Specialities of reinforced concrete model

A special source of material nonlinearity is the RC model for surface and line elements. 

Properties #

The constitutive model implemented into AxisVM for consideration the material nonlinearity of RC surfaces, beams and ribs is not a comprehensive material model. It does not track the cracking process of concrete and the yielding process of steel during the load history. It is just a simplified model to find the internal forces compatible with the current strains with consideration the nonlinear behaviour of concrete and steel. 

Application #

The purpose of this model is only to get a better approximation of deflections by verification in serviceability limit states. Application of this model to track the materially nonlinear behaviour and to get accurate internal forces by verification in ultimate limit states is not recommended. 

Recommended settings of nonlinear analysis #

There are 2 special properties of the RC model that induce two recommended settings of nonlinear calculation parameters. 

Convergence criterion #

On one hand, since the convergence in force cannot be expected to achieve, only the displacement convergence criterion is recommended to turn on.

Increment function #

On the other hand, since the behaviour of both concrete and steel material is linear at lower strains, convergence problems are not expected during the beginning increments of the nonlinear calculation. Therefore, the application of the predefined <bilinear> increment function is recommended for models with only RC material nonlinearity. This increment function starts with a load factor of 0.4 in the first increment. With this large first step, the whole linear part is computed in one increment and all other increments can be used to compute the nonlinear part.

Totally and partially nonlinear behaviour #

Further speciality of the RC model is the capability for partly nonlinear behaviour. 

ε-N nonlinearity #

In case of ε-N nonlinearity only the connection between normal forces and membrane strains is nonlinear. The bending and torsional behaviour is linear elastic without any coupling with the membrane behaviour. This option is recommended to walls.  

κ-M nonlinearity #

In case of κ-M nonlinearity, only the bending behaviour is nonlinear. But the calculation of the bending moments takes into account the linearly computed normal forces. This option is recommended for slabs.

ε-N, κ-M nonlinearity #

For 3D shell models the totally nonlinear model is suggested. It is calculated on the basis of a materially nonlinear fiber model with coupled membrane forces and plate moments. For the occurrence of normal force due to deflection, the geometric nonlinearity is also necessary to be turned on.

Shrinkage #

It is recommended to mention one more speciality of RC model, namely the partial consideration of shrinkage. Shrinkage is taken into account only for the bending behaviour. Normal forces are not influenced by that. The reason for this is that unreal high normal forces could appear in simplified models, in which the walls and columns are modelled with line and nodal supports.