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2024-01-16

Sections

You need a section to describe the properties of a member: The section properties and the assigned material properties affect the stiffness of the member.

Not every section that has been defined must be used in the model. This way, you can model variants quickly, without deleting sections. However, sections cannot be renumbered.

Name

You can define any name for the section and specify its cross-section properties. If the description matches an entry in the library, RFEM imports the stored properties. To select a section in the library, click the Library button at the end of the input line. The import of sections is described in the Section Library chapter.

Tip

When you enter a common description in the text field, for example "ipe 240", a list box appears for this section type, sorted by various standards.

For sections from the library, the section properties are set by default and cannot be changed. The shear areas and the dimensions for non-uniform temperature loads are exceptions.

For a user-defined section name, you have to define all section properties manually. You can then use the cross-section to determine the internal forces and moments. However, the design of this cross-section is impossible because no stress points can be defined.

Main

The Main tab manages the basic section parameters.

Material

A material must be assigned to each section. You can select it from the list of already defined materials. The buttons next to the text box allow you to select a material from the library or to define a new one (see the Materials chapter).

Section type

The 'Section type' is preset according to the usual classifications for sections from the library (see the Section Library chapter). User-defined sections are assigned to the 'Basic' type.

Manufacturing type

The section's production method is displayed for sections from the library. It controls certain design specifications; for example, the buckling curves of cold-formed hollow sections.

Sectional areas

The cross-sectional areas are subdivided into the total area 'Axial A' and the areas for 'Shear Ay' and 'Shear Az'. The shear area Ay is related to the moment of inertia Iz; the shear area Az is accordingly related to Iy.

The following technical article provides information on determining shear areas:
https://www.dlubal.com/en-US/support-and-learning/support/knowledge-base/000966

The shear areas affect the shear deformation, which should especially be taken into account for short, massive members. If you change the shear areas, you should avoid extremely small values: The shear areas are included in the denominator of equations, so numerical problems may arise.

Area moments of inertia

The second moments of area define the section stiffness with regard to loading by moments: The torsional constant IT describes the stiffness against rotation about the longitudinal axis. The second moments of area Iy and Iz describe the stiffnesses against bending about the local axes y and z. Axis y is considered to be the "major" axis. The warping moment of inertia Iω is used to describe the resistance to warping.

For unsymmetrical cross-sections, the second moments of area are displayed around the cross-section's principal axes u and v. The local section axes are shown in the cross-section graphic.

You can adjust the sectional areas and moments of inertia by means of factors that you define as section-specific 'structural modifications' (see the Structure Modifications chapter).

Inclination of principal axes

The inclination of principal axes describes the position of the principal axes in relation to the standard principal axis system of symmetrical cross-sections. For asymmetrical sections, this is the angle α between the axis y and the axis u (clockwise positive). The principal axes are called y and z for symmetrical sections, and u and v for asymmetrical sections (see the image Section Properties and Axes).

The principal axis inclination is determined according to the following equation:

The inclination of principal axes for sections from the library cannot be edited. However, you can rotate the section around a user-defined angle: Tick the 'Section rotation' check box in the 'Options' dialog section (see Section rotation).

Dimensions (for non-uniform temperature loads)

The dimensions concerning width b and depth h of the section are required for the calculation of temperature loads.

Info

The Section Values tab lists further parameters of the section.

Deactivate shear stiffness

Taking into account shear stiffness leads to an increase in deformation due to shear forces. Shear deformation plays a minor role in rolled and welded cross-sections. For solid and timber cross-sections, however, we recommend considering the shear stiffnesses for the deformation analysis.

Deactivate warping stiffness

The check box for considering warping stiffness is available if the Torsional Warping Analysis Add-on is activated in the Base Data. In this case, you can control whether the warping stiffness of the cross-section is applied in the calculation with seven degrees of freedom.

Section rotation

The section rotation describes the angle by which the cross-section is rotated. You can define the rotation angle α' in the Section Rotation tab.

For asymmetrical cross-sections, this tab also provides options to 'Mirror' the cross-section. This way you can, for example, put an L-section in the correct position.

When you import a section from the library or RSECTION, you do not need to take care of the section rotation angle α'. RFEM imports the angle automatically. However, for user-defined sections you have to determine the principal axis angle yourself and adjust the position by means of the section rotation.

Info

All members that have the same cross-section are rotated by the section rotation angle. If this is not wanted, you should define the member rotation angles for particular members.

Hybrid

The 'Hybrid' option is only available for sections of the 'Parametric - Massive II' type. In the Hybrid tab you can, for example, assign material properties to the components of built-up timber cross-sections.

Thin-walled model

With the 'Thin-walled model' check box, you can control the theory according to which the cross-section properties are determined for sections of the 'Standardized - Steel' and 'Parametric - Thin-Walled' types. For a massive section, for example, the shear areas and the torsional moment of inertia are determined according to a different method, since the analytical solution only applies to thin-walled sections.

Stress smoothing to avoid singularities

The stress smoothing is primarily suitable for composite timber sections in order to avoid singularities in the connection areas. In these areas, shear stresses often lead to stress peaks, which have an unfavorable effect on the design. This function allows you to achieve a better distribution of stresses.

Section Values

In the Section Values tab, the properties of the cross-section are listed in detail.

The section properties of parametric cross-sections are determined by RSECTION.

Info

FAQ 5400 describes the equations that are used to determine the section moduli Wy and Wz.

Statistics

The Statistics tab provides an overview of the members available in the model that use that cross-section. For example, you can use the 'Total weight' indication for steel schedules or cost estimations.

Stress Points

Stress points are required to determine the stresses acting on the cross-section. All library sections are provided with stress points at the design-relevant locations of the cross-section.

The Stress Points tab consists of several sub-tabs. It shows the coordinates of the stress points, the statical moments of area with corresponding thicknesses and warping coordinates (only for thin-walled cross-sections), and the unit stresses.

You can check the statical moments and stress curves in the cross-section graphic: Click in the column of the value, or select the type in the list below the graphic.

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