welding across a square hss

[marmon]

Imagine a skidder, whose frame is 2- 8x8 sq. tubes running parallel as the frame. Now outriggers are to be welded behind the rear tires, as a drill rig is now mounted on the back. The mount plates have 3 sides of the 8x8 cut out of them and are mounted from underneath perpendicular to the frame. The plates are to be welded on all three sides of the tube.

My question is shouldn't there be no weld on the bottom side of the tube?? Isn't that a no no.

Could someone also explain this principle better?
Thanks

 

[shin25]

Based on the information that you have provided, if you weld all three sides then I think your weld itself is going to look like an inverted U?

This will create eccentric loading on the weld and also will eccentrically load the tubing.

 

[marmon]

yes that is what i mean, except the U is not inverted as the plates are welded from underneath, anyways its the same thing.
Does anyone know a good example or an explanation in "laymen" terms, or even a rule of thumb that i can use to help the new welders understand, and some of the veterens as well?

 

[UcfSE]

When that weld is loaded, you could think of it as un-doing a zipper. It starts at the top and works its way down, instead of the whole weld working equally to distribute and resist the load. Once the top fails, the rest is on its way shortly thereafter.

 

[marmon]

ok thanks that works, but i got the question of well if the top weld fails then the two verticals are still there, and also that it would seem that the more weld the better I explained that it would cause a stress concentration spot after the top cracks as best i could. Also would it be wise to say to never weld perpendicular to a load? even if its not welded to a structural member? like say a cylinder ear on the side of a plate? Also i am trying to get them out of the habit of wrap-arounds. any ideas?

 

[blake989]

I believe what you are trying to describe is that if you are allowed by code to increase the capacity of the weld when it is loaded perpendicular to the weld axis....which in turn makes the "perpendicular" weld stronger than the two longitudinal welds. Intuition would lead one to believe that most of the load is then resisted by the perpendicular weld until it fails. Or at least this is what my highschool welding teacher used to say. Is it true? I don't know, but someone here will.