huangyhg

zen and welding
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
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.
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?
there are lots of other options; clamps can work well. talk to the project mep and see what s/he knows -
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
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.
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
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.
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?
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
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

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