Non linear FE calculation

You may now press the mesh button to create the Finite Element mesh, and then press the button to start the non-linear version of the iConc applet. After a few seconds of calculation, your screen should show a picture looking like Figure 18a.

CONGRATULATIONS ! You just performed your first non linear Finite Element calculation with the iConc programm.

a) Finite Element mesh	b) Deflected shape

 
Figure 18: Finite Element mesh in the iConc applet including the reinforcing bars

The deflected shape shown in Figure 18 looks similar to the shown after the linear elastic calculation, although there are some differences visible to the experts. There are more visible differences when looking at the concrete stresses, using the menu Results – Concrete relative stresses – Varying length. The result is shown in Figure 19.

Figure 19: Concrete stresses

This figure is not simple to understand, so let’s make a couple observations:

• All stress vectors are blue, showing only compressive stresses in concrete. This is normal, as concrete cannot carry tensile loads. Compare for example the linear elastic results of Figure 15, in which both compressive (blue) and tensile (red) stresses were shown.

• Some triangular elements are coloured in gray, dark gray and even black : this is the way for the iConc applet to tell us that these elements are subjected to large distortions or too large compressive stresses.

Let us now look at the tensile stresses in the reinforcing bars: Results – Steel relative stresses. The results are as shown in Figure (after rescaling the graph, as for previous plots).

Figure 20: Relative stresses in the reinforcing steel

The explanation for the concrete problems (which are also visible in the figure) lays in the color of the reinforcing bars: over most of their length, they are colored in dark red, which indicates that the bar has yielded (reached its maximum strength). Only small portions at the end of some bars are colored in red, showing that the bar has not reached its maximum strength. In other words, there is not enough reinforcement in the beam! We can now close the iConc window and return to the iMesh applet.