LD1
Epic Contributor
That is NOT where the strength comes from.You make my point by missing the point...!
You were talking about built up headers and beams...I remarked about the strength of structural plywood etc...and used TJIs as an example...you said they used OSB because it was cheap...That is not the primary reason...the main reason is it is stronger than typical lumber..and much less (thinner) material is required to achieve the same strength...to use regular lumber it would have to be structural grade lumber with very minimal knots etc. (clear) it the strength of plywood etc. that makes it viable...
On beams etc...to add strength to a built up beam using plywood "flitch" members...requires engineered fastening along with effective adhesive...not as strong as a beam with steel flitch plates but significantly stronger than just 2x's nailed together...
Again, you can achieve the same strength with a floor "truss", which DOES use dimensional lumber as well as alot of "air" between the top and bottom members.
You can take a 4-ply built up header.......with OSB or plywood in between each of the boards. This header beam will be 7.5" thick...(4 dimensional boards at 1.5" and 3 plys of 1/2" plywood).
You can simply OMIT the 3 plys of plywood and opt for FIVE plys of dimensional lumber instead to have the EXACT SAME 7.5" thick header/beam.
Now in reality the strength will be little to no different. But in terms of engineering and structural calculations......the 5-ply of dimensional lumber will be stronger and rated to carry MORE.
You can harp about the merits of plywood, grain structure, no knots, etc etc all you want. But it doesnt change reality.
You could build your engineered I-joists and use a 1x8 for the web (which is 3/4" thick) or you can use 3/4" thick OSB......
As long as the flanges are the same size and the joist is the same depth.....it wont have an effect on its load carrying stength. Therefore......OSB is no better than dimensional lumber, and is indeed used because it is the cheapest solution.....NOT because it is the strongest.