How can I design a ribbed plate in state II with RF‑CONCRETE NL and use the stiffness export from the add-on modules for this?

Answer

First, the ribbed plate should not be modeled with the classic rib member from RFEM, but with an eccentric beam member that is arranged on the bottom surface of the actual plate. Rib members cannot be calculated with RF-CONCRETE NL with regard to deformations.

The eccentric beam is then designed in RF‑CONCRETE Members. In the "Serviceability Limit State" tab of Window 1.1, you can activate the "Nonlinear calculation.". In the detailed settings for the nonlinear calculation, you can activate the export of stiffness from the nonlinear calculation.

Figure 01 - Detail Settings for Nonlinear Calculation of SLS in RF-CONCRETE Members

In the example presented here, the stiffness is exported "individually" for each LC calculated in RF‑CONCRETE Members. You can find more information about the options "Individual" and "Consistent for reference load" under the link below.

After the calculation in RF‑CONCRETE Members, the exported stiffnesses of the calculated COs are available in RFEM, where you can activate them in the respective COs for a new calculation of internal forces. To do this, activate the extra options of the respective CO. In the "Extra Options" tab, you can then activate the stiffness exported from the RF‑CONCRETE Members add-on module for a new determination of internal forces.

Figure 02 - Extra Options in Calculation Parameters of Load Combinations in RFEM

After recalculating the internal forces of the COs in RFEM (taking into account the exported stiffness from RF‑CONCRETE Members), you can apply them for design in RF‑CONCRETE Surfaces.

The following figure shows the deformations of the ribbed plate in RF‑CONCRETE Surfaces, taking into account the stiffness in cracked state from the design in RF‑CONCRETE Members.

Figure 03 - Deformations from RF-CONCRETE Surfaces incl. Exported Stiffness in State II from RF-CONCRETE Members

In comparison to Figure 03, the linear-elastic stiffness in uncracked state (state I) was applied in Figure 04 for the eccentric beam.

Figure 04 - Deformations from RF-CONCRETE Surfaces Without Stiffness from RF-CONCRETE Members

Notes on the procedure described above:

  • In this case, the calculation was performed in RF‑CONCRETE Members first, and the resulting stiffness was exported. This approach was selected because it was assumed that the eccentric rectangular cross-section will proceed to the cracked state (state II) first.
  • The procedure shown "only" describes one iteration and is therefore only an "approximation" since an uncracked plate was assumed for the calculation of the eccentric rectangular cross-section.
  • The shrinkage effect is applied as an external load in the NL calculation in RF‑CONCRETE Members. This means that, for example, an unsymmetric reinforcement would result in an additional curvature, even if the cross-section remained in the uncracked state. When calculating the plate in RF‑CONCRETE Surfaces, this effect of shrinkage on the member cross-section is not taken into account anymore.

Keywords

Concrete Nonlinear Nonlinearity Rib Deformation Deformations State II Cracked state Downstand beam T-beam

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RFEM Main Program
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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

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