huangyhg

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**

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