I'm working on a structure that is conceptually similar to a backpacking tent. It has a flexible pole structure which is covered by a high tensile fabric. The fabric will transfer wind load to the poles and also provide buckling resistance. I'm not clear on the best way to model this. I have TurboCAD which will only output .STL. My concerns are the correct elements to use for the fabric and the method of transferring the force between fabric and frame. I'm not clear on the difference between shell, membrane, and a very thin solid. Also using bonded contact between pole and fabric seems to present a problem in selecting a large quantity of elements on irregular surfaces (round poles from many angles). One thought is to build it as one piece and assign one material to everything but make the fabric portion thin enough to approximate the correct tensile for fabric. Any thoughts?
Thanks,
Paul
Comments
Regards
1) solid
2) shell (no normal stiffness)
3) membrane (no normal, bending or shear stiffness).
A shell and a thin solid can usually be used for the same things as each other. It often depends which is easiest to model. Shells don't have stiffness in the out-of-plane normal direction so they aren't as accurate as solids at joints, sharp corners and other details. They can also fail to converge on some curved geometries.
Membranes only have stiffness for in-plane loading and can't support any bending or out-of-plane shear loads. In Mecway, they're always 2D so there's no way to bend them anyway, but I think in other programs you can use them to model fabric with lower computer resources to solve than shells. You can approximate a membrane element by using a very thin shell element or possibly even a thin solid as you suggested.
This "membrane action" you're describing requires nonlinear analysis because the initially flat sheet will have almost zero bending stiffness so it won't support wind loads until after it begins to deform. With Mecway, that means you'll have to use solid elements for both the fabric and the poles.
For bonding the fabric to the poles, you can select all the outside faces of the pole elements and use them as the master surface. The edge of the fabric will only bond to the nearest face and ignore the opposite face as in Tutorial 4.9 "Mixed Materials with CAD Assemblies" under Help -> Tutorials. You can check that it's connected properly by clicking the bonded contact item in the outline tree and that shows little lines and circles indicating all the connections that will be made.
To select all outside faces of a pole, make only the pole visible, change to Select Faces mode, then press Ctrl-A.
I'm assessing various frame geometries and pole designs. I'll be taking the models to failure to determine how light I can make the assembly. To start, I'll be applying an even load over the surface. Once the basic geometry is decided, I'll add some point loads to check for premature buckling.
Regards,
Paul
Thanks.
I'm imagining a way to take the straight pole, then define an ending shape rather than a force. Then retaining the difference in mesh geometry as the starting conditions for the normal force based analysis.
Thanks,
Paul
There isn't a practical way that I know of to convert displacements from the solution to fixed displacement constraints in the input.