Applied pressure and traction to vary with Z dimension - Silo analysis

Tim_Gowing

Hello,
I would like to apply a normal pressure and traction to the surface of a shell structure; the loading is due to a bulk solid such as sand in a silo. The pressure is not a hydrostatic pressure. The element type could be changed to 3d if that gave further loading options.
The fall-back method is to split the silo horizontally into many short rings so that each ring can be given a different normal pressure and traction.
Is there a facility within Mecway to vary a pressure via an equation or coordinate?
Can a pressure and traction be mapped onto Mecway shell element faces, from an external file?
Can a local coordinate system be created to allow the traction to be applied to an inclined surface using the Pressure XYZ menu?
All suggestion please.
Tim

Tim_Gowing

Victor,
I've attached an example of how I can apply a traction to an inclined shell panel. The intention in this example is to apply a pressure that results in a 1000N reaction that is in-place, i.e. a traction.
In this example the Y-direction pressure and the Z-direction pressure needed to be applied to opposite sides of the shell elements. I would prefer to use only the shell faces within the silo. I have tried to swap the face side of the element and use a negative pressure; this gives a result but not the one that I intended, i.e. in-plane tension without bending.
Please clarify the Mecway rules for element faces and tractions.
Tim

Tim_Gowing

Ah, the half shell thickness offset, this is creating a bending moment when the pressures are both applied to the same side of the shell element!

Victor

Yes, a traction also applies a moment due to the shell thickness as you pointed out.

No, there isn't currently a way to define a position dependent traction using the internal solver. There is a position dependent normal pressure that you can define by a formula, but it sounds like you need both components to change, not just the normal component.

Another workaround, which might be just as tedious as your suggestion of strips, is to apply a formula-defined normal pressure load to the edges of the elements. I did some of them in the attachment. It's a bit tricky to select those interior edges. I did it by disconnecting each strip of elements then hiding them with the cutting plane. It's not quite a smoothly distributed force because it doesn't put any load on some of the midside nodes. But it does mean you can use a single pressure load with a single formula.

Tim_Gowing

Victor,
Thank you for clarifying the pressure / traction application method.
I am using the STEP file surfaces to apply the pressure loads but I come up against a normal direction problem; some of my pressures are applied to the internal shell faces and some to the outside of my silo model, please see the image attached. I then thought, ah, check out the element normals and yes they matched the pressure direction. I selected the inward facing normal elements and used the invert tool; this resulted in the element normals all being consistent, all facing outwards, but unfortunately the pressure load symbols are still mixed inward/outward (seeming to be driven by the geometry normal direction). I could revert to applying pressure load to elements but would prefer to stick to the geometry if at all possible.
Is there a geometry or element level at which I can fix a shell model for this issue?
Tim

Tim_Gowing

Victor,
The attached 'tank' model is where I have carefully built the Solidworks model so that the plane on which each side is sketched, is facing inwards. In Mecway I select the five geometry internal faces and then use this named selection 'Surfacexxxx[linked]' to create a pressure load normal.
I haven't looked into how the .step cad format defines a surface. Could the imported geometry be colour coded for sideA / sideB? Does it make any difference to Mecway in whether I select the sideA or sideB of the imported surfaces for subsequent load application on child shell elements?
Tim

Victor

The geometry view shows the back (typically the inside for a solid) of each surface in a darker color than the front so even if they weren't all aligned in Solidworks, you could see which ones were facing backwards and put a negative pressure on them.

For selecting, it currently only selects the front no matter which side you click on, hence the need for a negative pressure rather than a positive pressure on the opposite side, which you could do if you were operating directly on the mesh.

Tim_Gowing

Victor,
Thank you for your assistance. I am applying loads directly to the mesh for the duration of this job and on the next shell model I will look into whether I can align all of the surface normal directions, within the CAD stage.
Tim