Masonry Magazine December 2016 Page. 19
SCAFFOLD PLANKS AND DECKING
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By Mike Gilleran
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 la.minated 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 ofwood plank as they relate to performance. You should pay particular attention to the physical design properties ofthe 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
Product Performance Comparison
PRODUCT MOE DEFLECTION (inches)
A 2.00 3200 0.61
B 2.20 2900 0.55
C 2.00 2350 0.61
D 1.80 2200 0.67
modulus ofelasticity 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 offour 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
PERFORMANCE
(same load and span), you
(% of Fb allowable)
will notice that products with the same MOE will deflect
47%
exactly the same amount. In addition, the far-right column
52%
represents the product strength 64CJfr performance relative to its fiber bending strength. For instance, 6996 product A would perform at 47
16 I MASONRY � December 2016 � www.masoncontractors.org The Voice of the Masonry Industry