Erik Kostson
Ansys Employee
Good Morning

Of course.

In that analysis in order to predict failure in concrete material models (Menetry Willam material model), where it becomes very difficult to get convergence once the structure fails, we use an enforced displacement to push the slab. So what we plot there is the total reaction which is the total force applied on the slab.

Now what happens when the force drops means that it can not take anymore load (so it drops) since the response/graph is going down (what we call a softening response of a structure - this is due to the strain softening in concrete), and this is due to the fact that the slab has failed in punching shear (large plastic zone marked in red second image below, shows that a plastic cone has formed similar to what you see in punching shear failure like in the first image below)

So the largest force on that graph is the total failure load (so the largest reaction observed in the graph not its value at 1 s which is not relevant - only the largest reaction we need here) of the slab which in this case corresponded well with experimental tests. AFter that the structure softens due to the large plastic strain region showing a cone like shape just as in real experiments.