a nondestructive grading machine based on a relationship between strength and stiffness depends not only on the correlation between the two properties but also on the accuracy with which stiffness is measured by the grading machine. In this respect :a vibrational method of strength grading may have several advantages compared to mechanical stress-grading, A proposal is presented for determining the stress rating for yard lumber.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member