RFEM – Online Manual

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RFEM – Online Manual

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4.18 Ribs

General description

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.

Ribs are primarily suited for models with reinforced concrete elements: You can use the rib internal forces and rib cross-sections for design in the RF-CONCRETE Members add-on module. However, when you want to model a steel plate with a welded "rib", use a surface with an eccentrically connected member.

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.

Figure 4.172 New Rib dialog box (for model type 2D - XY)
Figure 4.173 Table 1.18 Ribs
Position of Rib

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:

On +/-z-side of the surface

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 4.121).

Figure 4.174 Ribs on positive z-side (left) and negative z-side (right) of the surfaces

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

User-defined via member eccentricity

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.

Figure 4.175 Display navigator: Rendering - Solid Model
Effective Width

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.

Connecting surface

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.

Effective width

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.

Please note: When the member length is modified retroactively, the effective widths are not adjusted automatically!

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 allow for a specific evaluation of the rib internal forces as well (see Chapter 9.5).

Stiffness Reduction

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.

Without activity of plate component

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.

Torsion stiffness activity

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