Structural Tubing Telescopic Connection

[iv63]

Two pieces of tubular steel sections (round or square or rectangular) are telescopically connected as shown on the attached sketch. How to calculate/determine minimum overlapping length in order to develop full moment capacity of the smaller section?
Thank you,
IV

 

[rb1957]

how are the two pieces going to be connected together ?
i'd anticipate bolts; a minimum of two rows would produce a reasonable bending connection.

how tightly do the two pieces fit together ?
obviously, if there is a lot of clearance, the two pieces will act more like a plastic hinge.

 

[dik]

If a sliding fit, and no fastener except for something to keep the parts together, then I would likely do a quick FE model, complete with air gap. I'm not aware of any empiracle methods.

Dik

 

[steve1]

Just a thought:

1. Calculate Moment capacity of smaller section = SxFb
2. Calculate a couple equal to M/d where d = overlap length
3. Calculate shear strees equal to Couple/Av where Couple equals M/d & Av equals shear area in vertical legs
4. Check shear stress against allowable
5. Recalculate overlap length and repeat until shear stress is less than allowable.

This analysis assumes a tight fit between the pieces.

 

[miecz]

I like steve1's approach. Although it may not have much effect, I would increase "d" by a bearing length, the shear divided by (.9Fy times an assumed bearing width). Question is, what's an appropriate bearing width. For a round tube, I think you can use the smaller tube diameter. For a square or rectangular tube, I'd use 3 times the outside corner radius of the smaller tube.

 

[Dinosaur]

I recommend you check buckling of the thin wall and I would probably cut the value of 'd' in your model in half of the actual overlap length. Look at a free body diagram of one end and it should be clear to you.

 

[civilperson]

Using six different HHS sizes (2" to 5.5"),I get a empirical result of: Overlap d = Twice the larger member largest cross section dimension. Seems to work with some flexing, (less than 4 degreees of alignment change at tangent ends).

 

[NS4U]

AWD D1.1 has limits on the fit up tolerances for telescoping peices

 

[NS4U]

AWS**