Hi,
I have a template of a 40ftx20ft RC slab in Concept that I sometimes use to make quick studies or to verify my hand calculations. While looking at the net axial stresses I noticed that they are not equal to zero; in fact ,the axial forces are in compression everywhere in my slab. How is this possible? Net axial compression in my slab will result in reduced reinforcement demand, similar to columns, which is unconservative in slabs. I would expect to see net axial forces only in PT slab or in RC slab with steps or T-beams, but not in a RC flat slab.
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Hi Karl, I have uploaded the model. The name of the file is RC Slab
This is exactly the behavior that you would see with a moment frame load with a uniform distributed load. See image below:
In your RAM Concept model, you will see that the sum of the lateral reactions sum to 0 and that the model is in equilibrium. It is true that net compression can reduce reinforcement requirements. If that is your concern, you can use the span segment option marked in the image below to ignore the compression in design. Note that this option would also ignore net tension force also, which would increase reinforcement requirements. You should use caution when checking that box for that reason.