3.1.2.2 Nonlinear method of check

Nonlinear method of check

In order to perform a design according to the Nonlinear method, a license of the RF-CONCRETE NL add-on module is required. This method is described in chapter 2.8. The program performs a physical and geometrical nonlinear calculation.

The internal forces for the nonlinear design are generally determined according to the second-order analysis.

Click the button to open a dialog box for checking and, if necessary, adjusting the design parameters. This dialog box is divided into the Options and Material Properties tabs.

This dialog section allows you to control which serviceability limit state designs you want to carry out: deformation, crack widths, as well as concrete and steel stresses. You must select at least one of the four check boxes.

You can also decide if the influence of creeping and shrinkage should be considered in the nonlinear calculation.

Detailed settings for the individual checks as well as for creep and shrinkage are specified in window 1.3 Surfaces (see chapter 3.3.2).

The Save the nonlinear stiffness check box allows you to control if the nonlinearly determined stiffnesses are also available for a calculation in RFEM.

The stiffnesses can by exported Individually for each designed load case. In the Load Cases and Combinations dialog box of RFEM, you can then assign the according stiffness from RF-CONCRETE Surfaces to each of these load cases. RFEM allocates the load cases automatically. If you select Consistent for reference load, specify the governing load case in the drop-down list below. In RFEM, you can then assign the stiffness that results from these loads to all load cases that are defined.

The consideration of nonlinear stiffnesses in RFEM is described in chapter 7.3.1.3 of the RFEM manual.

The settings in this dialog section influence the process of the nonlinear design method. You can find more information in chapter 2.8.2.4.

When modifying the precision of iterations, make sure that the Maximum number of iterations is higher than the point in the calculation process from which the deformation criterion is additionally taken into account. Click the [Details] button to open the Calculation Parameters dialog box of RFEM. In it, you can adjust the precision of the convergence criteria for the nonlinear calculation.

In the nonlinear calculation, the surface is divided into so-called layers (see chapter 2.8.2.1). The recommended number of layers is 10.

Furthermore, you can influence the convergence behavior with Damping: Damping controls the magnitude of the stiffness change in the subsequent calculation steps. For example, if you specify a damping of 50 %, the change of the stiffness between step 2 and 3 can at most be 50 % of the stiffness change between step 1 and step 2.

The check box allows you to control if the calculation in the plastic zone of the reinforcing steel's stress-strain diagram is carried out with a rising or a horizontal graph (see chapter 2.8.3.4).

In this dialog section, you can specify the stress-strain relations of the concrete in compression and in tension. A parabolic diagram for compression and tension stiffening for concrete tensile stresses is preset.

For Tension stiffening (consideration of the stiffening effect of concrete in the tension zone), you can specify the parameters used to apply the concrete tensile strength between the cracks in a separate dialog box. To open it, click the [Edit] button.

Modifications of the parameters are immediately displayed graphically in the diagrams.

The approach of Tension Stiffening is described in chapter 2.8.3.3.