Wood Scaffold Plank Performance

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. Nowadays, the product offering of wood scaffold plank is quite diverse. 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.

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. 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.

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 (Fb) value and the modulus of elasticity (MOE, or technically speaking, E). 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. The higher the fiber bending value, the more load the plank will handle before fracturing or reaching its breaking point. 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.

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. 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. 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.

The chart in this article illustrates the deflection and strength performance testing of four different wood plank products with varying design properties. 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. In addition, the far-right column represents the product strength performance relative to its fiber bending strength. 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. 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. Both products have the same MOE, yet testing shows there is a substantial difference in ultimate strength performance of the two products. This further exemplifies that both design values, Fb and MOE, should be reviewed when selecting a product.

For further clarification/experimental purposes, take, for example, two different kinds of EPS foam board insulation products of the same thickness and width (one being white bead board commonly referred to as Styrofoam, the other being rigid EPS insulation, often referred to as Blue Board). Span both products over two supports the same distance apart, then apply light pressure downward on each (by using your hands simultaneously). You will find they both have a similar stiffness. The resistance will seem comparable. This is the Modulus of Elasticity. Now follow the same test procedure, but apply even more pressure continuously. Both boards will slowly flex, but eventually the Styrofoam will abruptly snap, while the Blue Board will continue to bend or accept more deflection before it reaches its breaking point. This is because the Blue Board has a higher fiber bending value.

The lesson to be learned is this: if these were scaffold plank beneath your feet instead of insulated foam board, while the MOE value of the product is important to your comfort level, it’s the fiber bending strength that weighs more on your safety.

There is yet another aspect to be considered as well. Plank thickness governs plank performance, and the design values influence the ultimate performance to a greater extent. The masonry industry is accustomed to using 2 x 10 (1 1/2 x 9 1/4-inch nominal size) plank. Therefore, if you are considering a thicker manufactured plank, keep in mind that if it has low design properties, it will likely be heavier and therefore more difficult to handle. At the very least, it could pose a tripping hazard if mixed in a scaffold system with other plank types.

To further substantiate the above points, when engineers calculate and develop span tables for particular plank products, they have to observe both the modulus of elasticity and the fiber bending value and must do so for each plank size, type and quality. Of course, they must also observe the deflection limit of L/60 (length of span divided by 60), stipulated in the OSHA standards.

The next time you are doing research to purchase more scaffold plank, do not be fooled if you’re told, “The plank I’m offering you is a better grade because the MOE is higher; therefore, it’s the better product.” Remember, the official grade is “SCAFFOLD PLANK,” and the modulus of elasticity is merely one of the design properties, relating specifically to the stiffness rating. Of much more importance is the fiber bending value, which influences a plank’s ability to support loads before fracture or failure.

There really is a science to the performance of wood products, and this is especially true regarding the overall performance and life expectancy of OSHA-compliant scaffold plank. Consider both design values, not just MOE, and then make an informed decision.
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