Concentrated Force on HSS

[ToadJones]

I have a connection I am analyzing for a fixture or sorts (NOT a building connection)where a 2.375" dia. round HSS is slid through holes of the same diameter drilled thru both walls of a 6" HSS rectangular tube.

The 2.375" round is loaded as a cantilever so it will have concentrated forces resulting from the moment where it bears on the rectangular HSS.
I am trying to check this condition using Chap K in AISC 360-05 and in particular equation K1-1.
K1-1 refers you to K2-1 for determining the factor Qf.
I am getting an extremely low value for Qf killing my connection capacity.

Is Qf even applicable here?

ConnectEgr - you out there?

 

[ToadJones]

I am determining the capacity of the connection.
In using K1-1 and subsequently K2-1, the Qf value I get is only 0.03.
Nonsense.

 

[SteelPE]

The lowest I could see Qf getting to is 0.4. That would be at U=1. Otherwise your pipe should be overstressed no (or at least close to it)? Something isn't right if your Qf=0.03 that means your U = 1.366.

 

[ToadJones]

Steel PE-
Here is the issue.

In calculating U, Mr = the full available moment because I am determining the capacity.
Pa in my case seems to be = 0 (no required axial strength)
So U = Mr/S*Fc
Yes, you are correct. When I calc U, I get 1.366

 

[SteelPE]

And the reason why you are getting 1.366 is because the shape factor for a circle is about equal to 1.366. Interesting, then I would say you need a larger outrigger or don't add any more load to the existing outrigger.

The only thing I would say is that with Pr= 0 you don't really have a chord. The definition of a chord member = For HSS, primary member that extends through a truss connection. So I am don't think these equations even apply.

 

[ToadJones]

Looking at Chapter G, maybe my concerns are covered with the shear bucking provisions Chap G6.

 

[racookpe1978]

What's the wall thickness of the 6" HSS?

THAT area is going to resist the force of the round member ("up" on back side, "down" on the front side); but only about 1/3 of the bottom arc and 1/3 of the top arc should be counted as effectively resisting the force.

(There's no real resistance to the force on the tube at the 90 degree of the pipe/round HSS wall.)

If the round beam does not fail by bending, then the failure mode will be collapse (kinking) of the round pipe on the bottom at front face.

 

[ToadJones]

1/4" thickness.

"(There's no real resistance to the force on the tube at the 90 degree of the pipe/round HSS wall.)"

Not sure I agree with this.

 

[racookpe1978]

How could there be resistance to a vertical force, when the round HSS is parallel to the round holes in the square 6 inch HSS member?

IF the round HSS were welded, then you can take credit for resistance to movement through the fillet weld's 360 degree circumference. The (two ?) fillet welds will resist tension and compression at both front and back faces of the 6 inch HSS. But for a "loose" member (one not welded), the only resistance to a vertical force will be the "somewhat horizontal" lower front and upper back.

If your member is fillet welded between the two members, then the whole problem changes: Your loads are NOT spread through the wall thickness only, but through the entire leg width of the fillet. Your bending moment is resisted through a wider lever arm as well (6 inches across the HSS plus 1/2 of each fillet leg on each side, rather than 6 inches across the HSS minus half the vertical HSS wall thickness on each side.) IF - and this doesn't often happen - the penetrating member is also using a slide fit (a very snug hole) then the walls of the vertical HSS will also assume part of the load as well: Gives you even more surface area to absorb the force. Your bending force will still be maximized at the front face of the assembly though.


If there is a problem, then replace the round small HSS member with a thicker wall "pipe": I'm working today with a 1-1/2 diameter pipe (1.90 OD) with almost 7/16 wall.

What's your max load?

 

[ToadJones]

Losing ya here....

Its a propped cantilever as Hokie said.
As the cantilever pipe is loaded, there will be a reaction at both faces of the rectangular tube. Vertical reactions in opposite directions.

 

[SAIL3]

interesting problem..probably no exact solution....
in this case I would usually bound the problem.
Calculating the longitudinal bending effects on the pipe are pretty straghtforward.
The effects of the shear loading on the pipe at the rectangular
member is where I would have to do some digging
The round HSS member is somewhat restrained from deforming in the plane of the face of the rectangular member, as hokkie pointed
out, but just outside the face of the rectangular memb, the round HSS member may still be influenced by this shear loading and start to deform.
Rectangular HSS: I might look at it similar to a lifting lug and
calc max bearing stress accordingly...this is a
very conservative approach as the round pipe is
not as rigid as a solid pin..if I can not live
with these results, I would back-off and say the
pipe will deform locally at the bottom of the
hole, say a 60 to 90 degrees included angle and
calculate the bearing stress based on that arc
length.

Round HSS: use appropriate case out of Roark for a parial line
load on a pipe or ring...this could be a varying or
constant load..get max bending and shear stresses.
combine these with the lonitudinal bending stress and
get max resultant stress.
To somewhat validate my approach, I would consider the case where the pipe would actually be welded to the rectangular memb and try and visualise the differnt behavior between the two cases.
What about buckling?..ofcourse the longitudinal bending should be checked for this alone. For the buckling effects of shear loading at the support I am assuming that the constraint provided by the hole minimizes this and one is left with only the concern of max local combined stresses in the round memb at the support.
I am sure there is a more accurate analysis out there, but this what I got off the top of my head.

 

[connectegr]

Toad...
Is there a sketch of you condition in this thread somewhere? Is this ta the top of the column? Is the pipe welded around the circumference to the tube wall? Is all the moment resisted by the column, or is a counter force on the far side?

http://www.FerrellEngineering.com

 

[ToadJones]

NO WELDS ANYWHERE

see attached ....generic sketch

 

[RFreund]

I think I would do as Sail has suggested -

For the round HSS you have shear stress and bending stress due to cantilever the cantilever then I would look at Roarks rings equations to find the moment or shear for the 'crushing or kinking' force. I would provide more direction on this but I have not yet read that section.

For the rectangular if your concerned with bearing I don't see how checking this as if it were a large bolt would be unconservative. So if it works for that process it would be ok, no?



EIT

 

[BAretired]

The problem is similar to the design of saddle supports for a pipe. The following link may be helpful, but in your case the saddle width is only 1/4", much less than the minimum required for a ductile iron pipe (I'm not sure about a steel pipe).

http://www.dipra.org/pdf/pipeOnSupports.pdf

BA