Perpendicularly Loaded Plate Boundary Conditions

[TLHS]

I have seen this done both ways, and I suspect they're both valid and it's a question of the material being smarter than the designer and loads redistributing to match whatever assumptions have been made.

Let's consider the case of a plate welded to some sort of support where the structure is stable with or without moment restraint at the support locations.

I'm thinking here about, for instance, a plate loaded perpendicularly, spanning a distance and welded at either edge; or a plate loaded at the centre and welded on all four sides; or an annular ring supported in the centre; or basically any plate structure that can be stable if the boundary conditions are simply supported.

The two design methods I've seen in these cases are:

1) Assume full moment restraint at the weld. Size the plate for the smaller moment and size the weld to transfer the bending stresses into the support.

2) Assume simply supported at the weld. Size the plate for the larger moment and size the weld to transfer only the shear forces into the support.

Really, it's a question of flexibility. Presumably in the second set of assumptions, if the weld becomes over stressed you form a plastic hinge at the location and it rotates until the stresses redistribute into the plate.

I don't really have a question here, I'm more just interested in discussing opinions on how people treat these types of connections.

 

[BAretired]

That is where engineering judgment comes in.

BA

 

[corus]

Consider the worst cases. For the welds this may assume different end conditions than if you're looking at the plate.

Tara

 

[Ron]

It can be either as BAretired noted...judgment. Here's an example of one I did this week.

A square railing tube is welded along its sides to a flat plate for mounting with 4 lag bolts onto a wood header. The connection to wood is not intended to develop moment; however, the analysis of the tube requires development of its full bending moment at the plate in order to determine its allowable stress level (note that plate stiffness is greater than tube stiffness). So tube to plate connection is rigid and plate to wood connection is resolved by force couple.

Exactly as corus noted...

 

[rb1957]

i'd use "overlapping conservative assumptions" ... design the weld with the fully fixed boundary (as this puts the most stress into this joint, as opposed to a pinned boundary) and size the pale with a pinned boundary ... the trouble with conservative assumptions is that they are conservative somewhere and unconservative somewhere else, ie the fully fixed assumption loads up the edges but underloads the plate so if the fully fixed moment doesn't develop then the plate is undersized.

 

[SAIL3]

This is an interesting problem. I have often drawn comfort from the fact that a structure will eventally respond to loading according to the assumptions I have made through redistribution when stability is not an issue.
I have also analyzed a complicated problem by getting an upper and lower bound solution and then using engineering judgement decide on an answer somewhere in that range.
I would look at the plate supported at the edges in the following way:
Is it actually simply supported?....if the plate is not continous
over the support with 2 lines of weld, then it is not usually fixed. If the support is restrained from rotating and one has a full pen. weld to the plate then maybe one could consider it fixed. This is assuming small plate deflection where one is not relying on membrane forces.