4.18 Ribs

Ribs are a special type of members. To create a rib, a member must already exist. Ribs can be used to display T-beams in the FEA model by defining eccentricities and effective widths.

You can define a rib directly with the Ribs navigator shortcut menu or through dialog input. When you create a new member and you select the Member Type Rib (see Chapter 4.17), you can use the [Edit] button to define the parameters. It is also possible to access the dialog box below by using the shortcut menu in the navigator or the menu.

Generally, a rib is a member that is eccentrically arranged. The eccentricity is determined automatically from half of the surface thickness and half of the member height (Table 1.15 Member Eccentricities is not affected). You can also define it manually. The rigidity of the model is increased due to the eccentricity of the rib.

The following arrangement options are available:

The eccentricity as the sum of half of the surface thickness and half of the web height is automatically applied in direction of the positive or negative surface axis z. To display and check the surface axes x,y,z in the graphic, use the Display navigator (see Figure 4.122).

The rib is modeled without eccentricity. The centroidal axis lies in the center of the surface.

You can define the member eccentricity perpendicular to the plate plane in the New Member Eccentricity dialog box, or in Table 1.15 (see Chapter 4.15). Then you can assign it to the member.

You can check the rib position in the rendering mode without problems: In the Display navigator, select the two display options for solid models: Members → Cross-sections and Surface → Filled incl. thickness.

When modeling 3D structures, the effective width has no influence on the stiffness because the increased stiffness is already taken into account due to the eccentric member. The effective width only affects the internal forces. For 2D models (model type 2D - XY), however, the stiffness is controlled by the settings applied for the Stiffness Reduction (see paragraph below).

If an eccentrically connected beam is used instead of a rib, the RFEM model yields internal forces of both the member and the slab. But in reinforced concrete design, the member and a certain part of the surface are considered a single unit – T-beams. To determine the internal forces for the floor beam, the bending moment in the member must be increased by the product of the axial force in the slab and the eccentricity. To determine the axial force in the plate, you have to know the area where the axial forces are summed up. Therefore, you have to specify the effective widths as well as the surfaces.

The effective widths of the rib must be defined separately for the left and right side. In most cases, you can keep the Autodetect setting in the Connecting surface list of the New Rib dialog box. Only if more than two surfaces adjoin each other along the line of the rib do you have to explicitly determine the connecting surfaces.

The effective Width b₁ or b₂ can be entered directly into the text box or calculated automatically from the member length by selecting the options L/6 and L/8. When confirming the dialog box, RFEM determines the effective widths and fills in the values.

After the calculation, the effective components of the surfaces can be considered for the member results in the Display navigator: Results → Ribs - Effective Cooperation Surface/Member. The member result diagrams also allow for a specific evaluation of the rib internal forces (see Chapter 9.5).

This dialog section and these table columns are only shown if the model type 2D - XY has been set in the general data (see Figure 12.23). In contrast to spatially defined models where ribs can be taken into account as eccentrically arranged members in the FE analysis anyway, RFEM uses a different analysis approach for floor beams.

For the calculation, RFEM applies a substitute cross-section whose stiffness is determined from the member cross-section and the effective plate component of the surfaces. Thus, the stiffness of the plate is determined twice for eccentrically arranged ribs because it is effective in the substitute cross-section as well as directly through the surface elements. If the Without activity of plate component check box is selected, the stiffness component of the plate is not considered in the substitute cross-section.

This text box is used to reduce the torsional rigidity of the rib.