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Welding around Bending Area

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            Wontae,

                        Chris gave good advice.

If you are welding rebar, AWS D1.4 currently, I think, and the PCI Handbook used to tell you to stay two bar diameters or 2” away from the bend. I’ve seen “weldable metallurgy” bars as large as #7 that were welded too close to the bend snap like a dry twig.

                        I once designed a fitting. It was a bent plate which was welded to a perpendicular plate on the edge, then the assembly was galvanized. One failed, during construction, on a cold, windy, February night. Where were the bodies? I was lucky, because no one was under it when it fell. I redesigned the fitting for the rest that were yet to be installed. There were two mitigating factors. The fabricator had bent the plate to a smaller radius than I had called out on the detail. We had a metallurgist look at it, and he said that while it met A 36, there were many more inclusions than normal.

                        We also use a long piece of square stock that has two sharp bends and one not-quite-so-sharp bend. We ordered these one time as galvanized by mistake. Striking one anywhere would often make it snap at one of the sharp bends, which were close to the end but could be a foot or two away from the impact. We tried but could not get them to break when embedded in concrete. So I surmise that shock waves from impact traveled along the square stock and reflected at the end. Whatever the frequencies involved, an incident and a reflected wave added at the coldworked bend, causing the steel to shatter like glass.  I think the stock must have acted like a wave guide in air. Concrete encasing the piece must have had sound transmission qualities that were enough closer to those of the steel that some of the shock energy was dampened by absorption by the concrete. The shock wave was dissipated by the time it got to the sharp bend. The ungalvanized pieces never snap. Galvanizing introduces the possibility of hydrogen embrittlement, but it can be similar to welding in that it can heat already stressed steel to the point of failure.

                        I’ve also seen rebar that was not intended to be welded snap at a weld. I used to keep a section of #9 in the corner of my office to show people just that.

            Jim Getaz

            Precast Concrete Engineer