Welding Aluminum I-Beam

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Hello,I am designing a greenhouse and the main stuctural members are sections of aluminum ibeam. See attached image. There are 4 ibeams each consisting of three sections curved to form a 'cheated ellipse'. The material is Aluminum @ t6 temper (after the bending process).  My question for the learned members of this forum is, how should I join these ibeams. They general dimension of the ibeam profile is 101.6 mm width * 101.6 mm height * 9.6 thickness. I'm not a welder so any guidance would be greatly appreciated. How should the joint be prepared? In Autodesk Inventor I can weld these beams together. What I do here is add 2mm chamfer to the ends of each beam than add the bead. But how much chamfer? How much bead? Would brackets and bolts be a better method? Thanks,Jordan Attached Images
Reply:Do you already have a source to bend that I beam?
Reply:That looks really cool!
Reply:Perhaps you had better bolt this together. Sandwiching plates on the web, and straddling plates inside the flanges would be easier than welding. Flat-head screws for a flush profile would be nice as well.Only reason I would not suggest welding is done hastily or incorrect could be a huge mess. At least you could have a competent weldor repair holes if need be. Just use high strength filler like 5356. And the structure could be disassembled for transport for future purposes.I have made many structures like yours and the hardware should be grade 8 steel zinc plated, with zinc compound used in addition to prevent galvanic corrosion between the dissimilar metals.Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.
Reply:Originally Posted by shovelonPerhaps you had better bolt this together. Sandwiching plates on the web, and straddling plates inside the flanges would be easier than welding. Flat-head screws for a flush profile would be nice as well.Only reason I would not suggest welding is done hastily or incorrect could be a huge mess. At least you could have a competent weldor repair holes if need be. Just use high strength filler like 5356. And the structure could be disassembled for transport for future purposes.I have made many structures like yours and the hardware should be grade 8 steel zinc plated, with zinc compound used in addition to prevent galvanic corrosion between the dissimilar metals.
Reply:Originally Posted by shovelonPerhaps you had better bolt this together.I have made many structures like yours and the hardware should be grade 8 steel zinc plated, with zinc compound used in addition to prevent galvanic corrosion between the dissimilar metals.
Reply:Originally Posted by DavidbrownI agree that bolting is preferable, but the rest is questionable. Grade 8 is an automotive fastener, structural grade fasteners are generally rated ASTM ####. Anyhow, zinc plating is almost always the worst material when galvanic corrosion is a consideration, especially with aluminum. It is frequently used as a sacrificial anode (or cathode? Not sure) on aluminum boats.
Reply:Yes I do.
Reply:Hello Shovelon, You are correct about sandwiching plates. This structure was designed for easy dissassembly and welded beams would not permit that. I've made a note regarding the steel plating. I was thinking of using galvanized steel plates to bolt each section of the polygonal base together. However, since this base is Fiberglass Reinforced Plastics I may have plates made from this material. I'll get back to the drawing board and design plates for the web and flanges of the i-beam.
Reply:Hi MJD, If I used metal plates I would definetly add rubber between dissimilar metals. Howeverthere will still be contact between the fasteners and the beam especially if I thread the beams...I'd like these greenhouses to last a lifetime or more and am building with that in mind. If I use plates the material configuarion might be;1. galvanized steel plates and steel bolts but this will lead to corrosion issues.2. aluminum plates and stainless bolts but I still have dissimilar metals...3. What about fiberglass plates (FRP) and FRP bolts? Anyone? I'd have to do some math and see if FRP can withstand the stresses but this material would solve the corrosion issue. 4. aluminium plates and FRP boltsLast edited by oieuya; 12-01-2014 at 02:00 PM.
Reply:What are your thoughts on using Structural FRP treaded rod to fasten these members together?
Reply:Why don't you use aluminum fasteners?
Reply:Originally Posted by EcondronWhy don't you use aluminum fasteners?
Reply:I have a commercial building with all aluminum rafters and mezzanine joists.  30# roofing paper (tar paper) was used to isolate the alum from all grounds, contact with concrete, etc.  Stainless hardware was used for all connections.  Building was completed in 2000.  No signs of corrosion.30+ yrs Army Infantry & Field Artillery, 25 yrs agoMiller 350LX Tig Runner TA 210, spool gunLincoln 250/250 IdealArcESAB PCM 500i PlasmaKazoo 30"  vert BSKazoo 9x16 horiz BSClausing 12x24 lathe20T Air Press
Reply:Personally I would use rubberized undercoating on all the connections , let it dry then assemble with stainless fasteners and call it  good. If you are real concerned maybe a good urethane caulk applied in an oversize hole with some applied under the washers so it shtzz out may help. On aluminum semi trailer rails where a steel crossmember meets the rail a heavy clear tape is used  and a nylon washer is often used under the Huck fastener. Other times aluminum buck rivits are used. In time the buck rivits pop the heads off from stress.
Reply:Good thinking! I will add this to my list of materials to research. Not sure why aluminum fasteners suck but a litte reseach may uncover why.
Reply:I believe its because they have a real reduced shear rating compared to steel or stainless steel. I would think the strongest aluminum fastener would be a " hard" solid buck rivit. The only problem is they are a PITA to install.
Reply:Makes sense. I think I'll experiment with galvanized steel plates on the web and flange with rubberized undercoatings and stainless bolts. What gauge of steel do you think is appropriate for this application?
Reply:Thanks! I think so too. I still need to do structural calculations to ensure it will stand. I got the idea for this structure from the base of a chair found at the garbage dump!
Reply:Originally Posted by oieuyaWhat are your thoughts on using Structural FRP treaded rod to fasten these members together?
Reply:Originally Posted by oieuyaThanks! I think so too. I still need to do structural calculations to ensure it will stand. I got the idea for this structure from the base of a chair found at the garbage dump!
Reply:I agree with bolting using S/S fastners.  You could also use the di-electric grease as used on electrical conduit joints to reduce the chances of electrolysis.Dan D.Manipulator Of Metal
Reply:Originally Posted by storemanPlease use "reply with quote" under the person you are addressing.   It is hard to tell who you are responding to.