Preliminary Study of Proportional Limit as a Predictor of Ultimate Strength in Bending

Machine grading of structural lumber in the United States combines visual indicators and modulus of elasticity (MOE), determined from load-deflection measurements made by machine, as predictors of strength. The machine-based aspect of prediction relates MOE and strength through a statistical procedure called linear regression; thus the accuracy of strength predictions is influenced by the precision of the MOE-strength regression relationship. This study compares MOE with bending stress at the proportional limit (SPL) as predictors of modulus of rupture (MOR) by comparing the MOE-MOR and SPL-MOR regression relationships. One hundred and fifty Stud grade Douglas-fir 2 by 4’s and 125 Utility grade redwood 2 by 4’s from mills in Oregon and California were kiln-dried and tested on edge in bending to determine MOE, SPL, and MOR. The correlation coefficients from tests with Stud grade Douglas-fir were 0.88 for SPL-MOR and 0.66 for MOE-MOR. Correlation coefficients for Utility grade redwood were 0.90 for SPL-MOR and 0.68 for MOE-MOR. Though further research is needed before broad conclusions can be made, the regression relationships suggest that as a predictor of MOR, SPL is much more accurate than MOE.

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