Thickness for Weld Base Metal Strength 1

[ILoaThePats]

Hello all,

Consider a Flat plate, with a plate Normal to it fillet welded on both sides. Assume the Normal plate is loaded in pure shear (load applied parallel to the base plate, no bending).

When checking the base metal strength of the "flat" plate, do you consider the size of the fillet weld, or just the thickness of the base metal? I always check shear failure both through the thickness of the plate (using "t" in the calculation) and at the leg of the fillet weld (using "w" in the calculations). It seems to me that the flat plate base metal could fail in shear at either interface, but I've seen many people just check it using the thickness of the plate "t", and not the weld size "w".

Similarly, consider the example above but with the load applied to the Normal plate at some offset above the Flat plate, such that there is now some bending. How would you approach checking the base metal of the Flat plate? It seems that some interaction equation is needed because the shear load is resisted by 0.6*Fy, but the normal stresses due to bending are resisted by 1.0*Fy, but I've never seen it done this way.

Thanks,

John

 

[WillisV]

http://www.modernsteel.com/Uploads/Issues/January_2012/012012_si.pdf

See fillet weld design question/answer p2 of above.

 

[ILoaThePats]

Thanks for the source WillisV. I still think it's a matter of typical practice where the weld material is matched to the base metal, precluding failure in the fusion zone. If you could weld steel plate to a 'sponge' it might fail in the fusion zone.

Can I ask how you were able to locate the modern steel article so quickly?

 

[dhengr]

ILoaThePats:
Of course, your weld analysis calcs. are usually based on a weld material at least matching the base metal, so that it is not the weak link. And, most of the codes assume this as the norm, but they don’t preclude using a stronger/weaker filler metal. You may also have two different base metal strengths, then which one do you match? The analysis of the entire weld joint becomes more complicated as you move away from weld process and filler at least matching two identical base metals. You have to do enough of these problems to start to get a feel for what the likely failure mechanism will be. You should check the base metal at its strength, over its thickness, as one failure mode. You should check the weld throat, in shear, based on the weld filler strength, and maybe a little lower, if the base metals are lower than the filler metal. You might check the weld at its fusion zone, with some adjusted strength due to the mixing of the filler metal and the base metal, when the filler and process do not at least match the base metals, but then, the effective throat would still likely be the weaker link. And, there isn’t much guidance on what that adjusted strength should be. Obviously, it won’t be lower than the lowest value of the base metal or the filler, whichever is lower.