Damaged & Rusted Steel Trusses Designed to Span 60'. Repair Or Re-Fabricate?

"Vernon" wrote in message news: snipped-for-privacy@o13g2000cwo.googlegroups.com... | Here's a serious welding topic if ever there was one. | | Some time ago, I bought out a pile of salvaged steel trusses. Each set | consists of four pieces. The four pieces, bolted together and erected, | make a free span of 60' outside to outside. | | They are designed to go on 25 foot centers, connected by perlings, and | covered by sheet metal. | | Not only were they damaged when they came down but some of also have | some significant rust deterioration. | | What's of interest to me is just how little material went into making | them. >>SNIP

No, but I imagine they're the most common flavor of mild steel.

VT

Your point is well taken. If I can't find any marks (doubtful) I guess I could do a spark test. In any event, since they're welded, I'll persist in hoping they're mild steel.

How about if I heat an quench a piece of one of 'em. If the piece is still easily cut by a file then I can safely assume it's mild steel.

Yes?

VT

"Vernon" wrote: (clip) How about if I heat an quench a piece of one of 'em.(clip) ^^^^^^^^^^^^^^^ When you cut a piece, that will tell you a lot. If a hack saw or band saw handles it easily, it is not likely to be anything special. Next, easier than heating and quenching, why not just do a weld and bend test. That goes directly to the question.

Mr. Lichtman, When you say "just do a weld and bend test. That goes directly to the question" is the idea that if it doesn't crack you're good to go?

And are you say> "Vernon" wrote: (clip) How about if I heat an quench a piece of one of

Any low hydrogen process should do the trick. The material even is a low ally is too thin to require preheat. Make sure you fill all craters at the end of your beads. I would be inclined to use stick because of the size. Dragging a wire feeder back and forth for small welds can take more time than the weld. If there is significant loss of material you could reduce spacing to say

20 feet. Erecting trusses can be tricky in that people tend to set them up without installing enough cross bracing. The result is that they topple like dominos during construction. That's going to be a big building! In most cases trusses brace themselves so distortion is not a problem. If you are welding them while laying on the flat it is smart to turn them over three or more times alternating locations of welds. In some cases people stand them up on horses and then weld alternate sides. Slight bowing is taken out by the cross bracing at erection. Randy

I gather that the requirement to fill the crater is to prevent cracks from beginning at that point. Correct? David Todtman

"Vernon" wrote: When you say "just do a weld and bend test. That goes directly to the question" is the idea that if it doesn't crack you're good to go? And are you saying to weld something to one of the trusses and bend it? Or cut a strip of steel from one of them for testing purposes? ^^^^^^^^^^^^^^^^^ A traditional weld test is to make a weld and bend it so the filler metal is in tension. If it develops a crack you know the weld was not good. In your case, the idea is to duplicate the type of weld you are going to be doing in restoring these trusses. If you are going to weld up a crack , then, as a test, make the weld between two samples of the original material. If you are going to fabricate a patch or brace, then make the weld between a sample from a truss, and a sample of the new steel.

My main point was that you get directly useful information by testing sample welds. If you go at it by heat treating, you will still be left with questions about welding.

I'm certain;y no expert on this subject, but the one thing that stands out in my mind is, WHY are they all bent? Is it because they were abused then the they were taken down, or because they were too weak for thier intended purpose? I would make sure I knew he answer to this before I put them up somewhere...

Someone might have bent them to keep them from getting used for their original purpose. Could be because of the rust, fatigue, or simply because they dropped to the ground for removal instead of wasting time by gently lowering them down. If you have a job to remove a bridge and it doesn't involve salvaging anything, are you going to care what happens to the bridge while you remove it?

Randy to the rescue!

Thanks!

As to at least one of the questions posted after yours:

I presume they got bent by less than delicate dismantling.

Your points about alternating the welds are well taken.

However, tell me this, O Wise One...

Is it a viable plan to skip weld 'em until they're effectively continuously welded? I hope to know more about the approach after more carefully studying the original ones.

I got a quote today. Even with the high cost of steel it looks like the material won't cost THAT much. Of course I haven't figured out exactly how much what I'm gonna need. So there's still a possibility for sticker shock.

They really are masterpieces of maximum strength for minimum material.

I'm sure I'll have more questions later. But meanwhile, let's have a beer.

Vern> Any low hydrogen process should do the trick. The material even is a low

saying.

these.

appreciated.

Skip welding is a safe bet. When in doubt it is a safe tactic. I am guessing that you will use some of the worst trusses to salvage parts and replace damaged sections on the better ones? You coud torch out a bad section and replace it. A standard butt joint with some doublers scabbed over the welded butt would be safe. That Texas beer went down smooth. I hope you brought another 24 for yourself :')) Someone asked for confirmation about craters. Yes, they are future crack sites especialy in fatigue situations. Randy

Dude!

You're reading my mind!

But it is a really short book! ;o)...

I was just thinking about how to best salvage what's usable so as to not bear the expense of re-inventing the wheel.

Pretty soon I will take some pictures and post 'em on my web page.

But meanwhile, I'm wonderin' about sandblasting and repainting 'em and then reinforcing 'em as needed. In fact, if they're good enough to go in the air once straightened, I wondered about re-inforcing 'em after they're already erected... Is that a dumb idea?

If the flanges and diagonal braces clean up good I could weld reinforcing on 'em and then cut and add some additional 1/8" webbing and weld it in...

Given my penchant for overkill (Why kill it daid when you can kill it daid daid DAID!?) they'd probably weigh more than the Battle Ship Arizona by the time I'm ready to go up widdem.

Regards, Vernon

Randy,

I just noticed where you said "Any low hydrogen process should do the trick".

Assuming that the trusses are made of plain vanilla soft steel. Is there a down side to using only 7018?

I ask this because notwithstanding that it's hard to re-start I weld a prettier stringer bead with it than any other rod I've tried.

Vernon

7018 is the mainstay among local millwrights and mechanics. The primary reason is that if there is doubt about the material the lo-hi will take care of it and if it is not required the stuff will still work just fine anyhow. As a side note: Most people don't realize that wire feed welding is considered a low hydrogen process. Randy

Randy: when you say wire feed welding is a low hydrogen process do you mean flux-core or do you also mean mig with solid wire such as a S70 series wire with a mixed gas? It has always been my belief that welding with a solid wire tends to make a slightly harder weld bead as opposed to welding with a flux-cored wire!. I was told that the weld tends to cool faster and therefore slightly more brittle with a solid wire than with a flux core wire that leaves a slag deposit that allows the weld to cool a little slower making a more ductile weld bead.

However upon further reflection on your post; as long as you don't have water in your liner or are welding on damp steel it would be difficult to introduce hydrogen into the weld using wire feed.

I have read a few of the posts on what you are proposing. I am not clear on what the metallurgy of the trusses are. If they are mild steel you may get by with welding on them. If they are not and are something somewhat hard you may get brittle welds and the part will fail. This is caused by carbides being suspended within the metal. You can see this relationship in a Carbon Phase Diagram. This can only be controlled by controlling the cooling of the metal and the weld. These can be found in any engineering book under the time/ transformation tables. If the temperature of the weld is not controlled then it cannot reform into martensite or ferrite and you may get cementite or nasty things like that within the metal. This may not only be at the weld but may also travel into the HAZ (heat affected zone) and cause stresses to form between the parent material and "patch material".

The problem with using trusses that are already bent is the fact that they are already bent. This means at one point the material was stressed into the plastic region and thus had exceeded its modulus of elasticity (the thing that allows a material to carry a load without permanent deformation). If you look at a stress strain curve for mild steel you will see that up to the proportional limit (this is where the material leaves the elastic range and enters the plastic range) there is a linear relationship between stress and strain. If the material stays within the proportional limit it will simply return to its original shape after the load is removed. If the load is not removed and the proportional limit is exceeded then you will continue to strain the material with very little load applied. Mild steel is one of a few materials that will work harden within the plastic range for a small bit and then it will fail. So the material is already compromised and then you are going to attempt to weld on it which may only increase the odds of a failure.

If you do not have Machinery's Handbook buy or borrow one. Look at your truss and then calculate the moment of inertia. Find a solid homogenous material that has the same moment of inertia in the same plane and see what happens when you apply a load. You of course have to know what the weight of the roof will be when finished and also take into affect any snow or rain loads. You will also need to find out what type of load the wind applies to the roof. These are "distributed loads" and not point loads. You will get a figure in pounds per square foot. Find out how big the roof is and calculate your load. Add this to the other factors such as wind, snow, and rain. See what these loads do to the beam and then incorporate a safety factor. Then find out with your safety factor applied where you can put the supports to safely support the total load. Get a freeware program like Beam Boy etc to save you some time. If you need anything else drop me an e-mail.

-Steve

Steve,

Your post was beyond "informative". It was encyclopedic! Thank you!

And thanks to those who continue to keep this thread alive, including you with whom I've had a "stress-strain" relationship.

Tomorrow I'm off to Monterrey, Mexico to interpret depositions in a commercial lawsuit. I hope to continue to kick this tarbaby while there, as I have a new notebook computer, an IBM Thinkpad X40 ultralight.

It has "WIFI". The only thing is "Can I get it to work?"

I do indeed intend to determine if the trusses are dead soft mild steel. If not, I will definitely not attempt to repair 'em.

If I repair 'em or use 'em as jigs/patterns to build new ones I will probably TIG weld 'em. I have a Syncrowave 500 welder. That should provide perfectly fine welds with a minimum HAZ.

Regards, Vernon

snipped-for-privacy@aol.com wrote:

something

homogenous

support