Simultaneously, product C tests at 64 percent of its allowable strength under the same test criteria, meaning there is a 22 percent performance difference between products A and C. For instance, product A would perform at 47 percent of its maximum allowable strength before reaching the threshold, or when it will begin to fail. In addition, the far-right column represents the product strength performance relative to its fiber bending strength. Assuming the plank products are the same size, brand new, have identical moisture content (less than 19 percent), and have been tested under identical circumstances (same load and span), you will notice that products with the same MOE will deflect exactly the same amount. The chart in this article illustrates the deflection and strength performance testing of four different wood plank products with varying design properties. In fact, when developing span load tables for scaffold plank products, the fiber bending value often influences the ultimate performance more than the modulus of elasticity. Even if a product has a high MOE value, this does not necessarily mean it is a safer product or will support as much weight as a plank with a lower MOE but high fiber bending value. To put it in simplistic terms, the higher the fiber bending strength, the more weight a plank will support before fracturing or breaking, and the higher the modulus of elasticity, the more rigid or stiff the plank will feel beneath one’s feet. The modulus of elasticity is the measurement of the plank’s stiffness, or the association between the amount a board will deflect downward and the load which causes this deflection. The higher the fiber bending value, the more load the plank will handle before fracturing or reaching its breaking point. The fiber bending value corresponds to the board’s resistance against an applied load and the relative tension and compression of the wood fiber within the plank. Since scaffold plank is used “flatwise,” the two most critical design properties used to determine a board’s ability to support specified loads over given spans are the fiber bending (F b) value and the modulus of elasticity (MOE, or technically speaking, E). You should pay particular attention to the physical design properties of the product being considered, specifically, strength and stiffness values, which have been determined through extensive laboratory testing in accordance with the ASTM Standards for Structural Composite Lumber Products. Regardless of what wood plank type (manufactured or solid sawn) you are in favor of, it is important to understand the key elements of wood plank as they relate to performance. Aside from traditional solid sawn lumber planks that are visually graded to meet OSHA standards, there are various types and qualities of manufactured or laminated wood scaffold boards to consider. Nowadays, the product offering of wood scaffold plank is quite diverse. Wood scaffold plank is undoubtedly a critical component within scaffold assemblies, as worker safety depends on the performance of the planks on which they stand. SCAFFOLD PLANKS AND DECKING By Mike Gilleran
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