Weld Strength

[Armando23]

Hi,

I'm trying to determine the strength of a weld in a unique situation, please see attached. I have an aluminum sheet welded to a frame made with extrusions. The sheet is loaded with a pressure that will put a bending load in the welds. I know the bending moments at the welds but I can't figure a way of calculating the stress in the weld, I haven't been able to find an applicable example in any of my textbooks or the web. I think it can be simplified to T-joint with a weld only in one side, but I don't know how to calculate the weld strength for such case. Any help will be greatly appreciated.

 

[racookpe1978]

You have a very tiny 1/8 fillet weld all-around on the inside edge of a large, very stiff rectangular frame supporting a thin plate.

Your point load (magnitude is not labeled!) is on the center of that thin membrane, so the membrane is under tension that will "pull" the center of the membrane down, and the outside edges of the membrane "up" slightly. But you don't have any outside fillet weld around the frame, so you have to go back to the book for "point loads restrained (pinned) all around their edges." there is no moment resistance around the outside, only a shear force and a rotational force around the center of area of the fillet as the center of the membrane goes down and the outside edges of the membrane try to go up.

 

[Armando23]

You have a very tiny 1/8 fillet weld all-around on the inside edge of a large, very stiff rectangular frame supporting a thin plate.

Your point load (magnitude is not labeled!) is on the center of that thin membrane, so the membrane is under tension that will "pull" the center of the membrane down, and the outside edges of the membrane "up" slightly. But you don't have any outside fillet weld around the frame, so you have to go back to the book for "point loads restrained (pinned) all around their edges." there is no moment resistance around the outside, only a shear force and a rotational force around the center of area of the fillet as the center of the membrane goes down and the outside edges of the membrane try to go up.

Thanks, for the reply. Is there a formula or a mathematical procedure to calculate the strength of the weld in a situation like this? I've attached a simplified example illustration.

To give a little bit of background the frame in the first post simulates a VERY rough example of a shipping container cover. I'm tasked to asses the strength of the covers when loaded with snow. Normally our container covers have stiffeners inside and outside but every container has a different configuration of stiffeners. So I decided that if I remove the stiffeners and prove that the welds alone are sufficient to take the load, obviously the stiffeners will only make the weldment stronger.

 

[Armando23]

I forgot the attachment in the previous post.

 

[racookpe1978]

No, you first have to calculate the stress around the edges of the membrane sicne it is fastened to a relatively inflexible outside rectangular frame. THEN you can begin approximating the forces you need to resits movement with the weld.

What engineering reference do you have for membrane stress?

 

[Armando23]

I'm using this formula (attached) from Roark's Formulas for Stress And Strain. I also did FEA using shell elements and basically got the same answer as the formula. The weld strength is what's killing me.

 

[racookpe1978]

Roark's formula 8a is for a uniform load over the entire plate. But restrained around the entire edge of the rectangular plate, as is correct for your image.

But your example image file shows a point load at the center of the plate.

 

[Armando23]

It looks like a point load but it says pressure, so it is a uniform load.

 

[tbuelna]

Your two sketches don't show the same configuration. The first sketch shows the sheet overlapping the perimeter frame, with a fillet weld around the underside of the sheet offset from the sheet edges. The second sketch shows a fillet weld right at the outer edge of the sheet which is exactly the same size as the frame opening. Given the direction of the applied pressure force on the sheet, the arrangement shown in the first sketch would seem to give lower stresses in the weld, all other things being equal.

I would expect there to be significant stress concentrations around the perimeter of the sheet with the weld joint shown in the second sketch. There is an abrupt change in stiffness at the transition from the sheet edge to the weld and to the frame.