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In few answers I've been advised to reinforce OSB shelf with wooden beam for rigidity over time. Hence 3 interconnected questions:

  1. Isn't OSB supposed to be more stable to elements comparing to wood? Isn't that what's all the adhesive in it is for.

  2. Tables give me tensile strength of OSB few times smaller than wood. Can I replace 18mm x 36mm wooden beam safely with 2* 50mm strips of 22mm OSB screwed and/or glued together? That'd give me 50x44 "beam" section -- should be quite enough.

  3. Should OSB surface and edges be treated if I plan indoor use with no proximity of water. For it's tensile characteristics to remain same over time.

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Isn't OSB supposed to be more stable to elements comparing to wood? Isn't that what's all the adhesive in it is for.

Not be a wise-guy, but the adhesive is there to hold the little pieces of wood together. Dimensional stability is a secondary and very desirable benefit.

Tables give me tensile strength of OSB few times smaller than wood. Can I replace 18mm x 36mm wooden beam safely with 2* 50mm strips of 22mm OSB screwed and/or glued together? That'd give me 50x44 "beam" section -- should be quite enough.

I assume that you are considering stiffeners of 18mmx36mm strips with the 36mm dimension vertical and the 18mm horizontal and, likewise, the 44mmx50mm OSB strips with the 50mm vertical and the 44mm horizontal. (That's the way that you get maximum beam action.) You would get comparable beam action with the OSB beam solution and the two pieces that make up your 44mmx50mm stiffeners can be screwed - it should not affect the stiffness.

If, when you first place stuff on the shelf, it visibly sags, then use more stiffeners, or deeper stiffeners.

OSB comes in various stiffness depending on the glue and wood. Compared to pine it will sag from about the same or up to twice as much. I would assume that the stiffness varies with price.

You also have an issue of attaching the stiffeners to the shelf. Screwing or nailing the edges of the stiffeners to the shelf material would generally be discouraged, but they might hold well enough to hold the stiffeners in position. Gluing OSB has serious problems as well - common woodworking glue has trouble adhering to the adhesives (think plastic resins and wax) that hold the OSB together.

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Isn't OSB supposed to be more stable to elements comparing to wood? Isn't that what's all the adhesive in it is for.

OSB is more dimensionally stable when compared to wood. Like other engineered woods there is no grain direction so when its moisture content even does change it will not be significant in one direction or another. Plywood still has grain direction but the plies are glued perpendicular to eachother.

OSB, as you know, does not like direct contact with water, especially at its edges.

Tables give me tensile strength of OSB few times smaller than wood. Can I replace 18mm x 36mm wooden beam safely with 2* 50mm strips of 22mm OSB screwed and/or glued together? That'd give me 50x44 "beam" section -- should be quite enough.

It is still a sheet good. OSB is used as flooring but there is still a frame that it sits on. So as a sheet I would expect that it would bow over time across its length. 18mm is think so this might take a while but what you use it for would have a impact on that. I don't think you can compare laminated wood to a sheet good in terms of strength.

Should OSB surface and edges be treated if I plan indoor use with no proximity of water. For it's tensile characteristics to remain same over time.

Depending on what you mean by treated No there is no real need. OSB can be considered unsightly so people sometimes paint it when it is showing though. Depending on the quality and time some of the chips might break off.

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    OSB absolutely fails if in significant contact with water. I have seen a carpenter scrape OSB sheathing off a beam using his hammer claw like it was butter where the sheathing had contact with water behind vinyl siding on an improperly flashed wall. – Ashlar Feb 19 '16 at 22:36

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