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RE: WE-Electrode Strength

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>The design
>examples I've looked through state an electrode strength assumption
>then proceed without explaining why that particular electrode was used
>or is appropriate.
As a specific response the engineer should pick the strength of the 
electrode (the 70 of the 70xx designation) and generally let the 
fabricator pick the xx part, _provided_ you understand why and what 
design trade-offs are involved. It'll depend greatly on the welding 
process, but also on the welding position and equipment. The engineer's 
interest extends to such matters as ductility or moisture pick-up, which 
the xx spec covers. It's also important for the engineer to be at least 
acquainted with the workmanship issues involved and to understand why the 
fabricator wants to use a certain electrode and what the trade-offs are. 
The only way to find these things out is with a heart-to-heart with the 
fabrication people who pick these things. Lincoln Electric is an 
excellent resource in this area. They publish The Procedure Handbook of 
Arc Welding and Omer Blodgett's 'Design of Welded Structures.' These are 
both treasures; either one would be a bargain at 5 times the price.

I'd be real interested why no one's mentioned the weld design methods 
outlined in the AISC Manual of Steel Construction. Most of the capacities 
are shown for 70xx welds but appropriate reductions are shown for other 
grades if needed. Seems pretty straightforward. 

My practice (based on experience with welded pressure vessels) is to 
specify weld electrode one grade higher than the base material (70xx for 
60 ksi UTS), so full penetration butt welds properly made are stronger 
than the base metal. Fillet welds where allowed are proportioned for the 
design loads using any appropriate and conservative method, such as the 
AISC 'elastic' method or the coefficient tables for the 'ultimate 
strength method.' The 'elastic method' can be grotesquely conservative, 
but it's generally applicable to a wide range of weld configurations. 
ASME code rules for welding specify certain sizes and types (something 
like 'prequalified' joints, although in the PV biz all welding procedures 
used by each manufacturer are qualified by specific testing). Design 
stresses are based on the UTS depending on how the weld is loaded, pretty 
much like the AISC code.

Nuke plant lifting equipment, which uses AISC ASD methodology because of 
functional requirements likewise uses AISC weld analysis methods, which 
I've always found to be quite good, except for the conservatism in the 
'elastic' method.

I don't think you can overemphasize the need for good workmanship. 
Without it the welds you specify won't hold the loads you expect. Most of 
the weld problems I've been involved in have been workmanship related, 
frequently because the engineer didn't understand what his design 
demanded of the welder. It's important to talk with a fabricator and make 
sure he does what you intend and that you understand what he's doing and 
why he's doing it. I've always felt that 'Leaving it up to the 
fabricator,' means 'Do what you want, not what I expect,' and that's as 
silly than letting the fabricator switch member sizes or materials at his 
convenience.


Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw