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# Squared tube connectors

## Squared tube connectors

(OP)
Hello All,

Hope you have a great day!

I'm working on design of steel connector between two squared tubes (80x80x4)mm, the suggested connector is as per the below photo.
thickness of these plates is 6.0mm, length of 355mm. this tube will be subjected to torque value of 30 KN.m.

There is another suggested criteria, which is connecting these tubes using inner tube, lower in dimensions, it will be (70x70x4)mm, length of 400mm.

1. what are the structural calculations required to check these connectors?
2. what is the advantages of using the second method?

The prompt response will be greatly appreciated. Thank you all.

### RE: Squared tube connectors

You need to think through the load path for the connection shown.

Compression, shear and tension. Note: I don't believe there is a load patch (other than friction) for resisting tension in the shown connection.

### RE: Squared tube connectors

Agree with JoshPlum....the only way you can develop tension in the tubes (to resist the tension during bending) is by friction in the connection, and my guess is you don't have enough engagement to do that. Even if you calculate the moment and resolve that the bolts will resist the force couple, you still have to consider that clamping the tubes will increase their potential to flatten during bending. Further, the clamping force and coefficient of friction of the steel will dictate the amount of friction resistance you can achieve. If these parts are painted or do not have a blast prepared surface, you'll have relatively low friction resistance.

### RE: Squared tube connectors

This appears to be a tube splice to transfer torsion only? Assuming that to be the case, if you could get adequate torsional shear into the connection between the tubes, your tube torsion would translate to longitudinal torsion along the bolt lines. Check for torsional distortion of the shape as well; this will inhibit shear transfer across these surfaces. An internal tube will help for torsional distortion.

### RE: Squared tube connectors

I agree with Buggar's suggestion regarding longitudinal shear along the bolt line. For analysis, I think that you can visualize the bolted connection isolated from the tubes and loaded with torsion at each end. Each individual plate in the box will tend to want to warp but will be resisted from doing so by the longitudinal shear connection to it's neighbors. Torsional stiffness of the joint may be a concern. Thick, strip washer plates and/or slip critical bolting might be prudent.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

### RE: Squared tube connectors

Why do this when a smaller tube inserted into the other tubes at the splice can function as a backing bar for a full pen weld?

Mike McCann, PE, SE (WA)

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