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Re: CJP Weld in a Moment Frame

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On Feb 26, 2010, at 11:32 AM, Mark Johnson wrote:

Thanks for the reply. I think what you gave me is a way to calculate the force in each flange if it were welded with fillets. However, I have to take issue with several things: (1) I'm starting with a CJP (complete joint penetration) weld at both flanges, (2) it matters what the beam is made of (A36 or A992 or other), i.e. Fy needs to be in your formula, (3) it matters what plate material your attaching to (A36 or other) i.e. Fy of the plate needs to be in your formula.
Ordinary practice is that the base metal and the weld metal be 'matched' in strength. Typically the allowable weld stress is based on the weaker of the base metal and the weld metal. Sounds like your plate is weaker than the beam so you're trying to justify a build-up of weld metal to match the strength. If so you'll need the axial force in the flange to design the weld vis the formula given and then proportion the weld size by the ratio of the weld metal strength to the filler metal strength. I can't find anything like it in Blodgett, but that formula have have come from equating the force in the flange to the force in the weld and assuming that the base metal 'matched ' the filler metal. If I were doing it, I'd certainly make sure I knew where that formula came from before I started plugging in numbers.

Just by equilibrium, the weld necessarily carries the same stress as the beam flange in a full penetration weld like I think you're describing, but if the plate allowable is less than the beam allowable you need to make allowances. Just a guess, but if you're trying to spreading the flange force over a larger area by adding a fillet, I don't think that's going happen.

My situation is that I have a W8x40 A992 beam welded to a 1.25" thick A36 plate with CJP welds at the flanges. I maintain that doing this will NOT develop the beam. (Yes, I want to 100% develop the beam). I maintain that I need to add a fillet to the CJP (at least 5/16 fillet) in order to develop the moment capacity of the beam.
Good practice is to use fillets with the kind of full penetration butt welds you're describing to reduce the potential for fatigue at the transition and make sure the necessary weld metal volume exists. It sounds like you also want some additional strength. You can figure the additional strength provided by the fillets in the usual way, again basing the weld allowable on the base metal (plate) strength rather than the strength of the filler metal.

I've never been particularly wild about just dumping in weld metal to build up a joint like this, although I admit I've been tempted a couple of times. There are a number of pitfalls including thermal distortion of the plate (and subsequent cracking when that distortion is too highly restrained) and tearing resulting from through- thickness loading on the plate. You might want to take a close look at how the plate itself might respond. Blodgett has some fairly simple approaches to this kind of connection in _Welded Structure Design_.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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