A lot of very sophisticated material models was developed during last few decades. Such complicated events as cracking, damage and fracture of concrete can be simulated on powerful workstations. Despite the fact that actual power of PCs based on newest PIII processors is very high, demanding simulations take dozens of hours of computer time.

Let us present the following example for demonstration. It includes experimental investigation and computational simulation using microplane model.

The experimental program was focused on RC column loaded by compressive load with a small eccentricity (see Fig. 22). Especially the post-peak behaviour, size effect and confinement effect of stirrups were studied and the following conclusions have been drawn:

- No significant yield plateau was observed in force-deflection diagrams. Due to the compressive softening of concrete, specimens lost their load-bearing capacity very fast. See Fig. 22.
- Longitudinal compressive load causes high transversal expansion of concrete and leads to yielding of stirrups.
- The longitudinal density of stirrups influences the ductility and load capacity of RC columns.

Numerical studies were focused on 3D modeling of RC columns using finite element method. The nonlinear zone of the column was modeled by microplane model for concrete. Geometrically nonlinear element were used for longitudinal steel reinforcement in order to capture buckling of steel. The finite element model consisted of 4896 linear elastic space elements, 3456 nonlinear space elements including microplane model and 1619 nonlinear truss elements (reinforcing steel). Explicit time integration method was used. The computation was performed on a single processor PC - PII-Xeon 400 MHz, 512 MB, where 6000 time increments lasted for 61.65 hours. The computational effort needed for model with microplane material is evident, even if explicit method is used. The comparison of experimental data with simulation, presented in Fig. 23, reveals reasonable agreement in ultimate bearing capacity (peak value) but relatively poor agreement in the post-peak behaviour (the descending branch of the load-deflection diagram).

The following conclusions have been made:

- Microplane model is capable to describe concrete in complicated stress states which can appear in RC structures.
- Microplane model is very computationally demanding model, which should be preferably used with explicit methods.
- Parallelization strategy can solve the problem of large computational times.

*Daniel Rypl
2005-12-03*