4.2.3 Iteration Parameters

Iteration Parameters

For more information about this tab, see Chapter 2.4.9.

The locked setting options of this dialog section are not required for RF-CONCRETE Members: The FE division allowing for a fine control of the convergence behavior is used for nonlinear calculations.

Click the button to open the FE Mesh dialog box where you can adjust the global target length of elements of the FE mesh and the division specifications for members.

You can control the iteration process with the parameters in this dialog section.

The iteration process strongly depends on the cross-section shape, the structural system, and the loading. Thus, the number of iterations required to reach the break-off limits is exposed to strong fluctuations. The preset value of 50 iterations is sufficient for most practical applications but can be adjusted, if necessary.

The program determines the difference in stiffness on a node in the course of two successive iteration steps. The Damping factor represents the part of the stiffness difference that is considered for the new stiffness applied in the subsequent iteration step. By reducing the stiffness changes between two iteration steps, it is possible to counteract the calculation's oscillation.

The higher the damping factor, the smaller the damping's influence. If the factor is equal to 1, the damping does not affect the iterative calculation.

The break-off limits can be adjusted depending on purpose and function: Even if relatively roughly defined break limits (ε₁ = ε₂ ≤ 0.01) lead to sufficiently accurate results when calculating according to linear static analysis (beam deformations in SLS, for example), it is nevertheless recommended to refine the tolerances used for stability analyses (ε₁ = ε₂ ≤ 0.001). Example 3 in Chapter 9.3 illustrates the effect clearly.

With the break limit ε₃, you can additionally control the deformation change. This criterion observes how the size of the maximum deformation changes. The specified damping factor is also taken into account.

The loading can be gradually applied in order to avoid or attenuate an abrupt stiffness change within the individual finite elements ("adapting" the system to the loading). The aim is to avoid the generation of major stiffness changes during an iteration. When the loading is applied step-by-step, it is possible in the iteration step of a load increment to always fall back on the corresponding final stiffness of the element from the previous load increment.

This input field determines the number of individual load increments for the nonlinear calculation.

• Linear

• The load is applied in linear steps.

• Trilinear

• As you can only react to the load-dependent stiffness development with a correspondingly fine gradation when applying loads linearly, RF-CONCRETE Members provides a trilinear load application as an alternative. Thus, it is possible to respond accordingly to boundary conditions like creeping near the state of failure.

The trilinear load application is managed by a table: You have to specify two intermediate points that characterize the respective applied load ratio.