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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. 21).
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. 21.
- 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
3D beam 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. 22, reveals reasonable agreement in
ultimate bearing capacity (peak value) as well as in
the post-peak behaviour (the descending branch of the load-deflection
diagram), when nonlinear 3D beams elements are used to model the reinforcement.

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*Daniel Rypl *

2005-12-03