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I'm looking for information on construction details and how to determine appropriate size for box joists sheathed with OSB (similar to this document that describes such joists sheathed with plywood, although preferably with tables for joists that need to take less load and/or have shorter spans than the ones described in that document, as I'm hoping to find a design that will fit in a 200mm vertical space). Or more specifically: how can I determine the maximum loading and required construction details for a box joist/beam consisting of two faces of OSB joined with timber that will occupy 200mm of vertical space? (For the purposes of this project, I do not require compliance with my local building code, but I do need to have some assurance that what I'm building will be safe, so as long as somebody's building code would cover it I'll be happy...)

  • My understanding of Oriented Strand Board is that it doesn't have great shear strength, and so wouldn't make good material for a beam. – Daniel Griscom Jun 20 '16 at 1:14
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It's an intriguing concept. But it's harder than you think. It's not only an engineering problem, but a manufacturing one too.

I think your best bet is to build a standard pre-engineered truss design that already has the strength to meet code. And then, use the sheeting for added strength. That would take the engineering out of it. There's still manufacturing. By contrast, the guy in the link is using square trusses, which is the worst truss design possible. He is betting the farm on the strength of the sheet material.

Dimensional lumber is not straight. Even with well-chosen boards, you'll need to run it throught a jointer edgewise to straighten one edge, then through a planer to straighten the other edge and bring it to your standard width. Since your standard width must be the lowest common denominator of all the boards you can get, be prepared for this to gobble as much as an inch / 25mm. So you'd need to start with wider boards.

The good news is, with such a narrow truss, you could do a minimal jointing to get the long members straight on one side, then assemble the truss, then run the assembled truss through the jointer and planer to square it as an assembly.

To assemble the truss, you'll need to lay them all down on a flat reference table (where?) and frame them up. I would construct the truss members to fine tolerance, and screw and glue to maximize the strength of the truss without sheeting.

Then plane the second side of the whole truss, flip it over and give the first side a last pass through the planer to correct any irregularities. Gluing the sheet needs to be near perfect. And the manufacturing technique needs to be consistent.

Nails and screws will have no strength when affixing the sheet material. All the strength of the sheet material depends on the glue. This is no place for random hardware-store glue. I would probably go for an epoxy if I could keep it out of direct sun.

The people in the link used nails, but they used an awful lot of nails, and they are dealing in much taller beam sections than you are. They also said "glue it too", I am saying "glue it well". For you, space matters so precision and strength matter.

It's important the wood be fully dried when working with glues. It takes weeks to stabilize kiln dried lumber bought off the lot, and months to stabilize wet lumber.

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Your best advice is to take your design requirements to a building engineer and have them draw up something that will meet those needs and specify the proper materials.

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