Is this a do-able moment connection? 1

[KootK]

I'm trying to make a moment connection between two HSS members. They're radiused but we won't worry about that for now.

Welding isn't an option and the achitect is pushing hard for a clean looking connection.

I came up with an idea that is shown in the attached sketch. Is this even possible? Am I asking for nightmarish istallation headaches?

Any advice would be appreciated.

P.S. I know this would be a highly uneconomical connection.

Thanks,

Adam

 

[desertfox]

hi Kootenay Kid

Well I don't know what load your putting on but I wouldn't want to be any where near it,
It looks like your sliding a flat bar into a box section if my understanding is correct and if thats the case I have a question:- How do you weld something thats labelled 10mm ring 200mm down inside another box section?

desertfox

 

[Stillerz]

Can you explain this sketch?
I am having a hard time trying to see what is going on here.

 

[BAretired]

If you have a tubular section which fits snugly into the outer tube and laps, say a foot or two, the connection can take moment. Then, the only function of the bolts would be to prevent the two pieces from separating. Through bolts would work for that purpose.

If the pieces do not fit snugly together (which is usually the case with HSS) you would need to provide packing pieces to ensure a snug fit.



BA

 

[KootK]

Fox: the stiffener plates would break the HSS pieces into segments. Welding would be accomplished from the outside.

Stillerz: the sektch is of a moment connection between two HSS members. More specifically, it connects the two halves of a steel arch rib member that must be split for shipping and galvanizing.

BA: In my detail, I was using the single bolt to take up slack in the connection for serviceability purposes. Theres some other hardware that would prevent axial seperation of the HSS segments. What exactly are these packing pieces? Have you used that method before?

 

[Stillerz]

I'd at least replace the flat bar with a piece of HSS.
Are these "ring plates" in the shape of something similar to a block "O"?
How do you intend to weld the "ring plate" inside the HSS?

 

[desertfox]

Hi Stillerz

He doesn't intend to weld them on the inside he is going to split the box, I asked the same question, doesn't look a very good idea to me

desertfox

 

[KootK]

Stillers:

Yeah, the original had an HSS inside piece. I may go back to that once I get a chance to crunch some numbers. The ring plates would basiscally look like stiffeners or cap plates with 102 x 152 penetrations cut out of them so that the inner tube could pass through.

As I mentioned above, the ring stiffeners would break the HSS. The HSS on either side of the ring stiffners would be prepped with an angular cut and full or partial penetration welded. No welding inside the HSS.

Adam

 

[Stillerz]

I see it a little better now....not used to reading sizes in millimeters. So that "flat bar" is about 4" x6" ...thats pretty substancial.

I guess the porblem with it is, it isn't really a connection. The only thing holding it together is a set screw and some compression forces (i'm assuming)

 

[KootK]

Stillerz: as I responded to BA above, there's other hardware that provides an axial connection between the pieces. That's not a problem. My primary concern is whether or not it can be reasonably constructed in the field.

 

[Stillerz]

I have seen splices in Continental Pedestrian Bridges that were almost invisable.
They spliced HSS to HSS with a smaller piece of HSS inside and through bolts on either side. Where the main pieces of HSS butted on the outside, they must have been milled. The butt joint was almost invisable.
The problem is findig a piece of HSS to fit snugly inside the large a BS said.
I thought it was a very nice looking connection.

 

[KootK]

Desert Fox: Your comment isn't of much use to me unless you tell me what it is that makes it a bad idea. The assembly can be fabricated, albeit expensively (as I acknowledged in my original post). What problems do you forsee? I'm mostly concerned about erection tolerances.

 

[KootK]

Stillerz: that does sound like an elegant connection. Was it clearly meant to transfer moment?

 

[csd72]

Kootenaykid,

I have used a very similar type connection but with much larger sections where it was easier to get the welds done on the inside.

Another option is to have two cross bolts in sleeves to take the moment.

 

[Stillerz]

I suppose anything can be designed for moment.

 

[BAretired]

Kootenay Kid,

No, I haven't done that type of connection before and I would have concerns about fitting them together in the field. Very difficult with a snug fit. The outside member is 203 x 152 (8" x 6")outside. You don't give us the thickness. If it is 6.4 (1/4") the inside dimension is 7.5" x 5.5". A 7" x 5" member fits in easily but has too much play. The packing pieces would be, say 1/4" flats bent around the corners and welded to the inner tube, one at each end of the lap. When inserted into the outer tube, this would fit nice and snug.

You could even skip the end plate on the right hand member in your sketch and let the insert HSS extend both ways from the splice. I would certainly want to discuss such a detail with the fabricator before going too far with it because it will be a pain to erect.



BA

 

[KootK]

BA,

The thickness of the HSS isn't yet determined. I'm pretty keen on the packing piece idea though -- that's slick. I'll run it by some fabricators here and see if that might be the answer.

Kootenay

 

[KootK]

csd72,

Can you give me a little more detail about the cross bolt idea? It sounds interesting Is it one sleave on top of the HSS and one on the bottom, both running parallel to the HSS? Is this essentially a prestressed connection?

KK

 

[NomLaser]

Welding the inside pieces will be difficult to say the least. And as it was already said that you won't get the snug fit that you need to keep it rigid. Any chance that you can make your flat bar go through your end cap so that you can get at least 2 welds on it? I would be more worried about shear forces at the junction of the flat bar to the end cap than moment bending, then again it's hard to tell without expected forces.

 

[csd72]

Put the bolts so they are in double shear and spread them apart enough to get the moment capacity.

Sleeve is through the inside tube but unfortunately you cannot fully tension the bolts as it may crush in the outer tube walls.