Predicting the Bending Strength of Ladder Rail Stock

Ladders are a high liability product. Nondestructive testing of ladder rail strength could be used to sort the rails into different quality groups for improved reliability. Two hundred pieces of western hemlock ladder rail stock, 1-3/8 inches by 2-3/4 inches by 8 feet, were tested nondestructively for density, transverse vibrational modulus of elasticity (E), flatwise and edgewise E, edgewise quarter point E, and stress wave transit times. The ladder rails were broken on edge and the modulus of rupture (MOR) was calculated. Linear regression analysis of the nondestructive predictors of NOR were performed. Good correlation was found between MOR and static bending E, r2 = 0.5. Better correlation was found when multiple variables were used. Four examples of sorting schemes are introduced and the safety indexes for the original sample are compared to the sorted sample. The calculation of the safety index illustrates the increased reliability of the sorted product.

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