Zen and Welding

[Koz]

Hi all,

So I was wondering, how do we assure the quality of a special process when no on is looking? It's like the question, if a tree falls in the forest does it still make a sound if there is no one there to listen?

We've been having an on-going discussion concerning welding on a concrete thread. Out of respect to the rock-hounds, we are moving this discussion over here. If you feel like you are picking this up in midstream, don't worry, you are. But like all philosophy, it doesn't matter where you jump in or really what your level of comment is. Kirkegaard or Alfred E. Neuman, it won't matter.

So we kick it off by saying should engineers make do or get by because of job site conditions or locations.

We had a question concerning welding rebar. I maintained there are all sorts of "weldable" rebar if proper practices are followed. Others joined in that rebar wasn't weldable because you can't control the process.

Phewy.

To CWIC: Just the facts, Man.
1) Regardless of whether or not a jurisdiction implements UBC or IBC, the owner has the right to impose inspection. The design codes, like all others, are minimum standards. Likewise, buliding officials can impose restrictions on substandard practices. If we choose to blink so we can support the tax-cuts that cut our regulatory budgets that is a moral question. I think we need to do some soul searching here. Ah, philosophy.

2)AWS makes it the responsibility of the inspector to judge the ability of a welder. No one else in the design,build, assure team has that responsibility. If the inspector has any reason to doubt the capability of the welder, he can require retesting of that welder. If they're a bad welder, they will probably fail the test. Again, do we blink so we can milk the job for a few more weeks.

I have shut down shoddy inspection agencies in my time, because they were not performing RT in conformance to Code and wouldn't get into to line. They blustered just like an old welding foreman and tried to stone-wall me. They don't do much work in our area anymore. It isn't just welding that is out of control.

I know I haven't addressed all of the facts, but I have patience. As the I Ching says, "Perseverence Furthers".
Koz

 

[LPPE]

My $0.02 - I recently had a repair job where initially rebar was welded to stainless plates and other rebar. Needless to say, every weld cracked. Sure, if done properly, you can weld rebar. 99.9% of the time it is not done properly. Therefore, never weld rebar. Done and done.

 

[lapstruct]

what about when the electrical guy specifies a CAD weld to the rebar in a footing. From what I can tell, this is done regularly. However, I have seen research that this process affects the composition of the rebar and thus affects the strength. I usually just add another bar in the fooring where they are going to weld their ground wires. Is this what others are doing? What is another way to attach the grounding cables to the rebar if welding is a non-no?

 

[Focht3]

There are LOTS of other options; clamps can work well. Talk to the project MEP and see what s/he knows -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[Koz]

Hi all,

Glad to see you in the pool. Special processes include all operations where the quality of the product cannot be verified without destroying the product during evaluation. Special processes include welding, nondestructive testing, heat-treating, and plating. As such, these processes must be controlled by following tested and proven procedures using technicians qualified by testing and certified to perform the procedure. This is strictly enforced for welding and nondestructive testing. Technician qualification is not usually required for heat-treaters and platers. Codes have been developed to standardize tests and qualifications to demonstrate the process capabilities. We must follow the Codes, as a minimum, to ensure the quality of our efforts. When we do not follow these minimum requirements we are in a danger zone that most of us don't like to tread. The majority of sins will fall back to not follow the basic tenets of our craft.

Notes to Pylko. You have a material compatibility problem with the SS material and the reinforcing steel. This is not a standard reinforcing steel connection as recognized by the common welding Code AWS D1.4. Others have recommended using AWS B2.1 to qualify this kind of connection and I agree with them. You can't, however, extrapolate out from this unique situation to the welding of all reinforcing steel. The problem arises from not properly controlling the special process, not from the process or material itself.

The unique welding variables were not given, but I can imagine several welding scenarios where I would guarantee these welds would crack long before the first arc was ever struck. If you follow Code required parameters, however, your welds will be mechanically sound.

The stainless to reinforcing steel weld can safely be made, but you need to do more research regarding the appropriate filler metal to use and you may have a preheat requirement for the reinforcing steel that may not be compatible with the choice of filler metal or stainless base metal. This joint can be made, however. It just takes more research regarding the metallurgy and then test the results. There are no welding Codes that permit welding of reinforcing steel without mechanical testing of test weldments. The one exception is the "prequalified" joint of rebar to structural steel fillet welds. This prequalification does not include flare-bevel grooves where the reinforcing steel is laid on its side on a plate or flange. You have to test all special process procedures.

To Lapstruct
The reason why these unauthorized CAD welds are so bad is they are unauthorized and unaccounted for in our designs. The welds are not performed following a tested and proven welding procedure that can be reviewed by Engineering. They may be causing mechanical damage as you suspect, but they may not. A706 reinforcing steel will be more forgiving than any other type. Are the welders welding these attachments by arc welding or are they using capacitor discharge equipment? They might get away with capacitor discharge because it is very localized, if any, damage depending on the kind of rebar and given the overall mass of the rebar. An engineering evaluation could be made if we knew more about the welding procedure. The more reliable and easier way to go is the way Focht3 recommends by using mechanical connectors and not permitting unauthorized welds.
Fun!
Koz

 

[Ron]

At the risk of being on the Alfred E. Newman end of the scale, rebar can be welded and the results acceptable if proper procedures are followed such as AWS. Think about it....if you can lap two rebar side-by-side and tie them with a wire, it would logically follow that a physical connection (mechanical or metallurgical as in welding)could at least achieve the same objective.

 

[Koz]

Hi Ron
Not a metallurgical connection. Mechanical, yes. And it isn't a special process because we can count the wraps of tie wire, measure the gauge of the the tie wire, and calculate the actual strength of the connection without having to destroy the connection.

Why concrete guys choose tie wires over welded joints (like indirect butt joints)or vice versa, I don't know. I just know welding of any of the many reinforcing steels available takes special consideration many concrete people aren't taking. If these folks were taking the proper considerations, we wouldn't be talking about cracking stainless to rebar welds or unweldable rebar here. Special processes require special controls i.e. tested procedures run by qualified technicians.
Fun!
Koz

 

[LPPE]

I think the point is in constructability and labor saving costs. Its labor intensive and time consuming to properly weld rebar, therefore resulting in higher costs to the owner. It seems only logical to me as a responsible designer to provide a cheap and easily manufactured solution to any design problem. Overall, forget welding rebar. There are many cheaper alternatives that will provide on the whole much more satisfactory results.

 

[jheidt2543]

The COST/hr for putting a union ironworker in the field here in the midwest US has been over $40/hr for some time and is bumping $50/hr in many areas. How many welds/hr. can be made vs. the other types of connections? How much do the mechcanical connectors cost installed vs. welding the same connection?

 

[Koz]

Hi folks,

Thank you for presenting the biggest difficulty in welding reinforcing steel, the cost! I've often wondered about this. With the exception of A706, the other bars generally require some kind of preheat, considering diameter and carbon equivalent. Preheat equipment can cost less than $75, but it still takes time. And tacks require preheat, too. And it is a lot easier to verify the quality of a mechanical connection.

But one of our local premier design firms really has a prejudice for welded over mechanical connections. And I got a call just a few weeks ago to develop a welding procedure to weld crossing rebars. It's not allowed by Code! I suppose there isn't a mechanical connector for this configuration. I'm curious about the cost differential. I'm going to ask some questions. Thanks for the thoughts.
Koz

 

[Koz]

Hi Folks,

I looked at mechanical connectors for rebar today. I talked with a field erector, too.

We do a lot of seismic construction out here. I looked at a connection this erector has for a new building for a local University. This school is about 1/4 mile from a fault that is considered scarier than those in San Francisco. The designers out here have a new toy called an "Unbonded Brace" or an "Unrestrained Buckling Brace". A rose by any name... but it is like a seismic shock absorber. The steel stretches and compresses during a seismic event, dampening the accelerations, leaving the moment frames alone. The testing shows these things are pretty bomb-proof, figuratively speaking. However, they need huge embedments to react against, like 30-ton of rebar and concrete at the base of each connection. Fun! So the designer is using mechanical connectors at the horizontal reinforcing steel in the footing to embed connection. The reinforcing steel is shop-welded to the embedment.

The reinforcing steel is A706 so it is easy to weld in both shop and field. No preheat is needed until we get to about 32 deg F. But as was noted here, it costs about $50 per hour to put a union tradesman on the job. My field guys confirmed the same numbers out here in the West. The mechanical connector replaces the welded direct butt joint, which demands the most welder skill of all of the common reinforcing steel joints. And the joint can be made up by unskilled, but trained "rod-busters".

I haven't found out the cost of the connectors yet, but this group of designers loves to weld. Connectors look like a real smart, economical move. I don't know if there are mechanical connectors for crossed bars or lap joints.

I'm very glad we're finally talking about the reinforcing steel connection in light of the cost to make the joint rather than some prejudice that reinforcing steel can't be welded (irrational) or that it can be welded in the shop and not in the field (operator or management issue, not material or process).
Take Care and enjoy the Wonderful World of Welding!
Koz

 

[Womprat]

Koz
Check with your local municipality or Department of Transportation, rebar connectors that are on some kind of qualified products or approved products lists have been tested or at least evaluated. Some of these products could be installed by untrained anybody.
Womprat

 

[Koz]

Hi Womprat,

I believe these connectors have been tested and are very effective time savers. They are good things. And I think that the reason designers are using these mechanical connectors is because they can presumably be installed by untrained anybodies. It costs approx $50/hr, fully burdened, to get an Ironworker on site. An untrained rod-buster out in the field, however, costs around $30 and they learn-while-they-earn on your project.

The quality of the mechanical connection can be evaluated by measurement. Unlike welding or nondestructive testing, bolting is not a "special process". Someone can physically determine the cleanliness of the joint before the connector is assembled. Someone can check the tension applied to the connector fasteners with a calibrated torque wrench or proof testing the assembly equipment at the start of shift. Now whether or not these checks are an actual requirement, I have no idea, but they are effective checks. And whether or not a "connection procedure" is imposed (similar to the bolting procedures required by AISC for shop and field installed structural fasteners) for these hybrid structural steel-concrete connections I don't know. I'd ask for one in the document submittal at the start of the job.

I'm a nut on training and procedures. I won't let the quality of my work depend upon the next guy a contractor happens to pick-up off the street. And that is what happens when we don't train. This training including a brief course outline and the hours attended should be documented for outside review.

Bolting procedures are now routine in quality structural steel. And if you're in a portion of the industry that isn't requesting these procedures, you should start. Documentation is pretty routine these days and it shouldn't cost you anything as far as your supplier's price is concerned. There has been a lot of shoddy bolting in the past. By shoddy, I mean, rusty fasteners, TC bolts without the correct factory applied lubricant, washers on the wrong side, improper tensioning because of un or ill-calibrated equipment, max inside-diameter nuts assembled to min outside-diameter bolts, no quality verification. IBC requires special inspection of slip-critical and pretensioned bolts, but not snug-tight. I don't think the Code imposes this requirement on mechanical concrete connections. It wouldn't be out of line to request this documentation, however.

So I'm with you, Womprat. If we let these anybodies do the work, we pick-up more responsibility to make sure their work is done correctly. I think you're right on the money.
Koz

 

[SweetNlow]

The strength of two bars next to each other is not a factor of tie wires. They snap apart if you wiggle them 5 times. The strength comes from the bonding of the concrete to each run of rebar, a "lap length".

Welding weakens steel by making it brittle, therefore its main function to give ductility to concrete is gone. Preheating with skill reduces that problem. Skill: not required to the same degree with CADWELDS. The system is used to save space when too much steel would croud the forms in a column.

 

[Koz]

Hi SweetNlo

Thanks for catching my brain freeze on the wire. I did a "Doh" with appropriate slap on the head as soon as I hit the submit button. I applaud Alfred E. Neuman because it is a "What Me Worry" kind of world. The marriage of steel and concrete is a good one.

The embrittlement problem is not unique to reinforcing steel. A rapid quench creates a martensitic grain structure that, while very strong, has low ductility. Embrittlement is a misnomer. I have a customer who is welding common A992 wide-flange shapes. But these shapes have flanges 5.5" thick. The rapid quench of these "Group 5" shapes requires preheat around 350 deg F. They form structural truss chords and are being welded in the field so the skill level required to preheat is common. Propane weedburners are the tool of choice. The fieldhands are using digital thermometers to monitor the operation, also common to their craft. These same craftsmen hold D1.4 certification for welding reinforcing steel. I suspect the preheat knowledge is uncommon to the supervisory personnel of concrete contractors (personal prejudice).

We also preheat reinforcing steel to permit hydrogen from the weld filler metal or the atmosphere that gets trapped in the weld metal opportunity to out-gas. This, again, is true for virtually any steel that is welded.

Reinforcing steel has a sufficiently high Carbon Equivalent (CE) to require the preheat. Except that A706 has been alloyed so that it does not require the preheat needed for other reinforcing steels. It has a specified CE of .55 or less and AWS D1.4 does not require the preheat for this level of CE. A706, the "weldable" rebar, is very forgiving and readily weldable by qualified welders and others who can "read the bead".

So welding does not weaken the steel. A misapplied welding procedure will reduce the mechanical properties of the steel. Shoddy welding technique by an untrained welder will introduce discontinuities into the weld reducing its strength. Both of these conditions are excursions from good practice and can be fixed by training. Special Process stuff that is not intrinsic to the material or form.

But what are the Cadwelds? Are these mechanical connectors or welds? I don't know the terminology and I'm interested.
Thanks
Koz

 

[Koz]

Hi SweetNlow,

I hit the submit button to fast, again.

The A992 truss chord will form a martensitic grain structure with a rapid quench, too. It is a little different from the martensite of the reinforcing steel because it has less carbon. It is an academic difference.

And the A706 up to #6 bars require no preheat, #7 to #11 must be no less than 50 deg F. for welding to meet Code, and #14 to #18 require 200 deg F. These temperatures can be reduced if the CE is less than .45. And if the base metal is below 32 deg F, it has to be preheated and held to at least 70 deg F. while welding. Sorry to get so detailed, but I never know if concrete folks have ever looked at the welding Code, even though it is required by ACI and IBC.

Doh!
Koz

 

[Rich2001]

Koz, Cad wleding is an exothermic reaction type of weld.

For steel Thermit welding is utilized.

Thermit Welding
Thermit welding (TW) is a welding process which produces coalescence of metals by heating them with superheated liquid metal from a chemical reaction between a metal oxide and aluminum with or without the application of pressure.

Filler metal is obtained from an exothermic reaction between iron oxide and aluminum. The temperature resulting from this reaction is approximately 2500°C. The superheated steel is contained in a crucible located immediately above the weld joint. The superheated steel runs into a mold which is built around the parts to be welded. Since it is almost twice as hot as the melting temperature of the base metal melting occurs at the edges of the joint and alloys with the molten steel from the crucible. Normal heat losses cause the mass of molten metal to solidify, coalescence occurs, and the weld is completed.

The thermit welding process is apply only in the automatic mode. Once the reaction is started it goes to completion.

Cad Welding is simillar but it is for joining copper alloys together, i.e, grounding cables.

 

[Koz]

Hi Rich2001,

Thanks for the info on the Thermit(e) weld. I had to write a repair procedure for a local group installing light track rails for this process. It appeared to be very user-friendly and they sure welded a large joint fast. The integrity (most of the time, as evidenced by my repair procedure)was excellent. Do you (or anyone) know anything about the grain structure of the base material after the Thermit weld? My prejudice is that it would be coarse-grained like an electro-slag weld. I know it is not electro-slag. And is this coarse-grained (maybe?) welding process suitable for reinforcing steel which needs ductility to perform after welding?

I also got to thinking about preheat skills and requirments after I logged off last night.

To SweetNlo: While "weed-burners" are relatively inexpensive, they are not part of the welders personal toolkit. Preheating equipment is furnished by the contractor. And welders will not know what the proper preheat should be. That information comes out of Engineering. That information is buried in the Codes we use, but welders aren't responsible for interpreting Codes. Ergo, welders may have rules-of-thumb they can use for preheating but that is art not engineering. And we aren't following a proven procedure when we depend upon the art.
Koz