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What is "strength axis" on OSB boards? Question

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OSB boards often have a stamp that says "strength axis" with an arrow, usually parallel with the longer dimension of the board.

What is this strength axis? Should it be parallel or perpendicular to supporting joists? Why does OSB have a strength axis? I thought the point of OSB was supposed to be that it has randomly oriented strands so that you don't have to deal with things like wood grain direction.

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The key is in the name, in this case.

Oriented Strand Board (generally shortened to OSB) means, literally, that the wood chips are (roughly) oriented so that the majority of them have the grain of the wood running the long direction of the panel.

That strength axis should be perpendicular to the framing, the same way a board would be oriented, with the long fiber of the wood crossing the framing at or near 90 degrees.

https://www.innovativepanel.com/wp-content/uploads/2015/11/American-Plywood-Association-Load-Span-Table.pdf

Includes a table (2a) for OSB Sheathing, and a separate table (2b) for OSB Structual I Sheathing, and a third (2c) for OSB Sturd-I-Floor

These tables include information about floor loadings with the strength axis perpendicular to the supports and parallel to the supports.

Those loadings are significantly smaller for the "parallel to the supports" case in almost every condition; strength where load is governed by shear is sometimes not reduced, or only slightly reduced.

Examining the case of 32/16 sheathing (table 2a) supported at 16":

  • L/360 loading is 188 pounds per square foot (PSF) in the perpendicular case
  • L/360 loading is 41 PSF in the parallel case. (21.8%)
  • L/180 loading is 376 PSF perpendicular
  • L/180 loading is 82 PSF parallel (21.8%)
  • Bending is 209 PSF perpendicular
  • Bending is 54 PSF parallel (25.8%)
  • Shear is 207 PSF perpendicular or parallel. (100%)
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