Wood joists smaller than actual standards 1

[Alan CA]

Hi everyone,

I encountered wood joists which their actual size is 1.5 by 9 inch (exactly). They are SPF No.2.
Why does it measure this way and how would these joists be checked using the building code tables? Would these be considered deficient 2x10? Or buffed 2x8?

 

[skeletron]

I would recalculate the section modulus for the reduced-depth section and then find the reduction factor (ie. S_true / S_standard) then apply that to the moment capacity. Other concern would be: if these are existing, does the current SPF values for bending strength still apply? For a 2x10 floor joist, I probably wouldn't lose sleep over it unless there are some other deficiencies noted.

 

[jayrod12]

That is almost precisely a 2x10. It was probably milled at 1/5x9.25 but with a bit of shrinkage from drying you end up with 9".

 

[phamENG]

What jayrod said. I've encountered this a lot where crawl space companies convince people they need 17 dehumidifiers cranked to the max under their house. Measured a 2x10 at 8.5" deep once. I've seen continuous girders under continuous floor joists lift off of their foundation piers because the joists (which were nailed to the girders) shrank so much they lifted the girder (which was not nailed to the CMU...). The best part was the other engineer who insisted it was the 300# pier sinking...

 

[JoshPlumSE]

Alan -

You do understand that the actual size of a 2x10 is 1.5 x9.25, right? Just take a look at the NDS supplement Table 1A.

If it's 1.5" by 9.0" then it is a little bit small. That might mean it shrank more than expected. Regardless of why it's this size, you can still use the NDS exactly as you would before. You just use a slightly different area, moment of inertia and such. But, anywhere the code refers to nominal size (like the basic design values) this would still be considered a 2x10.

 

[ChorasDen]

Part of the shrinkage consideration is dependent upon where within the log the 2x_ was cut from. See the image below, gives a relatively idea of the expected shrinkage depending on grain orientation. Wood tends to shrink twice as much in the tangential direction when compared to shrinkage in the radial direction.

 

[ChorasDen]

One thing to add, make sure these joists weren't field ripped. If the product has been ripped down from larger stock, that will affect the visual grade, and may affect the allowable design properties for this particular material.

 

[Mike Mike]

thanks for the insight phamENG, never seen anything like that before

why would field ripping affect the allowable design properties ChorasDen?

 

[phamENG]

Mike Mike - visually graded lumber is graded based on rules like grain size, grain angle, knot size, and knot location.

If a a 2x10 is no2 with a 4 inch knit along the centerline and a 2x6 can only have a half inch knot at its edge, ripping it down will lead to a 2x6 or so with a massive edge knot that drops it to less than no2.

(I made up sizes for the example)

 

[ChorasDen]

Pham has it covered, but basically, the grading rules allow for certain defects, at certain locations, and are limited to a certain size within the structural member. If you rip down a visually graded member, it will need to be regraded to ensure it conforms the to the allowances/requirements of the grading standard for the new product depth.