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Is this a do-able moment connection?

Is this a do-able moment connection?

Is this a do-able moment connection?

(OP)
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

RE: Is this a do-able moment connection?

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  

RE: Is this a do-able moment connection?

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

RE: Is this a do-able moment connection?

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

RE: Is this a do-able moment connection?

(OP)
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?

RE: Is this a do-able moment connection?

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?

 

RE: Is this a do-able moment connection?

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

RE: Is this a do-able moment connection?

(OP)
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

RE: Is this a do-able moment connection?

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)

RE: Is this a do-able moment connection?

(OP)
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.

RE: Is this a do-able moment connection?

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.  

RE: Is this a do-able moment connection?

(OP)
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.

RE: Is this a do-able moment connection?

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

RE: Is this a do-able moment connection?

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.

RE: Is this a do-able moment connection?

I suppose anything can be designed for moment.  

RE: Is this a do-able moment connection?

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

RE: Is this a do-able moment connection?

(OP)
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

RE: Is this a do-able moment connection?

(OP)
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

RE: Is this a do-able moment connection?

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.

RE: Is this a do-able moment connection?

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.

RE: Is this a do-able moment connection?

If I might, a trial fit of the pieces on the shop before shipping would probably be a very good idea. Easy to solve problems at that stage and adjust fit.

You said that the pieces were galvanized. Hence, before shipping but after galvanizing (which will affect clearance).

 

RE: Is this a do-able moment connection?

(OP)
NomLaser: I could pass the flat bar through the end cap.  These are pretty small loads however.  I figured, with a thick cap plate, it would be okay as shown and probably more economical to fabricate.

RE: Is this a do-able moment connection?

Hi Kootenaykid

Well my concerns are alignment after welding as the parts are bound to distort, I have no idea what load your putting on this except that its the centre of a arch how do you know the joint is strong enough.
Looking at the plate section sliding into the larger box section with the clearence between the two,the rings that support the flat plate become point load supports that  will transer all the load from one box section to the other with very little bearing area.
This in turn will transfer the load to the 8mm fillet weld on the righthand side of your joint which means there's a bending moment on the fillet weld as well as shear and at the far side of the joint where you split the box into two to put a stiffening ring in again your relying on the totally on the weld.
How confident are you that the load transferred in the joint won't just rest on one edge of a single stiffening ring and the rest carried the bottom of the adjusting screw because you will never get those 2 support rings perfectly level, which from the sketch is what is required.
What are you going to do during welding as the box sections are galvanised, which after welding they certainly won't be galvanised anymore and I am fairly confident that attempting to weld galvanised steel gives off toxic fumes.
I think what BAretired is suggesting is sound in that by packing all round the joint so its snug it will transfer the load from one to the other over a much larger bearing area then your drawing suggests.
hope this helps

desertfox  

RE: Is this a do-able moment connection?

(OP)
Good idea Kelowna.  They're planning to do a full scale mock up on the shop anyhow. The whole framing assembly is pretty complex geometrically.  By chance, are you CWMM or 4DEng?

RE: Is this a do-able moment connection?

(OP)
That helps a lot Desert Fox -- thanks.

Alignment after welding is a good point.  I was planning to have the assembly galvanized after all of the welding is performed.  With no closed sections, I had hoped that this would be possible.

I intended to use the set screw only to keep the thing snug under service loads. In an ultimate case, I'm positive that the inner tube would be resting on one edge of each ring as you described.  The connection would have to be designed to work exactly that way.

As for the rest, you've basically described the precise load path that I had in mind.  The loads are small which works in my favor.  Everything that you've pointed out would certainly have to be checked.

Based on the advice provided in this thread, I am hoping to go with something more practical and less dubious analytically.  We'll see what my architect thinks...

RE: Is this a do-able moment connection?

Yes basically thats it.

RE: Is this a do-able moment connection?

(OP)
Cool.  Is it a challenge to get the bolt hole and sleeve to line up?

I wonder if you could manage it as a slip critical connection for light loads?  If that were the case, maybe I could use an oversized hole with a plate washer.

RE: Is this a do-able moment connection?

Hi KootenayKid

Thats a much better idea provided its strong enough to take the loads.

desertfox

RE: Is this a do-able moment connection?

Thinking outside the box on the connection you sketched.

-Why isn't welding an option?  It would be super easy to make a slick connection.  Maybe you already said and I missed it.

-What about a bolted end flange with stiffeners.  It wouldn't be invisible, but sometimes architects like them.

 

RE: Is this a do-able moment connection?

(OP)
Thinking outside the box about a connection inside the box!  Sorry, couldn't help it...

Welding is out because we'd have to metalize or otherwise touch up the galvanizing after welding.  These aren't really exposed to the elements so the architect questions how well the galvanizing and touch up will blend.  Basically, we don't want the touch up work to be apparent.

Yeah, I tried to sell a bolted end plate moment connection too.  No...such...luck.

RE: Is this a do-able moment connection?

I think your original connection would work, but I would allow a bit of tolerance for the tongue insertion, then use several set screws to align it and take the bending reactions.

RE: Is this a do-able moment connection?

Is there any possibility of a bi-axial moment on this connection?

RE: Is this a do-able moment connection?

I agree with BAretired...an internal sleeve would be simpler and more effective.  Even if you had to construct a matching sleeve from plate and machine or grind the edges to fit, it wouldn't be that expensive.  Shimming an undersized sleeve could be done as BA suggested; but that could be a bit tricky in the field.  A close fitting sleeve would be relatively idiot proof and would take care of any biaxial considerations as well.

If the sleeve is thick enough, you could drill and tap it to receive the alignment/keeper bolts, or you could use a rivet nut/Nutsert or similar to allow one sided bolting.  That would keep clean lines to, of course, satisfy the architect.

I have used this approach in aluminum structures and it works well.

RE: Is this a do-able moment connection?

KootenayKid,

The attached page shows a typical sleeve connection for HSS members for cantilevered sign posts which take moment. The sketch seems similar to what you are trying to accomplish. It is from a book called Engineering Sign Structures by Benjamin Jones. I've never designed one so I can't be much help other than background info.

 

RE: Is this a do-able moment connection?

Hi Adam

No, I do not work or worked for CWMM or 4DEng. I just happened to be living in Kelowna at the time I joined the forum and it was as good a username as any other.  

RE: Is this a do-able moment connection?

how not make the inner tube walls thicker, and just drill thru the outer tube, and tap into the inner tube and use studs ?

i'd use 4, two on the sides (as shown), 2 on the top and btm faces, interpitched.

and this is going to look neater than retouched weldments ??  CSK bolts no doubt.

RE: Is this a do-able moment connection?

"how" should be "why" (of course)

sigh

RE: Is this a do-able moment connection?

rb1957, BA, Ron have it.  Suggest using Unbrako countersunk bolts.  Some of the other proposals ignored the requirement for no site welding.

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