The North American plywood industry has been using the same wood veneers and the design values for many years. Thus, the tables of load capacities for North American plywood concrete forming panels have been unchanged for decades.
Changing wood resources, however, have forced North American plywood manufacturers to seek out new sources of veneer and new overlays to provide not only a smoother, more durable product, but also to provide greater panel strength and stiffness. Additional pressure from foreign imports has prompted APA–The Engineered Wood Association to review the concrete form design values for products manufactured by its North American member manufacturers.
Plywood load capacities are calculated either from panel test results or from the engineering design properties of the veneers used to make them. The plywood capacities are plugged into equations that tell the engineer how much pressure the plywood can safely resist. This calculation process takes into consideration design deflection, bending strength, and shear strength.
The published load-span tables traditionally show the load capacity in two columns with the headings L/360 and L/270, representing two deflection limits. This means if the span is, for instance, 12 inches, the permissible design deflection meeting the L/360 criterion is 12/360 or 0.033 inch.
Those using the tables often do not recognize that the values in each cell may not actually be controlled by the L/360 or L/270 deflections, as the column headings imply. Often the cell entry is determined by bending strength or by shear strength. That means the load may exceed one of the two design strengths of the panel before it reaches its deflection limit—not a desirable situation.
As a general rule, the calculated design capacities of concrete forming panels are controlled by shear strength when the loads are relatively high and the spans between the supports are relatively short. Bending strength generally controls the design capacity in the “middle spans and loads” and panel deflection usually doesn't control until the spans are longer.
APA has traditionally published deflection and shear equations that account for support width—which affects panel load capacity—when calculating plywood load capacities for bending and shear. For shear strength, the standard published equation for three spans is ws = 20(Fs)(lb/Q)/L, with L being the clear span. The smaller L gets, the higher the panel's load capacity based on shear. A similar relationship between wider support width and decreased deflection was determined by testing at APA in 1971 and empirical adjustment factors were incorporated into the currently published equations.
For specific forming systems, specialized support widths are factored into load tables prepared for those systems. It is easy to understand that when the support widths become wider, the load-carrying capacity of the panels becomes greater. For instance, the support width traditionally assumed in the table calculations is 1½ inches. If a set of forms is constructed with 3½-inch-wide supports, the increased width of the supports decreases the span and leads to less deflection at center span.
Although it is logical to expect that a plywood panel's ability to carry load will increase as the supports get wider, calculated panel load capacity based on bending strength has never benefited from an increase in support width. This has remained so, in part, because of the inability to test the capabilities that calculations might predict.
The traditional way of dealing with the bending stresses on a three-span condition (four supports) is to use a standard engineering formula. The three-span equation from APA's Concrete Forming brochure looks like this: wb = 120FbS/L2. (It has been altered from more traditional bending formulas to permit using units in inches to yield load capacities in pounds per square foot.) The problem with this bending formula, however, is that it provides no adjustment factor to account for support width. The span, L in this case, is always the center-to-center distance between supports. The bending moment calculated when the panel is loaded is used to determine if the panel is within its design limits: the higher the moment, the higher the stress in the panel. A graph of the bending moments prepared with this equation looks like .