In addition to our technical support (e.g. via chat), you’ll find resources on our website that may help you with your design using Dlubal Software.
Frequently Asked Questions (FAQ)
Customer Support 24/7
Shear panel and rotational restraint options should be activated after having enabled the stability analysis in Details of the add-on module. Effective lengths and intermediate supports will also be available.
RSBUCK / RF-STABILITY calculates at least one critical load factor or one critical load and an assigned buckling shape. The effective length is then back calculated from the critical load (see here ). Since this analysis is not carried out for individual local components, but only for the entire structure, the resulting load summary factors refer to the global structure and not to the local elements. However, it may happen that the structure fails globally for some load branch factors (but fails locally as well (depending on stiffness and axial force state).
The calculated effective lengths should therefore only be used by the members that buckle in the respective buckling mode. In the case of the global failure of a structure (see example Figure 1), it is thus difficult to draw conclusions regarding the buckling behavior of individual members.
Figure 2 shows a structure where the rear columns buckle. Therefore, it is recommended to use only the effective lengths calculated for both columns.
Again in general: Buckling lengths from the RSBUCK module are only valid for a structural component in the respective direction if the related buckling shape clearly "bulges" the member in relation to the other in the respective direction. It is clear that the axial forces also have an impact on the results here.
AnswerFor a vault-free cross-section, a proof of the complete system is recommended using theory II. Order - internal forces and local imperfections. The entry of the effective lengths and nodal bearings (with staff sets) can thus be omitted. However, please activate gamma_M1 for cross-section verification.
Unfortunately not. Intermediate supports and nodal supports have to be set manually. The default configuration for effective lengths is 1,0*member length and for set of members, one nodal support with definition u_y' and phi_x' is defined on each side of the set of member.To speed up the process of defining nodal supports, you can pick more nodes in one step by using the highlighted function in the attached picture.
The length LT represents the buckling length for torsional buckling and lateral-torsional buckling under the systematic centric pressure. The length LT works similarly to Column D and Column G: a critical and buckling load is calculated for the design of a pure compressive force according to EN 1993‑1‑1, Section 184.108.40.206.
The length L W, however, is no effective length but the length to which the distribution of moments and shear force is related with the boundary conditions k z and k w (columns J and K) as well as the lateral intermediate supports. This length should i. d. R are not changed.
AnswerAll effective length factors are preset to 1.0 by default. There is no calculation of the factors based on the model.
The effective lengths for the buckling about the minor axis are synchronised with the defined intermediate supports from Window 1.4. If the areas show different lengths, no values are entered for the values Lcr,zw and LT.
The modules perform the eigenvalue analysis for the entire model with a certain axial force state. Depending on the number of eigenvalues required, the programs provide results of crictical load factors with the corresponding buckling shapes for an eigenvalue, and effective length about the major and minor axis for each member per mode shape.
Since each load case LC and each load combination CO often has a different axial force state in the elements available, there is a separate respective effective length result for each load situation of the frame column concerned. The effective length, which causes the column in the frame plane buckles sideway in the buckling shape, is the correct length to be used for the analysis of the load situation.
However, this result may be different because of various load situation in each analysis, the longest effective length of all analyses performed applies in the design on the safe side equally for all load situations.
RSBUCK uses a momentary representation of the axial force distribution in the respective load state. The axial forces are increased iteratively until the critical load case occurs. In the numerical analysis, the stability load is indicated by the fact that the determinant of the stiffness matrix becomes zero.
If the effective length factor is known, the buckling load and buckling mode are determined based on this. For the lowest buckling load, all effective lengths and effective length factirs are determined.
Example: Hinged column with a length of 20 m, cross-section HE‑B 500, self-weight load
For the first buckling mode, you obtain the effective length factor of kcr,y = 2.92 for the buckling about the major axis. For the buckling about the minor axis with a buckling load of 651.3 kN, you obtain an effective length factor of 1.00.
If you set the expression for determining the buckling load Ncr = π² * E * I / Lcr² to Lcr and apply Ncr = 651.3 kN and Iy = 107,200 cm4, you obtain the Lcr,y of 58.4 m, which results in the effective length factor kcr,y of 2.92.
In RSBUCK, there are two effective length factors determined for each buckling mode and buckling load.
In order to obtain the correct effective length factor for the deflection perpendicular to the y-axis (buckling about the major axis), it is necessary to calculate several buckling modes (mode shapes). The correct value is displayed in Window 2.1. In the example, it is the third buckling mode with a buckling load of 5485.5 kN. For this load, the effective lengths and effective length factors are determined as follows: kcr,y = 1.0 and kcr,z = 0.345.
In the case of a quadratic cross-section, two equal effective lengths result as the stiffnesses in both directions are the same.
In RSBUCK and RF‑STABILITY, the lowest critical load is calculated first. This is obtained, for example, for a hinged column (Euler buckling mode 1, IPE cross-section) for the buckling about the z-axis. With this buckling load, the effective length Lcr,y is determined retrospectively.
In order to obtain the correct effective lengths for Lcr,y, it is necessary to also consider the second buckling mode (mode shape). For this, specify at least two or more buckling modes for the calculation in the calculation parameters. In the second buckling mode, you obtain a higher buckling load (sway about the y-axis), from which you obtain the correct buckling load Lcr,y.
As shown in the example, RSBUCK or RF‑STABILITY requires the calculation of several buckling modes (mode shapes). Thus, you can obtain results for the individual directions (see Figure).
Did you find your question?
If not, contact us via our free e-mail, chat, or forum support, or send us your question via the online form.
Your support is by far the best
“Thank you very much for the useful information.
I would like to pay a compliment to your support team. I am always impressed how quickly and professionally the questions are answered. In the industry of structural analysis, I use several software including service contract, but your support is by far the best.”