huangyhg

welding rebar to steel?
i have a 14"x14" concrete column supporting intersecting steel beams. this needs to be a moment connection from the column, through the steel top plate, and to the steel beam.
in an effort to avoid the clutter of anchor bolts to connect the plate to the conc, i wanted to bend the rebar 90 degrees, and weld it to the steel top plate, then extend the other end as long as needed for development length.
sro:
here in seismic zone 4 - ca, tying in existing framing to new grade beam cages, etc. is very common for retrofits. rebar to hsla steel is performed daily for embeds as well.
most eor's and building depts. are now requiring procedure (pqr) qualification to confirm the compatibility of the materials. previous wps's that have been qualified typically are not permitted. procedure qualification is not an expensive undertaking although some contractors whine and complain that they are requested to do so.

if this is new construction you should specify a grade of rebar that is weldable. (i believe the normal non-weldable grade of rebar has a higher amount of carbon which makes it more brittle for welding applications)
in addition, be careful about welding a bar too close to a bend. typically it is recommended to terminate rebar welds a minimum of 2 inches or 2 bar diameters from the start of the bend.
threaded rod would give you less strength for standard a307 bolts or grade 44 ksi steel so i would use 60 ksi rebar steel. anything higher strength could cause embrittlement problems.
hope that helps!
consider using deformed bar anchors that are butt-welded to the plate with a stud gun. the quality is more reliable.
sro your comment: "i wanted to bend the rebar 90 degrees, and weld it to the steel top plate, then extend the other end as long as needed for development length". if i understand you correct, then this is a bad detail. when the load is applied, it will simply "unzipper" the welds starting from the bend. a better detail is to drill a hole through the plate and insert the bar and weld both faces of the plate. it is possible to insert the bar only part way through or say 2 x the plate thckness.
i am not entirely convinced by pxc suggestion is necessary. sure it seems to be a more positive anchorage to weld a bar to pre-drilled holes but the strength essentially comes from the weld. a long fillet weld, mostly on both sides of one leg of a "l" shape rebar should have more weld than that around the circumference of a couple of holes. one leg of the "l" in this case could be fully welded to the steel member. the weld should be strong enough allow the bar fail first before any unzipping or any detackment from the host steel
hi all,
i'm a little surprised no one has mentioned the governing welding code here, aws d1.4.
it has sections on allowable stresses, allowable reinforcing steel and allowable filler metals. for the flare-bevel joint described above, the code will only allow stresses of 30% of the nominal tensile strength of the filler metal, similar to a fillet weld in lrfd structural welds. effective weld sizes must be determined per code similar to flare bevel grooves joining sections onto pipe or tube steel.
i'm not sure of the design, but the reinforcing steel is 60 ksi ys and the structural shape is 50 ksi ys. moment frame connection are typically complete penetration welds to develop the full strength of the materials. code requires (i assume a706 "weldable" rebar) 70 ksi filler metal to match the lower strength structural steel. the code makes no distinction between a706 and several other reinforcing steels available. they are all "weldable" when following a proven welding procedure in conformance with the code. my tests have shown tensile strength to be at least 125% of yield strength is easily attained for welded a706 and a615 gr 60. but it is your money.
d1.4 is referenced and discussed in the thread noted by sro.
speaking from the projects i have involved (all outside usa) the standard site practice is to disallow welding of rebar on site unless the situation is unavoidable. welding in a fabricator yard is a totally different story because the work will be properly prepared, executed and experienced and qualified welders are available.
unless sro's design is prepared and included in the design of the structural shapes allowing the fabricator to implement the welded bars before the delivery of the structural steelwork, the other alternative would be to put the detail of the welded rebar on the reinforcement drawing leaving the contractor to carry out the welding on site.
may be sro should make it clear which route he is likely to embark on. it is just not a standard practice to ask a steel fabricator to weld steel reinforcing bars routinely on structural shapes even though there is no technical reason not to and i am sure many such cases have been done in the past.
i seldom deal with structural shapes and welding myself and wish to understand a bit more from koz's comment.
would i be correct in thinking as the code allows only 30% of tensile strength from the filler material in the welding with a rebar this effectively requires a designer to specify an equivalent of 330% of rebar cross sectional area for the weld. also the end result is that the full capacity of the rebar can be utilized in the design without any loss in the welding process, presumably obtainable from any site application as long as the proper procedure is followed and experienced and qualified welder is used (i am leaving out the problem of achieving the right facilities on site as a separate problem).
i am not sure why development length is irrelevant here. one leg of "l" bar is anchored to the structural shape and koz's remark convinces me that the anchorage can be achieved by designing to aws d1.4. the remaining leg is cast into the concrete column and surely the moment in the interconnected steel shapes cannot be transferred into the concrete column without the embedded rebar able to develop it full strength. concrete cracks out around bars only in compression and the reinforcement problem here is mainly tensile (because if needed we always can put rebar in the compression zone with the amount equal to the tension zone to beef up the combined compressive resistance with concrete to avoid a compressive failure).
finally there is a potential gap to be bridged in our understanding of joining steel structural shapes with reinforced concrete. this is because, unlike steel which behaves homogeneously in both tension and compression, concrete is an inhomogeneous material not permitted to have strength in tension (or its small tensile resistance is ignored) in the design. thus under a loaded condition the tension side of the neutral axis has no concrete but in the analysis of the structure it is acceptable to use only the full concrete section for resisting moment and all reinforcement is ignored. in other word in the frame analysis we always use the second moment of area of a rc beam not corresponding to its service condition. hence there is an error in the analysis. although many codes also allow cracked sections to be used but this is rarely done in practice because the uncracked (full) section approach works well as long as every
hi all
to bbird's question regarding weld area.
i think your numbers are right. aws d1.1 and aisc lrfd set the same limits noted in my post for fillet welds of structural to structural. you're have to have fillet welds long enough to resist the allowable load whether they're 3/16" or 3/4". and then you can only use 30% of the filler metal uts. this is standard practice. anyone who designs a ledger angle runs into this same consideration. it gets slightly more complicated for a flare-bevel groove since you have to consider the "effective" throat of the weld. look at your aws d1.4 for accurate information.
the loss of strength because of welding that you speak of is already considered in the 30% reductions and the load resistance factors of aci. and the weld is stronger than the reinforcing steel. sound direct butt joints always fail in the reinforcing steel with large amounts of necking of the bar observed. the structural steel has the least strength of the joint and it is your limiting factor.
regarding the welding of the reinforcing steel, the steel doesn't care where it is welded. shop or field, daytime or nighttime, california or katmandu (base material quality aside and don't weld when it is less than 0 deg f or in the rain, etc and read the code) it doesn't matter. follow code parameters and the weld joint will pass the required mechanical tests. done deal. if someone can show me why the steel is sensitive to the geographical location of assembly please enlighten me. otherwise, this is just noise.
code parameters include proper preheat, proper cleanliness, proper joint preparation. these parameters are generally performed by non-welders or overseen by supervisory personnel that have no understanding of welding codes or requirements, so they cut corners when no one is looking. the mundane aspects of setting the proper power source settings and having the skills to manipulate the torch properly, all controlled by the skilled welder, are only part of getting a good weld. if the welds are failing, it is because someone is cheating. it is not due to the welding processes or the base materials.
of course your comments regarding the tension side versus compression side of reinforced concrete are correct. the reinforcing steel takes the tension. and designs frequently consider the resistance of the section using the cracked moment of inertia.
but a seismic or ballistic event is not simple tension nor compression. it is acceleration more like waves. so that hard reinforcing steel bashes around in the concrete. eventually you just have a steel rod sitting in a dust filled hole. that's what i think, anyway.
what do you think?
koz
koz:
the "noise" you refer to is not based upon geographical location, only regional practices and standards. shop welds typically have better control over welding parameters, environmental conditions, favorable part positioning, etc. as i travel the planet, often there is no qc in the field (again, a regional standard).

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