HarryvL wrote: ↑Sat Mar 23, 2019 9:24 pmA hand calculation assuming a straight slip surface at 45 degrees shows that collapse occurs at a load factor of 0.5.
So why this 16% error in the analysis? Well part is always related to lack of mesh refinement. However previous analyses showed that even for coarse meshes rather accurate collapse loads can be predicted.
The answer is in the translation of undrained shear strength to yield stress. The von Mises stress (S_vm) for pure shear (Tau) equals S_vm = Sqrt(3) * Tau. So a von Mises material with yield stress Sy has a shear strength of of Sqrt(3) * Sy and not 2 * Sy as is the case with Tresca (= Mohr Coulomb with Phi=0) material. So by simply doubling the undrained shear strength to get a yield stress, we introduce as much as a 2/Sqrt(3) ~ 1.155 (or 15.5%) error in the collapse load.
This simple case study shows that the macro accurately predicts collapse, but that care should be taken with translation of measured soil parameters into model parameters
I have done as much testing as I can and am able to reproduce the beam failure load of 2.37 (see: https://forum.freecadweb.org/viewtopic. ... 60#p308289) accurately:
Well I completed the functionality to use the reinforcement ratios from an elastic analysis as a starting point for the collapse analysis and it indeed gives a much smaller reserve strength.HarryvL wrote: ↑Tue May 14, 2019 9:01 pmThe reason for the excessive reserve strength is the fact that the maximum reinforcement ratio required at the bottom of the beam is applied over the full height. When I complete the functionality for varying the reinforcement ratio by integration point I will rerun the analysis with the minimum required reinforcement following from the elastic analysis.