Custom Steel Section 2

[timbercharlie]

I am working on a project where the owner wants to make a custom (home made) steel rectangular HSS. I have currently designed these members as HSS, ASTM A-500 GR. B. The cost of rolling the HSS on a radius is blowing the budget so we are trying to investigate options. The members are full arches - inverted "U" and are to be one piece. We want to consider welding cut plates (strips) to form an arch... (2) plates would have the arch (radius) cut and the other (2) would be curved along the ID and OD and fully welded along the 4 edges. My concern is the possibility of residual stresses induced during the welding and subsequent cooling of the section. Wouldn't you have to anneal the section to relieve it of the welding stresses? Has anyone ever done this sort of thing? Thank you.

 

[shobroco]

How big are these arches? I can't believe that you could cut the plates & weld them for twice the cost of rolling them, never mind for less than the cost of rolling them. Is there no one, or only one, shop with the capability near you?

 

[dhengr]

There should be no problem in doing what you want to do, depending on a bunch of details which are in short supply in your problem description. Many box shaped sections can be fabricated without post weld heat treating. If you can make it work with a std. HSS, you can fab. it into an arched shape. Radiused corners require some extra plate prep. and you want to avoid groove welds if you can.

 

[SAIL3]

Assuming the plates are of reasonable thickeness a qualified fabricator should have no problem with this and would not require annealing. As dhengr pointed out avoid full pen welds where possible...more cost effective and less prone to heat distortion.
To put your mind at ease add a note on the fab dwgs..."sequence welding to avoid distortion"....a qulalified fab shop should do this anyway as standard pracice.

 

[connectegr]

As all have said, this is not a problem. Tight curves are frequently made with fabricated tube sections, for monumental stairs and other miscellaneous architectural items. A qualified fabricator will know to sequence the welding to avoid distortion. PJP welds can be used if the wall thickness is adequate to provide the 1/8" land.

http://www.FerrellEngineering.com

 

[timbercharlie]

Thanks everyone for your input! The sections are 6"x8"x1/4" and 6"x10"x1/4" with min radius at 8'-8". Well, those sizes are for A500 GR.B and the fabricator will surely want to use A36, so I will have to upsize them a bit. The whole point of this effort is to save money. Shobraco, I too am surprized that this proposal is a valid cost saving alternative, but I still have to get more into the details.

The welds would be along the edges of square cut plates... please refer to attached sketch. Do you consider this a full pen weld? Should we lap the plate in the corner and weld on the outside only?

We are in the NW and from my research, I could only find one outfit that can do this size of rolling (near Portland, OR), but we do have fab shops that can make the welds... it's going to boil down to cost.

 

[connectegr]

The welds will depend on the stress. Will 3/16 fillets be sufficient?

http://www.FerrellEngineering.com

 

[timbercharlie]

connectegr, that is my next step. Once I determine the feasibility of this option (which I now have), I will be doing the weld design. I will most likely start with a 1/4" fillet and go from there. I also have to do research on how to design this weld and to make sure to consider all of the forces that are acting on the member. There will be torsion on the tubes, so I have to consider that as well. Do you know of any publication to refer me to that might help? Thanks!

 

[mijowe]

how much plate are you going to start with to be able to cut out the curved faces? how much waste will be left when you are done?

 

[SAIL3]

Option 1 may be difficult for fit up...maintaining edge to edge contact of the plates..
Otion 2 would require a prepared edge and with the thickeness of pl of a 1/4" a partial pen would leave you with very little weld.
Anyway, since you mentioned that it will also have torsion, I would be hesitant to use either one..they would be ok for bending and shear. Torsion, causing rotation of the member x-section could tend to open up the root of these welds..not good.
Because of the torsion I would use option 2 but with a square grove(butt) weld with a backing bar. This would also faciliate fit-up. This is a FP weld only because of the presence of the torsion.
If you want it to look pretty, grind flush after welding.

 

[dhengr]

I think SAIL3 pretty much hit the nail on the head. But, you might also consider the following: (1) I would like to know much more about what these arch structures really are, how they are loaded and magnitudes, how they’re braced, how the torsional loading is applied, do they have vert. legs/columns under the 180° arch, how is the arching action supported and reacted, etc. (2) If there is torsion, how much and how concentrated? However you cut it, with torsion, there can be a tendency for two corners to close and two to open up, and you never know which ones, so all four must be considered as important. I don’t think the exact type of weld is the important thing, they all will have roots which can be adversely affected. The important thing is that your weld detail leads to a good, sound, root pass or root condition. (3) I would like to see the moment, shear and torsion diagrams and stresses in these arches so I could study what to do with the fab. and welding details. (4) Remember that flange tension on the inside of a curved member has a stress component which pulls the flange away from the webs, and this stress must be included in designing the welds. Flange compression on the inside of a curve presses the flange into the web, generally not a problem because it is a compressive bearing stress, but that stress must still be considered in the weld design. On the outside of the curve this flange/web condition is the opposite in terms of pulling the flange away from the web.

My first choice for a cross section would be the following, assuming this was acceptable aesthetically. It is totally honest from the fab. and design standpoint and is most economical, so I’d fight for it. I would use 6" (+/-) wide flange plates and press or pull them to the webs for fit-up and welding. I would set the webs at 5" (+/-) wide, out-to-out, leaving .5" (+/-) of flange outstanding to receive a nice clean fillet weld. This has the potential of good, but simple, fit-up and welding, and as good a root condition as you are going to get short of a fillet on both sides of the web, and you have access problems on that account. I would cut the webs out of plate, in two or three pieces, 90° or 60° of arc. This gives you some economy btwn. plate size/waste and splice welding and clean-up. In cleaning up the cut web pieces, you could grind or nibble a 1/16th or 1/8th inch bevel on the webs, to pick up some weld throat, but this is expensive and not needing in the above cross section. This would be needed in your option #2, and possibly in SAIL’s backer bar scheme. As SAIL mentioned, your option #1 is an almost impossible fit-up problem out in the fab. shop. But, easy to draw.

Omer Blodgett has several real good books on weld design and the design of fabed. structures.