Question on specifications

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I've started again on the world's strongest carport.I think I've just about used up all my free questions with the engineer - it was a love job for him anyway.Part of the structure (a horizontal bit) has a 100mm (4 inches?) channel specified. My problem is 100mm is too much - I've got about 70mm to play with.Now I could go down to 2 smaller dimension channels back to back.Questions:1. What dimension smaller channels mounted back to back would equal the strength of the 100mm?2. Would I bolt the channels together? If so, I'm assuming 10mm bolts at, say, 400 spacing would be fine - total length of piece is 2400mm.3. Would I bolt and weld? If I did this, I'm assuming I would not need to weld the whole seam?I hope the above makes sense.ScottLast edited by scott brunsdon; 03-11-2007 at 09:14 PM.
Reply:Pictures. Now.Craftsman 230a Buzz-box,  Lincoln 140T & 180T, Century K2789, PUROX W202 O/A14" cheapo chop saw that cuts straight and square!A toolbox of the cheapest Chinese tools money can buy"Real" tools all old reliable Husky/Craftsman/Proto stuff
Reply:Yeah, I figured drawings would help. It's not going to happen - beyond my IT skills. Just found out also that the minimum channel available here is 75mm. Might have to go with that. I need it to be butt welded to 75mm SHS (I think you call it 'square tube'). Problem is the square tube has rounded edges, so there is going to be some fill where the legs of the channel meet the square tube. Is that much fill going to be a problem?This might be making even less sense now.
Reply:This might be making even less sense now.
Reply:By two channels back to back you were thinking of making your own I beam so to speak??  If so that's been done before. I couldn't about the strength ratio at all tho.
Reply:I need it to be butt welded to 75mm SHS (I think you call it 'square tube'). Problem is the square tube has rounded edges, so there is going to be some fill where the legs of the channel meet the square tube. Is that much fill going to be a problem?
Reply:Gidday Sandy.That's what I was hoping.Zap, pics are a problem when the thing isn't built - haven't even got the steel yet because I'm still trying to work out what I need.Going down a size or two on the engineers specs doesn't worry me hugely when I think back to the flimsy structure that was there previously. The engineer more or less admitted the new structure would be strong enough to park a car on top of. Scott
Reply:Going down a size or two on the engineers specs doesn't worry me hugely when I think back to the flimsy structure that was there previously. The engineer more or less admitted the new structure would be strong enough to park a car on top of.
Reply:Hard to explain, but the perimeter of the roof is 125mm (6 inch)channel on edge with mitred corners. So it's damned heavy.The sheet roofing and guttering is all concealed within this so that when viewed from the street, the channel on edge is all that will be seen. (Exactly how it will be concealed is something I'm still thinking about.)Dimensions are 7 metres x 2.4 metres.Have a look at the pic.At one end - the house end - the carport will sit on that white wall. There is a 100mm (6 inch) high post sitting there now on the corner closest to the camera that the perimeter frame sits on. Then see behind the bamboo there is a 75mm post? One corner also sits on that.Now, look down the end of the driveway. See that beam? It's about 200mm lower than the wall, so the carport sort of sits about 300mm above that beam. (How I achieve this is what I've been pondering today.)From the street, the 125mm channel frame should look like it's sort of floating.I'm reasonably confident it's going to look pretty good. Attached Imagesfountain 04.pdf (88.1 KB, 33 views)