structural tubing telescopic connection
 

structural tubing telescopic connection
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

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.
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
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.

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.
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.
using six different hhs sizes (2" to 5.5"),i get a empirical result of: overlap d = twice the larger
awd d1.1 has limits on the fit up tolerances for telescoping peices
aws**