Shell Element Modeling 1

[theshearstud]

Hi all, I have a few questions regarding modeling:SAP2000V.15

1) When modeling shell elements as a concrete slab, some shell areas shade pink, and others red. What is this implying?

2) I have a large floor plan with many columns. I drew the shell area element blanketing all interior columns. How do I get the program to connect the shell elements to all interior column nodes without redrawing the entire slab and connecting it manually to each column node.

3) It seems when using the POLY area drawing function, sap2000 meshes the slab on its own for analysis. When using the rectangular drawing function, the slab is not meshed after analysis. Is this correct? Will I need to manually mesh the slabs drawn with rectangular area?

Thanks again for the help

Nick

 

[theshearstud]

I have found the answers to my questions:

1) Pink shaded area signifies that local positive 3 axis goes into the page (area is drawn in a clockwise fashion). Red signifies that local positive 3 axis goes out of page (area is drawn in a counter-clockwise fashion).

2) I do not believe so.

3) SAP2000 will only automatically mesh shells drawn with 5 or more nodes.

 

[stressed]

By default, SAP2000 shell element (membrane has different default and behavior) will transfer shear and moments at corner locations. So if you model the floor as one giant 4 node rectangle, it would transfer load only at the 4 corner joints. You can select the slab/floor and assign auto-mesh options "Mesh areas based on points on edges" or General mesh option to automesh the floor in order to pick up the columns. If you have beams framing into the columns, then you can mesh based on Intersections of straight line objects. Alternatively, you can add gridlines to intersect at each column location and draw the floor using the "quick draw area" tool which divides/meshes the shell at each grid intersection

It's not true that SAP2000 will only automatically mesh shells drawn with 5 or more nodes. SAP2000 can automesh with 4 node quads or 3 node triangles.

You are correct that the color of area object indicates if whether the normal axis is +3 or -3. You can activate the area local axis using the set display options dialogue. Depending on your version of SAP2000, local axes 1, 2, and 3 will be signified by the color of the arrows red (local 1), green (local 2), blue (local 3). In older versions, SAP uses white instead of green for local 2

 

[theshearstud]

Thanks for the input stressed.

 

[VanJohnson]

Hi everyone.
Why is it that the moment due to slab loads on a beam are so small after analysis, as compared to results obtained in manual analysis?

 

[theshearstud]

Van

If you could provide a little more information about your model it will be easier to attempt an answer. But my first guess is that Your manual analysis doesn't recognize 2-way slab behavior while your finite element model does.

 

[VanJohnson]

I modeled a 6m X 6m 2-way slab. I also did a 6m beam with triangular loading, representative of the loading that would go on each of the beams bordering the slab. The moment on the separate simply supported beam (which represents my manual analysis) was far higher than on the bordering beams.
I however discovered that that when I modified the area load type to 'uniform to beam', the moments matched.
Thank you.

 

[VanJohnson]

Is it correct to assume that slab meshing is done only when one is interested in the stresses in the slab but can be ignored otherwise?

 

[Tweety3]

Great answers

 

[stressed]

VanJohnson, regarding your last question, the answer is "no". Without meshing properly, you will not transfer load between slabs, columns and beams. You have to have area meshes to transfer load. Your slab stresses would be wrong and you wouldn't have correct support points or load transfer. A proper mesh is critical for proper results for slabs as well as connecting beams and columns.