3.4 Ribs

The ribs defined in RFEM are preset. Ribs represent a special type of member consisting of a beam and an effective plate cross-section (see RFEM manual, chapter 4.18). RF-CONCRETE Members imports the rib internal forces from RFEM and uses them for the design.

You can adjust the effective widths b_eff in columns D and F. A recalculation in RFEM is not required because the system stiffness remains unchanged. The calculation of the cross-section properties and the integration of the rib internal forces are carried out automatically for each change of the effective widths.

This column shows the numbers of the members defined as Rib member type in RFEM.

Columns A and B show the cross-section numbers (see Chapter 3.3). If different numbers are displayed, the member is a tapered member.

Columns D and F indicate the effective widths for the left and the right side of the member. The values for b₁ and b₂ taken from the New Rib dialog box in RFEM (see RFEM manual, chapter 4.18) are preset here. The rib internal forces are determined based on the integration widths for the pro rata internal forces in surfaces.

The effective width controls the cross-section design in the form of an equivalent cross-section. Therefore, you can adjust the values for b_eff. Reduced effective widths result in reduced member internal forces, which affect the design in RF-CONCRETE Members. Increasing the integration widths is not allowed.

The effective widths can be modified directly. Alternatively, it is possible to adjust the widths with the button that opens a dialog box managing the parameters of the equivalent cross-section called Unsymmetric Floor Beam. When entering the widths b_l and b_r, follow the sketch of the cross-section.

To check the effective widths, select the Show rib effective widths for determination of internal forces check box. The table will be extended by two additional columns.

By selecting the check box, the [Edit Rib] button becomes accessible. It enables you to adjust the rib parameters of RFEM, thus affecting the system stiffness. However, this also means that the internal forces must be recalculated.

In the cross-section graphic below the table, changes are represented dynamically. The graphic shows the equivalent cross-section used for the design.

If the rib leads to problems in the design, a corresponding note appears in this column.

The following items must be considered for a correct design of ribs:

• The rib's local z-axis must be parallel to the local z-axis of the surface.
• The rib's local z-axis must be orthogonal to the xy-plane of the surface.
• The surface type of the connected surface must be Plane.
• The cross-section type of the rib member must be a Rectangle.
• When sets of members are used, a uniform rib type must be defined for the entire set of members.
• The material of the rib must be identical to the material of the surface.