2.5 Effective Lengths - Members

This module window is subdivided into two parts. The table in the upper part lists the buckling length factors and equivalent member lengths for buckling and lateral-torsional buckling of all members to be designed. The effective lengths defined in RFEM or RSTAB are preset. In the Settings section, you can see additional information about the member whose table row is selected in the upper part.

With the  button you can select a member graphically to activate its row in the table.

Changing entries is possible in the table as well as the Settings tree.

The effective lengths for buckling about the minor axis z are aligned automatically with Window 1.4 Intermediate Lateral Restraints. If the intermediate supports divide the member into segments of different lengths, no values are displayed in columns G, K, and L of Window 1.5.

You can enter the effective lengths manually in the table and the Settings tree. You can also define them graphically in the work window by using the button. This button becomes active when the cursor is placed in the text box (see Figure 2.24).

The Settings tree includes the following parameters:

• Cross-Section
• Length of member
• Buckling Possible for member (corresponds to columns B, E, and H)
• Buckling About Axis y (corresponds to columns C and D)
• Buckling About Axis z (corresponds to columns F and G)
• Lateral-Torsional Buckling (corresponds to columns I to K)

For the selected member, you can define whether a buckling or a lateral-torsional buckling analysis is generally to be carried out. In addition, you can adjust the Effective Length Factor and the Warping Length Factor for the respective directions. When changing a factor, the equivalent member length will be adjusted automatically, and vice versa.

It is also possible to define the effective length of a member in a dialog box that you open with the button [Select effective length factor]. You can find the button below the table.

For each direction, you can select one of the four Euler buckling modes. You can also set a User-defined effective length factor. If an eigenvalue analysis has been carried out by the RF-STABILITY or RSBUCK add-on module, it is also possible to define a Buckling mode for the determination of the factor.

The stability analyses for flexural and lateral-torsional buckling require the ability to absorb compressive forces. Therefore, members for which such an absorption is not possible due to the member type (for example, tension members, elastic foundations, rigid connections) are excluded from the outset. The rows are grayed out in the table, and a corresponding note is shown in the Comment column.

The Buckling Possible check boxes in table row A and in the Settings tree offer a control option for the stability analyses: They determine if these analyses are performed or omitted for the member.

With the check box in the Possible column, you decide if a member has the risk of buckling about the axis y and/or z. These axes represent the local member axes, with axis y being the "major" and axis z the "minor" member axis. The effective length factors k_cr,y and k_cr,z for buckling about the major or minor axis can be selected freely.

In the 1.3 Cross-Sections window, you can check the position of the member axes in the cross-section graphic (see Figure 2.18). With the [Jump to graphic] button you can also access the work window of the main program. There, you can display the local member axes by using the member's shortcut menu or the Display navigator.

If buckling is possible about one or both member axes, you can enter the effective length factors in columns C and F, and the effective lengths in columns D and G. The same is possible in the Settings tree.

To define the effective lengths graphically in the work window, use the button. This button becomes available when the cursor is placed in a L_cr input field (see Figure 2.24).

When you specify the effective length factor k_cr, the program determines the effective length L_cr by multiplying the member length L by this factor. The input fields k_cr and L_cr are interactive.

Column H shows which members are included in the analysis of lateral-torsional buckling.

To determine M_cr by the eigenvalue calculation method, an internal member model with four degrees of freedom is created. These degrees must be defined by the factors k_z and k_w. With both factors interacting, it is possible to determine the support conditions for lateral-torsional buckling (for example, lateral and torsional restraint).

The factor k_z controls the lateral displacement u_y and the rotation φ_z at the member ends.

• k_z = 1.0 restrained against lateral displacement u_y on both member ends
• k_z = 0.7le restrained against displacement u_y on both ends and restraint about z left
• k_z = 0.7ri restrained against displacement u_y on both ends and restraint about z right
• k_z = 0.5 restrained against displacement u_y and restraint about z on both member ends
• k_z = 2.0le restrained against displacement u_y and restraint about z left; right end free
• k_z = 2.0ri restrained against displacement u_y and restraint about z right; left end free

The factor k_w controls the torsion about the member's longitudinal axis ϕ_x and the warping ω.

• k_w = 1.0 restrained against rotation about x on both member ends; free to warp on both sides
• k_w = 0.7le restrained against rotation about x on both ends and warping restraint left
• k_w = 0.7ri restrained against rotation about x on both ends and warping restraint right
• k_w = 0.5 torsion and warping restraint on both member ends
• k_w = 2.0le restrained against rotation about x and warping ω left; right end free
• k_w = 2.0ri restrained against rotation about x and warping ω right; left end free

The abbreviations le and ri refer to the left and right side. The abbreviation le always describes the support conditions at the member start.

You can model a lateral and torsional restraint by using the factors k_z = 1.0 (support in y with free rotation about z) and k_w = 1.0 (restrained against torsion about x with free warping). Because the internal member model requires only four degrees of freedom, defining other boundary conditions is not necessary.

If the lateral-torsional buckling length L_w or the torsional buckling length L_T differs from the length of the member or the buckling length, you can define the lengths L_w and L_T also manually in columns K and L or graphically with the  button.

In the final column, you can enter user-defined notes to describe, for example, the equivalent member lengths.

Below the Settings table, you find the check box Set input for members No. When you select it, the subsequent settings will be applied to All members or to selected members (enter the member numbers manually or select them graphically with ). This option is useful if you want to assign the same boundary conditions to several members. You can find an example in the Knowledge Base on our website. 

Settings which have already been defined cannot be changed subsequently with this function.