Plywood and LVL Strength and stiffness
Generally in plywood panels, the outer most veneers have the greatest influence on strength and stiffness. Increasing the thickness of the face veneers will increase the strength and stiffness characteristics in the face grain direction. Thin face veneers with thicker underlying cross band veneers will tend to give the plywood more equal strength and stiffness characteristics in both directions.
Structural plywood usually has thicker face veneers than those used in concrete formwork plywood or marine plywood. Structural plywood transfers load in all directions, however because of the veneer arrangement, most structural plywood has far greater strength and stiffness in the direction of the face grain. It is specified that structural plywood be supported such that the face grain is parallel to the span. Supporting plywood with the face grain direction at right angles to the span is not recommended. (Note: the face grain direction of plywood is normally parallel to the length of the plywood sheet).
Concrete formwork plywood and marine plywood usually have thinner face veneers over thicker underlying cross veneers and are designed to have similar strength and stiffness properties in both directions; characteristics that are important in concrete formwork and boats. Actual structural properties for concrete formwork plywood both along and across the grain are specified in the EWPAA design manual “Plywood in Concrete Formwork”.
LVL (and I beams) usually have unidirectional veneers. That is, all veneers are parallel to the beam length and there are no cross veneers. This optimises their performance as a beam, spanning in one direction.
NOTE: Generally, different plywood types are not directly substitutable for the same structural application. For example, equivalent stress grade and thickness structural and marine plywoods will not perform the same in a flooring application. The structural plywood will usually be stiffer and stronger when supported with the face grain direction parallel to the span, than will the marine plywood.