The ultimate bearing capacity of foundations is analyzed by elastoplastic FEM through step loading. As the loads increase

the foundation status changes from elasticity to plasticity gradually

and finally the numerical non-convergence occurs. The solution of a classical Prandtl example shows that different yield criterions affect the results greatly; and only the use of the proper yield criterion can lead to the precise results. Under plane strain condition

the best solution can be obtained while Mohr-Coulomb inside-tangent circle yield criterion and associated flow rule

or Mohr-Coulomb matched D-P yield criterion and associated flow rule are used; and it can be applied to the analysis of practical engineering.