Tie 4x4 to 2x joist with hanging strength not supportive strength

Question

I want to attach a short 4x4 to a 2x12 joist which will be part of a treehouse's floor. The 4x4 will be hanging down and not touching the ground, so the force will be pulling and not the usual force of the 4x4 being attached to the ground holding up the treehouse against gravity. Is there a tie that is made for this? I tried looking but couldn't find one.

For some additional context, this is to be part of creating a dynamic uplift arrestor for if a wind gust pushes up on the treehouse on the sliding side (2-tree configuration). It will involve a strong metal plate running between two such 4x4s to "secure" it to the spreader beam that the sliding joists are resting on using UHMW treehouse sliders for the non-fixed side. The sliding configuration is as recommended by a professional treehouse builder from treehouses.com due to the fact that the trees can move differently in the wind, so that part is not in question, but the uplift aspect was not addressed.

I'm open to alternative approaches for creating this arrestor as well (one thought is to use some steel cables attached to the joist, but I don't love that idea because the tolerance can't be kept as tight since lateral movement may occur and that needs to be accommodated.

Answer 1

Hanging a 4x4 could be done with carriage bolts. Depending on the movement required it would be one single bolt or an array of bolts, and the bolt's required sheer rating would depend on the total load divided by the number of bolts.

The sheer strength of a bolt assembly is calculated as follows:

• Tensile strength probably at least 100000psi (check the specs by the manufacturer)
• Multiply by 0.6 to get Shear strength
• Multiply by cross section area of major diameter of bolt (e.g. 0.5in bolt has 0.44in^2)
• Multiply by number of bolts (e.g. 2, 3 or 4)
• Divide by safety factor of 5x for dynamic use

100000 * 0.6 * 0.44 * 2/5 = 10560 lbs

If you have 1/4in bolts, it is 1/4 of the above number (1/2 the diameter, so 1/2^2 the area).

If movement is not a factor, it is wise to use multiple through bolts even if the permissible load exceeds the requirement with just one.

Of higher importance is the bearing ability of the wood, considering the species. I have seen numbers of about 400lbs per lag bolt in deck building, so likely more for a through bolt if that's what you are using.

Pre-drill and keep the holes at least 1in away from the wood edge.