Thermal Cutting in the Field

[JenB]

I have a structural steel project where the contractor is requesting to do some thermal cutting in the field, specifically to make the slot in HSS braces longer and/or wider. Our specs of course say that thermal cutting in the field is not allowed, but due to the quantities of steel and difficult nature of this particular project, they are asking if they can enlarge/ lengthen the slots in the HSS braces with thermal cutting to make erection easier because grinding "is taking too long." My biggest concern about the thermal cutting is the increased risk of stress concentrations, especially at the end of the slot beyond the gusset plate where there is no weld or gusset plate to reinforce the HSS brace. Has anyone allowed thermal cutting of major structural steel members in the field before, and if so, what additional constraints did you put on the practice for the contractor to follow and/or for the inspector to look for/ test? Thanks for any input that anyone may have to share.

 

[TehMightyEngineer]

My first concern is field cutting with a torch is going to leave a very rough edge that will most likely not be straight. Thus, they'll likely have to grind this smooth anyway and now you'll be left with an oversized and uneven cut. Is this acceptable?

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Lomarandil]

We have contractors asking to flame cut bolt holes pretty regularly (in our case, for temporary construction structures).

Some contractors do a pretty good job of it -- tough to distinguish from a shop hole. I don't know all the standards in play here, but certainly if they were able to produce a hole that met the AISC standard practice requirements, not an issue.

However, I've also seen many field and flame cut holes that don't even resemble round. Definite points of stress concentration rather than a smooth surface for the bolt to bear on. We've evaluated those on a case-by-case basis, taking into account connection demand and the magnitude of the irregularity. However, in a permanent design application, I'd suspect you would want to be more cautious.

Sorry I can't be more helpful here. In short, I wouldn't take it off the table, but you're right to put some extra thought into it.

 

[boo1]

drill them

 

[Teguci]

If this is for an overhead crane runway with lots of high stress reversals, then I'd insist that the steel be sent back to the shop to be corrected. If this is a 3-story brace for wind loads, then I have a tough time seeing a crack propagate at the end of the cut zone to the point where it would cause failure of the member. Maybe out of plane bending would cause some stress concentrations at the end of the cut, but a thermal cut isn't likely to cause a sharp stress riser. Worst case, tell them to drill where the end of the cut will be and Mag Particle test the area after they are done cutting.

The HSS is probably sized for compression buckling at the mid point and probably has lots of extra beef at the connection zones. It will probably look ugly, but if that doesn't matter, don't be unreasonable.