huangyhg

torch cutting steel in the field
i'm working on a project where some beams were not coped properly for some connections and need to be modified. the contractor would obviously like to torch cut the steel in the field rather than send them back to the shop.
i have been trying to find out why it is normally not recommended to field torch steel. almost every spec i've ever seen has verbiage to this effect in it. as far as i can tell, it is more for the quality of workmanship typical of field cutting than any mechanical reason (heat effects on the steel, stresses induced from torching, etc.). is there any reason anybody is aware of that we would not want the beams torched in the field?
it is not only for aesthetic reasons but for stress related reasons and who is available to ensure the torch is used appropriately and not where stress is critical. as such, when the job site doesn't have an engineer on site all the time, it makes sense to limit these elements of latitude.
a couple of things to note.
1. steel changes properties when the color of the heated area is a glowing cherry red. obviously this is very subjective and thus a heat-stick should be used to determine the temperature of the steel and make sure it is below say 600-1200 degrees farenheiht. the properties may make the steel more brittle and subject to fracture and fatigue cracking. this is extremely important since you have to know how the beam was designed and where the maximum stress occurs.
2. once you've torched steel, you much chip and grind down any roughened edges as these become stress risers and any areas very close to the cut also usually have micro cracking which can propagate given the right circumstances.
so torching steel is a pretty serious issue that needs to be done with the right people involved.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
a cutting bed in a fabricator's shop will use the same principle as a torch to cut the steel, heat.
in fact if you send the piece back to the fabricator, most likely he will use a torch.
you do not mention if the piece has to fit anywhere, if that is the case, the distortion from the heat could affect the fit, but then there is also distortion cutting on the shop floor and welding.
in my oppinion, the issues on site are more to do with quality of the final product. personally i have no problem with cutting steel on site as long as there is a good guy doing it and the edges are ground smooth afterwards. if you have any doubts about the person doing the cutting, ask for a test on a spare piece of steel. that will give you and indication of how good or bad he is. if it looks good, it should be good.
been there; done that.
as one of my colleague used to say: "ironworkers with torches make me nervous".
i agree with qshake and kelowna; you need the right people -engineer/inspector and ironworker.
with a36 steel, torch cutting in the field is not as critical as with higher carbon content steels that are more prone to brittle fracture. they require preheating and tight temperature controls to limit stresses due to the cutting procedure.
grade 60 resteel is the same - no bending or cutting in the field.
mike mccann
mccann engineering
msquared48 -
notably sound advice with one minor detail....it is extremely difficult to maintain pre-heat in the field. especially when the contractor doesn't understand the benefits of a simple wind blind. i've been there and done that. very aggravating.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
does anyone know of any formalized "standards" or "procedures" for torch cutting, either in the field, or in the shop?
if you have fatigue issues, there are some requirements in appendix 3.5 of the 13th aisc manual.
if you "heard" it on the internet, it's guilty until proven innocent. - dcs
aashto/nsba steel bridge guide specification for fabrication
i didn't see anything on cutting but they do discuss pre-heat.
i assume we are talking about an oxy-acetylene torch here rather than arc-air gouging. in this case it would be impossible to cut without preheating.
as for maintaining preheat often the opposite case is true that the heat can exceed the maximum interpass temperature due to the preheat and the welding/cutting operation. here you're cutting with a oxy-acetylene torch so of course its hot - a skilled person is needed to cut neatly with the correct gas ratio and not to overheat the steel beyond the reasonable heat affected zone of the cut.
a-36 steel was mentioned above. i thought all the steel shapes today were a-50 because of all the alloys in the recyled steel being used to make the new shapes.

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