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Assuming an over-strength column composed of one jack and two king studs - how much load can a single jack stud support - assuming #2 stud grade SPF for the header and jack.

I've found a table that has SPF compression values : Fc perp of 425 psi, and Fc parallel of 1320 psi.

If the header, jack stud contact area is 3.5 x 1.5 (5.25 in^2), then is the maximum load on the SPF #2 header 5.25in^2*425 psi = 2231 lbs? Does this calculation have a safety margin? If not, what then is in practice maximum load on the jack stud?

general idea

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All structural design is done by one of two methods: 1) Ultimate Design, or 2) Working Stress Design.

1) As you might guess, ultimate design is based on “failure” of the material. When that method is used, a safety factor is then used based on the members importance. (Usually 20% to 100%...based on the designers experience and code minimums.)

2) Working stress design has already accounted for these safety factors.

The values you list are a bit confusing, because for compression perpendicular to grain for “working stress design” is 385 psi for Douglas Fir-Larch and pine and Spruce are much less, (they list 425) but compression parallel to grain for “working stress” is 1500 psi maximum.

This is all based on many factors, a) species, b) grade, c) use (repetitive or single use), d) size of member, etc. I suspect they have given you a safe number to use.

So, to answer your question: the maximum load for your 2x6 jack stud (I call them trimmers) is: 1320 x 1.5” x 5.5” = 10,890 lbs.

However, it sits on a plate and the header sits on it in perpendicular compression, so the maximum load it can support without crushing the plate is: 425 x 1.5” x 5.5” = 3,506 lbs. (neglecting the nailing into the king stud.) Note: I used their value of 425. You can substitute 385 if necessary.

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  • Thank you for correction - as it the header plate is the limiting value - not the 2x4 jack stud. Didn't provide details of the project (although you've been helping me with it!) - but once wet snow comes, that single jack isn't going to be enough. The idea that the nail to king stud will reduce load is an interesting approach. Possibly nails and 3" GRK #10 screws might be an easier solution. I'll explore that and possibly post a specific instance of the problem.
    – pathfinder
    Jul 26, 2018 at 20:01
  • @pathfinder Actually, nailing the jack stud to the king stud will INCREASE the allowable load.
    – Lee Sam
    Jul 27, 2018 at 3:24
  • I've bonded the jack and 2 kings w/nails, screws and wood glue (paranoid). The column should support 5800 lbs, with worst case design load of 2800 lbs @ 70 psf roof load. Following up your idea, I recalculated my loads and this time include a missing factor of 0.5 (since there is a jack on each side), so design load pressure is 266 psi ignoring any help the king provides.. Still added a 2nd jack to the one header that had only 1" of support which prompted this question....
    – pathfinder
    Jul 28, 2018 at 18:29
  • @pathfinder Usually the trimmer size is not the governing structural factor for the opening. Usually we start with the header. The header size is based on span, species of wood and grade of wood. As an example: for a 5’ opening a 6x10 Douglas Fir with a grade of #2 or better can support about 10,000 lbs. (What is the header span? Species? Grade?) If each trimmer is required to support 5,800 lbs., then the total load must be 11,600 lbs. Is this correct?
    – Lee Sam
    Jul 28, 2018 at 19:43

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