What torque value for bolts that are holding a steel beam to reinforce a wood floor beam

Question

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.

Answer 1

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.