Plate boundary condition

[LOKSTR]

What boundary condition ( Fixed or Simply supported) should be used for a 2.5" thick steel plate welded to W beams( spacing c/c is 2') below with 1/2" fillet weld.

The size of plate is 2'x 2' and it is uniformally loaded.

The plate is welded on 2 opposite sides and free on other 2 sides.

In reality, the boundary condition is somewhat in between fixed and SS, but we want to use fixed condition to minimize plate thickness.

2. Will the boundary condition change if we weld the above plate on all 4 side instead of 2 sides?

 

[JStephen]

I would treat it as simply supported on those sides where it was supported.
If you treat it as fixed, the maximum moment may be at the edges, and if the plate thickness is driven by bending, then the welds wouldn't be adequate.

 

[jayrod12]

if you were to weld all 4 possible edges 3 on top of the W beam and the fourth being the tip of the w beam flange to the u/s of the plate then I would make the argument for fixed.

If you are only welding the one edge (or even the 3 edges) I would be inclined to call it simple supported.

2.5" thick plate spanning 24" must be supporting some significant loads, do you really want to scrimp on plate thickness?

 

[KootK]

Expanding upon JStephen's recommendation, I think that you absolutely want to design the plate as pinned. That will help to ensure that your plate is stiff enough that you won't put your fillet welds into significant bending which is a faux pas: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

Provide a sketch. The arrangement is not clear.

Offhand, I would say that a 1/2" fillet weld on a 2.5" thick plate is closer to hinged than fixed.

2. Welding on four sides changes the boundary conditions considerably.

BA

 

[LOKSTR]

See the attached elevation.

 

[KootK]

Definitely pinned based on the torsional flexibility of the W-beams if nothing else. In this instance, I would ditch the stiffeners and run the plate an inch or so past the beam webs on either side. That will make things mechanically simpler, reduce torsion in the beams, and reduce prying forces in the welds.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Correction: you may indeed need stiffeners if web crippling/yielding is an issue. If that is the case, you might be better off with web doublers.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

Welding on the two free sides would have little effect unless you add a member between beams at each free edge. That would help substantially by allowing the plate to span in both directions. It would use less steel than the detail shown but fabrication cost would increase slightly.

BA