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?
- 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|
Answers to Comments and new info
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.
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.
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.