Pre-stressing a steel arch?

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I'm making a structure that will hold shade fabric above a deck.  It will consist of multiple arches, each consisting of two uprights bridged by a basic Gothic arch.Four of these will be connected to frame three sections, which will have criss-crossing arches inside them, like in a Gothic cathedral vault.  Here's a picture of a cathedral vault to show what I mean:The design is driven by cosmetic concerns.  The thing won't have to support any weight other than the shade fabric (this is Southern California), and it's also reasonably sheltered from wind.  The structure is getting made out of 16 ga 1-1/4" square tubing.  These are two of the curved pieces.  They'll sit on top of 8' straight pieces.Now, I know there's vertical pressure coming straight down from the weight of the steel arch -- that's not a concern.  But there's also (potentially) lateral pressure -- the arch pushing out on the tops of the vertical pieces as the load gets distributed downward.  My gut feeling tells me this won't be enough force to change the verticals from being vertical.  But as I've been thinking about it, it's occurred to me that I could 'pre-stress' the weld of the two curved pieces to essentially fight the lateral force.  In other words, I'd weld the two components of the arch as though they were covering a slightly narrower span, so that the steel arch pieces would be trying to pinch the verticals inward once the verticals were bolted down to the wider span.But again, it could easily be that the weight we're talking about (two 16' lengths of 16 gauge 1-1/4" tubing) just won't produce much 'bulging' of the finished shape.  Can anyone hazard a guess as to whether I'd benefit from pre-stressing the pieces when I weld them together?  I can always build the first one and see if it pushes the uprights out of vertical.  But I thought I'd post the question on the off chance that this was an easy question for someone out there.My goal is to avoid having to add a horizontal piece (I guess it would be a wire) at the base of the four main arches.Jack OlsenMy garage website
Reply:One of the beauties of the Gothic arch is that the vast majority of the forces are downward vs a Roman arch where there is a good bit of lateral forces being applied. With the weight you are talking about, I doubt there will be much of an outward force trying to bulge the verticals out. I'd fab them as you originally intend to. If there is any significant bulge, I'd just use thin aircraft cable to pull them back into place, or very small rods for your tension pieces.The only thing I'm wondering is if your 1 1/4" columns will be large enough to take all the ribs of the vault. If you do what you show in the cathedral pict, the side columns will have 2 side arches, 2 cross arches and a center arch ( see red circle ). If your material is all 1 1/4" you will have some complicated cuts to get everything to come together on one small  column. Note in the pict each rib comes down on it's own thin column, or in other cases, the column is larger in diameter to take all the thin ribs into one "trunk".I've always loved Gothic architecture. Looks like this will be an interesting project. I can't wait to see how it looks when completed. Attached Images.No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Thanks!As I'm currently planning it, there will actually be three vertical 1-1/4" pieces where you put the red circle. I won't have the window-forming arch that's in the picture, or the half-dome-ish set of ribs at the very top of the picture.  My idea is more simple than that.  I thought about having all three of  those arch pieces sit on a larger vertical column, but then decided to just have them each sit next to each other, possibly linked with a steel decorative piece holding them together where they diverge in the three different directions.I'm not sure how it's going to look when it's done.  I've looked through hundreds of pictures of different kinds of structures like this, and I haven't yet found anyone who has done the Gothic vault pattern with steel.  It's certainly not because it would be difficult -- so it might be because it's going to turn out ugly.    But my goal was to have a steel structure that didn't look as utilitarian (or as industrial) as steel tends to look when it's used for a simple barrel arch or an arch with reinforcing pieces.If I had the ability to use a program like Sketch-Up or even the skill to draw a good picture of what I'm planning, I would know better how it will look.  But learning to do either of those things would eat up more time than it'll take  to build this thing.Last edited by Jack Olsen; 03-26-2013 at 10:25 AM.Jack OlsenMy garage website
Reply:I've seen simple gothic arches used for garden trellises, but they are usually just a simple "tunnel" without the cross members. I've also seen quite a few Gothic revival arches done in steel and iron. Either as a beam with cutouts, or in some occasions as trusses. An gothic arched scissors truss comes to mind right away. As do iron age train stations. In the case of the station, they reorient the arches so they get a wider shorter roof than what they got with a cathedral where the idea was to have the roof soar to great heights as opposed to covering the most floor space with the train station. I'll dig a bit if I have time today and see what I can find online or in my files.I'm not sure how what you have planned is going to work without the window arches. There's a reason those "cross arches" are there. If you form an arched "tunnel" running down the length, then intersect it with arched "tunnels" running 90 deg from the window arches, Where they intersect is where you get those cross arches. In effect they are the same lines you'd have if you had to cope two arched tubes together if this makes more sense. Without the window arches, you'll simply have a "tunnel". Not sure if that is the look you are going for or not. Keep in mind the diagonal cross arches will be a different size/shape than the main arches. That shape is determined by what would be your nonexistent window arches ( see tube cope idea  above) based on the spacing between the columns in both the X and Y directions.These greenhouses show what I think you'll sort of end up with as the basic form without the window arches. If you just draw a line from the corner to the center of the ridge between the two "ribs" that would be your diagonal cross members.http://www.gothicarchgreenhouses.com/customers.htmYou might also look at fabric structures. There are all sorts of tensile structures that give you that gothic arched look, but don't use diagonal ribs. The tensile quality of the fabric gives you that "arched" look. It's been quite a few years since I did anything with those types of structures. They were real big as project ideas where I went to college for architecture, since the college originally specialized in textile sciences. Autocad could quickly generate  the approximate forms if you created arches or columns and told it to do a 3D shell for surface modeling.https://www.google.com/search?hl=en&...w=1366&bih=675http://www.aformny.com/styled/blog/structures.html.No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Jack,So you are planning to make an arched 'tunnel' that has 8 ft tall 'posts' and then those pretty tall arches on top of it?Four 'arches' that will make three 'sections', right?Each arch of two posts and two arch segments will weigh approximately 50 lbs (8 ft section 'post'+ 16 ft section 'arch' of 1-1/4" square 16 ga steel @ 1.047 lbs/ft x 2 = 50.26 lbs)  Times four arches = 201 lbs of steel.Plus the weight of the fabric.This project is not just a patio umbrella.  btw, looks like some smooth bends in the 'arch' segments.  Did you do that, or have a shop roll it?  The best laid schemes ... Gang oft agley ...
Reply:Thanks, DSW.  You found more examples than I did.  Still, I've never seen the X pattern used (except, of course, in cathedrals).And thanks for the calculations, Moonrise.  I think with the cross arches forming the triangles they do, it should be pretty strong.  But then, we'll see,I'm bending the steel myself with a cheap Harbor Freight roll bender.  The only trick has been finding enough space to do the 16' lengths on my tiny lot.I did something similar for my front patio.  It has far less strength, but has held up without any issue for a few years now.I don't use it often, but it's really handy when I need it.  I also did a bench crossmember with it.  Jack OlsenMy garage website
Reply:what kind of fabric is that Jack?
Reply:Coolio shade fabric, if I'm remembering right.Jack OlsenMy garage website
Reply:Some more image searches that might give you some ideas there Jackhttps://www.google.com/search?hl=en&...29%3B220%3B238https://www.google.com/search?hl=en&...w=1366&bih=675https://www.google.com/search?hl=en&...fabric%20vaulthttps://www.google.com/search?hl=en&...mg.reKBBg-xzkshttps://www.google.com/search?hl=en&...mg._VvyvNJJ0EwThe reason you aren't finding more "cross arches" except in cathedral like structures is that form follows function. The cross ribs are a natural function of how you create a masonry structure that spans a distance and allows light in from the sides. Maybe some of these picts will make the reason clearer. Attached ImagesLast edited by DSW; 03-26-2013 at 04:35 PM..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Very interesting thread!
Reply:Originally Posted by Jack Olsen In other words, I'd weld the two components of the arch as though they were covering a slightly narrower span, so that the steel arch pieces would be trying to pinch the verticals inward once the verticals were bolted down to the wider span..
Reply:Neat project. How hard was it to make the same radius as those  setting on the ground  ???  I could see doing one but multiple maybe more challenging . Looks good.What dies are in the bender ???  Are those from SWAG ?
Reply:Yes, they're the Swag Offroad dies.  Their replacement driveshaft is also a huge step forward for the HF tool.  I've also got all their pieces on hand to use the HF pipe threader to make it even easier, which I might do when I get back into town.  It's a real workout for your shoulders, even with this light weight stock.Doing the arches was pretty simple.  If you look carefully at the pieces of hardboard I'm using to protect the deck itself in the picture below, you can see two curves -- one that I scribed out using a 17' long string, and one where I extended the string to 17'10".  The criss-crossing ribs will have a slightly wider arc to them than the straight ones, which makes more sense if you think of the sides of a triangle involved in their respective spans.  I just bent-and-tested until the first piece matched the line on the hardboard.  Then I kept track of where the top die was, in terms of threads, and knew I had to aim for the same ballpark the next time.  The thing that made it even easier is that 16 gauge is bendy enough that you can 'adjust' the finished product with some strategic applications of your own body weight.Jack OlsenMy garage website
Reply:Cool, thanks for explaining. I didn't look that close. Can't wait to see the finsh product.
Reply:something very important is missing from all those interior shots of gothic arches, the flying buttress's on the outside that keep it together.  there is absolutely a lateral force generated from a gothic arch, in gothic cathedrals that force was counteracted by the flying buttress's. often they would but a big stone statue at the top of the buttress whose purpose was to provide downward thrust to help counteract the lateral force.if the youre just making the arch, ie not having the arch sitting on top of columns, and have the ends of the arch bolted down or otherwise secured in a lateral direction you'll be fine.
Reply:Somewhat true. Not all gothic arched cathedrals used flying buttresses, though most did. The taller and narrower the arch, the more vertical the forces are. The wider/flatter the arch, the more outward thrust they generate. Also the thinner and "lighter" the cathedral was, the less mass they had, thus the less weight that needed to be transferred from out to down. At the height of the craft, buttresses were almost unneeded in many cases because the material had become so slender and the arches so tall. Damage to many of the exteriors of cathedrals in Europe during the bombings of WWII proved they were really much stronger than many originally believed. Another thing to keep in mind is that part of the reason for those buttresses is that stone is very poor in tension, but strong in compression. Hence the buttress to "lean" against the sides to help offset the tendency of the main arches to want to push out. Steel on the other hand doesn't have that limitation to the same degree.The one issue that might need to be dealt with is buckling. Tall slender straight columns tend to want to bend under vertical or offset loads if the height to thickness ratio is wrong. A curve can either help or exaggerate this problem. I doubt the loads involved will overstress the columns however as they are quite small from the sounds of it..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:thanks JackI can't wait for you finished result.
Reply:Thanks, Rusty and DSW (again).  The thing that I'm building doesn't have a whole lot in common with actual cathedrals.  The total weight of my structure is only going to be about 500 pounds.  A cathedral weighs about 100 million pounds, and a single cubic foot of limestone is about 170 pounds.  So the forces a limestone arch would have to contend with, even if it was as small as mine, would be very different.If my math is right, my columns will have to support 17 pounds each, and only a portion of that will be producing lateral force.It brings me back to my original question.  Mild steel tubing is both fairly elastic and also somewhat malleable.  I am able to permanently bend these pieces, but if I welded the top joint for a span of 19 feet instead of 20 feet, it would continue (I'm assuming) to exert some 'pinching' force, which could be used to balance out whatever the lateral force the weight of the arch generates and acts on the tops of the columns.The problem is that right after bending the first two pieces, I had to go out of town for a week.  So instead of simply welding the arch and columns together and seeing what happens, I'm sitting here wondering about what might happen instead.  I'm sure that there is an engineer somewhere who (with the right resources) could tell me that the arch will generate X pounds of force pushing the columns out, and that welding the thing for a span that is X number of inches shorter than my actual span would produce a sort of equilibrium.  Without that, I suspect it will be pretty simple to build one arch, see if there's any spread, and then re-cut that top joint and re-weld it a little bit tighter (maybe more than once) until I hit that same equilibrium point.  I'm sure that over time there will be additional force applied, since the thing will be subject to some other forces and maybe the steel will lose some of its elasticity as it gets moved over and over again by the wind.  But an engineer might also simply answer that the lateral forces will be negligible when they're put up against the strength of a 1-1/4" square tubing that's welded into this shape -- especially with nine other arches welded alongside it, each reinforcing the other.  I just don't know.  And like I said, I'm out of town so I can't simply put the thing together and find out.Jack OlsenMy garage website
Reply:Just to confuse you more Jack, I've been thinking on this and am wondering if the tensile quality of the fabric wouldn't want to collapse the structure IN rather than out. If you put enough tension on the fabric to get it super tight will it want to possibly pull in on the members... You've played with the fabric out front before, so you've got a better idea how the stuff reacts than I do. ( I couldn't remember who did that fabric canopy here until you posted up that pict earlier) One thought is that maybe you do each "side" as a seperate piece. That way the fabric pulls from the ground up to the ridge from both sides, vs going from side A to side B over the ridge and tries to pull the whole thing into the center. Your arched members almost act as a "bow" in the 1st scenario. I don't know... I'll have to think more on this...The little bit that we did at college that actually had "practical" applications on tensile fabrics all had to do with larger heavier tent like structures. ( School was big on "pretty" pictures and drawings of stuff like this and very poor on actual practical applications and limitations for the most part... ) I know the large party type tensile tents I've erected usually take a fair amount of "cranking" to get them set up and tight. Most use 2" ratchet straps, but are pole support type units vs frame units. I'm starting to wonder if you might have to guy( sp) this thing to get the right tension..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:what you could do is stiffen the legs, then add bracing at the top of the arch to counteract the hinge action. sounds like the loads are light enough that it shouldnt be terribly difficult.
Reply:I'm back in town, and -- as is often the case when I get out my welder -- I'm in over my head.  I'm also up to high -- working all along, I'm lucky that I haven't fallen off a ladder yet.  The top section is getting put together first, then I'm going to lift it up and position the legs underneath it.Up in the air:The arches are only raised 10" up now:The pattern (sort of) visible:I've cut the height of the whole structure down by almost half from my original idea -- which was to use the proportions of the 'classic' Gothic arch.  Now it's going to be 16' high.  But it's still, well, pretty big...Judging by how the thing's behaving so far, I think I'm going to need aircraft cable in tension spanning the parallel arches.Jack OlsenMy garage website
Reply:Looks great. Time to rent a scissor lift.
Reply:Looking good. I was thinking a set of bakers scaffold. The scissors lift might be helpful in raising the whole unit up however..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:I was gonna say get a roll around scaffold. Looks nice. Are you gonna have any open uncovered area left ???     Yea, I know about working alone too. My wife is disabled so that sucks . Physically that is; ok maybe a little mental. From being married to me.
Reply:I don't think Jack can get a Scissors lift back there to that patio.How are you going to raise the structure?Ed Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridgeOriginally Posted by Broccoli1I don't think Jack can get a Scissors lift back there to that patio.
Reply:Originally Posted by DSWSure he can... He just needs a big enough crane to go over the house. Some of the small ones for interior use will fit thru most 36" doors. Steps and super tight turns would be an issue however.
Reply:does the shape  allow it to go around that awning? or is the design change to keep the shade structure below the awning?There are no problems. There are only solutions. It's your duty to determine the right one.Hobart Handler 210Airco 225 Amp MSM Stinger
Reply:I'm still working out the lifting part.  This afternoon, I'm hoping to get some primer on it (and maybe paint).  Ed, I don't THINK it requires a permit, since the platform is less than a foot above grade, and pergolas (un-covered structures not made out of flammable materials like this) are specifically exempted.  But then, my research on local code is admittedly optimistic.  I hope my neighbors are charitable with me.ThorsHammer, the awning is coming down.  That was always part of the plan.Jack OlsenMy garage website
Reply:Today was a LOT of work.  But if I get some time this weekend, I'll find out if it works.Jack OlsenMy garage website
Reply:I like the way you think. Looks like something I'd attempt.  Best of luck tomorrow. Be safe.
Reply:Hey, don't you need a strobe light on top ???    Get the wife out there too.
Reply:Today I wore my hardhat...Because while the kids napped, the pergola roof went up, up, up.I wanted a way to distribute the load -- so it wouldn't only be pulled up from the center -- so I turned the idea upside down, and ran the cable through an old piece of steel, and welded two arms out on the sides to push up in concert with the central line.  Having the cable run through the tubing made it surprisingly stable, considering that I was essentially pushing the load up from below.Jack OlsenMy garage website
Reply:It looks like a big job. Was it harder than expected?. Looks great so far. I won't let my missus see this one for sure.
Reply:The first surprise came when it fell.  It was only a couple of inches up, but the three lag bolts holding the winch to the boom/mast tore out of the wood.  It was a scary, loud, and very visceral reminder that there were several links in the 'chain' of what I'd put together that could individually fail and lead to a collapse of the whole thing down onto the idiot below.  The big problem with the crane I'd come up with was that there was no redundancy.  If the point where the chain was fixed below the steel lifting assembly failed, the whole thing would come down.  If the mounts for pulley up on top failed, the whole thing would come down.  If the central crossmember that was taking most of the load for the ladder-shaped component failed, the whole thing would come down.  You get the idea.So good sense prevailed at that point and I added the two vertical pieces on either end of the ladder/cradle thing.  At that point, I would raise it a foot and then re-fasten the pair of 2x4s on either end so that if the crane gave out, it would only come down that short distance.  Stopping every 12" also let me readjust the fore-aft and left-right balance of the thing so it stayed level and wasn't encouraging the boom to lean in any one direction.'Good sense' might not be the way to describe it, really.  Maybe 'less stupidity' is more on the mark.  But it didn't come down.  And I knew that if it did there wasn't much danger of it injuring me, sine the spider-like shape of the whole thing put the impact points along the perimeter of the deck.  If the crane failed, I knew the steel would do some damage to the fence and walls.  But that's better than doing damage to the builder.  (Then again, if the mounts for the pulley came out, then a pretty heavy piece of steel would have come down toward me.  But my eyes rarely left that part of the crane.)OSHA would tear me  new one for doing it the way I did.  And I'd encourage anyone reading this to not try it the same way.Last edited by Jack Olsen; 04-07-2013 at 11:41 AM.Jack OlsenMy garage website
Reply:Too bad some of us weren't closer... I'd drop almost anything to go hang out at Jack's place for an afternoon. It seems many of us are the "I'll do it myself" type, me included. Your projects, as always, serve to inspire me to bigger builds. Just know that if it wasn't 1500 miles one way, I'd have been there."The beauty of the Second Amendment is that it will not be needed until they try to take it." - Thomas Jefferson If the Lights are a Flashin', STAY OFF the tracks!!! It might be me at the Throttle...
Reply:You gotta play safe. If danger is possible GET WIFE !  I like the BLUE MILLER matching chairs too. All this work and the wife is gonna tell you she's selling !
Reply:Jack, What a killer project, the stuff you have done is awsome! The porsche is the icing on the cake!
Reply:Today I got the last of the basic welding/grinding done, so I was able to take off the Masonite cover and hose the deck itself down.  There is still a LONG way to go before it's done, but it was nice to see it cleaned up.  I won't have the rest of the brown paint until Thursday, so there's a lot of primer still.  Jack OlsenMy garage website
Reply:Cool ! Very impressive.  No one injured, no deck damage, and no fire . You can't beat that. Looking forward to the finish.    Hey, if your writin job gets slow  you could go into production on these.
Reply:looks awesome!
Reply:Way to go Jack. It really looks good.
Reply:I just noticed you said "kids"When did the other one come in?