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Re: Typical Stud Size for Embedded Plate

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Have you seen/heard of Metwood (r) beams for residential construction? They're popular here near the mfr plant. They stitch weld A615 (according to the rep I asked) rebar - up to #9s - into the top and bottom flanges of 14 gauge cold formed C joists with variable flanges. Then, two assemblies are welded together to form a box beam. These are used in place of engineered lumber or hot rolled steel to support gauge steel and wood structures, and concrete elevated decks, in residential and some light commercial work.

I was suprised to find out that they used A615 rebar, given the issues with weldability, though it is a shop process. and of pretty good volume based on the frequency in which I see them in buildings.

Harold Sprague wrote:

As stated in my post, it is an issue with the variability in the carbon content (or more appropriately Carbon Equivalents (CE)) of the more commonly available A615 rebar. AWS D 1.4 is the document that dictates welding for rebar. The varying carbon Equivalents of rebar from lot to lot require that you either assume a CE in excess of 0.75 or you have to track the mill certs relative to the rebar lot and the welding procedure. A CE in excess of 0.75 requires a preheat of 500 degrees F. This will require continuous special inspection.

If you have ever watched A 615 rebar being welded PROPERLY, you would avoid welding A 615 rebar.

The way to avoid it is to require A706 rebar. But availability can be an issue especially if the lot is small. The better precasters will use A 706 rebar. The problem is that you had better get this verified by a special inspector. I would also require submittal of the mill certs. At first glance, you can not tell A 706 rebar from A 615. You have to know the mill stamps on the bar.

The simple method is to use deformed bar anchors which are manufactured using weldable steel. These bars can be fusion welded with a stud gun or you can stick weld them on. Deformed bar anchors physically look different than rebar.

Whether or not you punch the base steel and plug weld, fusion weld with a stud gun, or just fillet weld makes no difference if you are welding a low CE steel. The QC is just so much more simple. I have just personally seen way too many brittle welds from welding A615 rebar. They may look good visually, but they fracture very easily.

The only exceptions on preheat are thermit or pressure gas welding. But that opens up another whole range of issues including expense.

Whatever you decide know and monitor: qualifications, testing, and inspection. A good guide is the US Army Corps of Engineers UFC 3-320-01A written by Bob Shaw. Bob is a contributor on this forum and will correct me if I am wrong.

Harold Sprague

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