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Question up front: What torque do I use on these bolts?

See Answers to Comments and new info below.

Details: I’ve got a 100-year-old home that has a 6x6 wood beam running down the center of the main floor. It’s supported on both ends by 6x6 beams set on top of the concrete basement footing. It runs ~32 feet long with 3 steel support columns spaced about 7.5 feet apart. It’s creaky but it works. In an effort to reinforce the beam, because replacing it would be too difficult, we’re looking at running a C5x9 steel channel beam down the length of it and bolting it to the side of that 6x6 beam. Bolts would be 5/8-11 A325 bolts spaced 12 inches on center, washers on the nut ends (bolt heads are resting against the steel beam).

This will eliminate future sagging and give the beam adequate strength to support additional load as we reinforce the floor and add weight to the house via other improvements in the structure. Like, sister'ing all of the 2x6 floor joists that are currently approx 20in on center. Some are more, some are notched, some are just broken. Goal is a strong beam to start to make a strong floor. I know 2x6s are not even in the code for floor joists, but it's what I've got to work with. The house is 100yrs old and nothing is even close to modern code.

Back to the main question I have, what to torque the bolts to? It seems like 110-150 FT LB. Does that sound correct?

Notes:

  • Steel Beam C5 X 9.0 (H5 x W1.885 x FT 0.32 x WT 0.325)
  • Wood beam is untreated, 100yr old lumber.
  • Bolts ASTM A325 Type 1, which seems to align with SAE Grade 5
  • plain metal, non galvanized/non stainless.
  • Size 5/8th 11 TPI

Best info I could find using the Torque (ft lb) = Diameter (in) * Clamp (lb) * Tightening Coefficient (0.20 for dry, 0.15 for lubricated)

Diameter TPI Tensile Strength Min PSI Proof Load LB Clamp Load LB Torque Dry FT LB Lubricated FT LB
5/8-11 120,000 19,200 14,400 150 110
  1. Source for table
  2. Details of calculations

Answers to Comments and new info

  1. Will the steel beam rest on top of the support columns and end beams?

    • Yes, planning on building up the existing 6x6 support columns to sit under the ends of the steel.
  2. C-channels on both sides, sandwiching the wood beam, or just one on one side?

    • Originally was just looking at one side but considering doing both sides (sandwich) to allow for more uniform support on the beam.
  3. As Isherwood noted, “Tension in the bolts provides friction between the wood and the steel” is critical and why I wanted to make sure I had enough torque.

    • I hadn’t said what size washers but was planning on 3”, the larger size to spread that crushing load on the wood.

Thanks for all of the feedback. I suspected the crush strength of the wood would be critical. I have a scrap section of the beam that I can conduct a crush test on. I’ll use that to inform any washer|wood connections.

If I stick with a single sided beam|wood, then I’ll tighten to “greatest force without deforming and damaging the wood”, and above crush test.

If I go with beam|wood|beam, sandwiching it, then I’m sure I can use much higher torque as the clamping pressure would be spread over that 5-inch cross section of the steel beam against the wood. Probably still not requiring the max 110-150 FT LB, but more than just the washer crushing area.

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    Will the steel beam rest on top of the support columns and end beams? Commented Aug 23, 2023 at 0:40
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    C-channels on both sides, sandwiching the wood beam, or just one on one side? Sandwiched, you have the beam spreading the load on both sides. If you have washers bearing on wood in a one-side situation, the point where the wood under the washer crushes will be a limit.
    – Ecnerwal
    Commented Aug 23, 2023 at 0:52
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    And honestly, you really only need the bolts to hold the steel beam in place. The bolts are gonna hold the beams together in shear, not in tension. Even if your bolt holes in the wood beam are slightly oversized, I doubt there will be any slop, due to the number of bolts you'll need to use—they won't all be centered the same. I would go with the old German torque value: güdentight.
    – Huesmann
    Commented Aug 23, 2023 at 12:34
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    @Huesmann, that's not quite right. Tension in the bolts provides friction between the wood and the steel, which can dramatically reduce shear load on the bolt. In fact, steel-steel joints rely almost entirely on this friction.
    – isherwood
    Commented Aug 23, 2023 at 12:50
  • The beam isn't what you should be looking at to reinforce by drilling holes in it. It should be the support of the joists, which means not doing both sides is out of the question. And so is torque, as the steel should (is to) be sufficient to transfer the load as if the 6x6 wasn't even there. - Your floor is bouncy because its 2x6s on 20"c, not because it's sitting on a 150yo 6x6 with a hardness of 7.
    – Mazura
    Commented Aug 24, 2023 at 22:09

1 Answer 1

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You have done a lot of research as to torque specifications. However they all relate to the strength of the bolts and nuts. That is not the issue.

The torque specification you are looking for is dependent on the wood beam. How much torque can be applied to the nut is unanswerable. It is dependent on the density of the wood, the moisture content, the size of the washer and I'm sure other factors.

The goal is to achieve a proper attachment of the steel beam to the 6x6 wood. This would have to be answered by an engineer. The answer would require giving them much more information than you currently have regarding the wood beam.

In real world application, your goal is to add strength to the wood by adding the steel.

The common process of attachment, without pre-engineered specifications calculated from known qualities, would be to use large washers, 2-3" under the nuts. The nuts should be tightened to a point where they are tight, but not causing the washer to crush into the wood more than maybe 1/16" .

What is that torque number? Unknown...However the steel is attached to the wood with a force that is at a point where it is greatest without deforming and damaging the wood. That should be your goal.

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    The final paragraph sums it up as I'd have said it. Tighten until you're seeing compression of the wood, indicating significant tension. The tension creates useful friction between the beams.
    – isherwood
    Commented Aug 23, 2023 at 12:50

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