Area mesh and linear analysis modes on sap2000 v18 1

[cesaramorim]

Hello dear engineers,
I've recently started using sap2000 and I have a few doubts

About meshes/area divide/fem:
When I define polyareas 2 things my happen, either I define a rectangle or triangle and get something like this

Or I draw a area with more sides and get this

What does it mean?

I've read it's about meshing, but I couldnt quite grasp it, because I defined the 5-sides area with general automesh, and while viewing the analysis model the fem nodes would show up

So what's meshing about? I believe it creates the linear elements that are used to analyse the model. But is a mesh different than using a area divide?

About linear vs non linear analysis:
Also I've understood that the only way to get elements working exclusively on tension, like cables, is to put loads on non-linear analysis.

However in the CSI knowledge base there's the following
"Nonlinear analysis methods are best applied when either geometric or material nonlinearity is considered during structural modeling and analysis. If only elastic material behavior is considered, linear analysis methods should suffice..."

Maybe I'm getting something wrong, but wouldn't a cable work first linearly? What's the point in needing non-linear loads to define a element workig exclusively with tension?

 

[theshearstud]

From what I remember:

When you draw an area element with four nodes only, the program will not automatically "auto mesh" your element (assuming you have not assigning an auto mesh. When you draw an element with more than 5 nodes, the program will mesh your element, whether your assign a meshing or not. I think best practice to is draw 4 node areas and assign a meshing.

Meshing should produce the same results as "divide area". Divide area might be useful when you want to connect adjacent shell elements (as if you assign an automesh to adjacent shell elements, they may not connect when the mesh is created). Divide area is also useful when you need to apply loads to the area.

Meshing is required for accurate results. It's been a while since I took FEM, but the shell element forces are obtained from the mesh nodes. So If you have a large, 4 node simply supported plate with an area load, you will not see any moments or shears in that shell.

Not sure about the cables. My guess is that since the stress/strain diagram will be different for tension and compression cases, a non linear analysis is required.

 

[cesaramorim]

@theshearstud
Thank you very much, the area/mesh explanation makes perfect sense, and is accurate with the tests I've made. I will perform some more tests about the linear/non linear analysis