Steel Buckling Resistance

The SSD task task_steel_buckling_resistance_small ‘’Steel Buckling Resistance’’ contains following tabs:

  • Design

  • Loadcase

  • Selection

  • Text Output

  • Graphical Output

SSD provides two types of the task ‘’Steel Buckling Resistance’’.

  • Beam elements (Structural lines)

  • Design Elements

Design Code

The Design Code tab is the first step, where the design code should be selected. In version 2022, following design codes are available:

  • Eurocode 3

  • DIN 18800 / OEN 4300


Depending on the selected design code, the task will show up corresponding options. For e.g. Eurocode 3 you can select between method 1 and 2, you can modify the partial safety factors, change the imperfection factors for the buckling curves, change the cross-section parameters etc.

In case, that the SOFiSTiK defaults do not fulfill the users expectations, then the user may easily change the defaults if required.


The Loadcases tab will show all loadcases with an action for ULS (“Design (D), (E), and (A)”).


Next steps are necessary to define an ULS loadcase:


All loadcases without type ‘’Design (D), (E) and (A)’’ won’t be shown in the loadcase list. Loadcases that are generated via MAXIMA (superposition of internal forces) are not available in the selection list. Superposition loadcases do not provide the loads information which is required for the eigenvalue solver.


The Selection tab is the most important tab in the task. It contains two tables:

  • Structural lines [task task_steel_buckling_resistance_small ‘’Steel Buckling Resistance (Beams)’’]

  • Sequence of Structural lines [task task_steel_buckling_resistance_small ‘’Buckling Resistance (Beams)’’]

  • Design Elements [task task_steel_buckling_resistance_small ‘’Steel Buckling Resistance (Design Elements)’’]


The table for Structural lines shows all available structural lines with steel material. The table for Sequence of Structural lines is used to generate a sequence of the existing structural lines.

In case, if you are using the task task_steel_buckling_resistance_small ‘’Steel Buckling Resistance (Design Elements)’’, then in the tab Design Elements only the design elements that contain sections with type “Steel cross-section” will be shown. Composite, reinforced concrete and other types will be filtered out.

E.g. you can convert two or more structural lines to one design element.


The requirements to create a sequence of structural lines are:

  • the structural lines are connected,

  • the new design element (structural lines sequence) should be straight.

If this conditions are NOT met, then you will get a warning in BDK.

In the table bellow you will find the description for Structural lines and Sequence of Structural lines table columns.

Description for the table columns


Identification number of the design element. If nothing is specified and if the ID number is not already used, BDK will try to use “SLN Number” = “ID Design Element Number”.


Identification number of the structural line.


Sequence of structural lines.

In the “Sequence of structural lines” column, you can need to define more than one SLN (“,” character is the separator, “-” will take into account all loadcase number “from - to”). Below you will find few input examples.

If you want to create a sequence of structural lines 1 and 2:


If you want to create a sequence of structural lines 1, 2, 10, 11 and 12:



It is highly recomended that you always define a custom design element number for “Sequence of structural lines” (ID) (e.g. 1001)


Length of the structural lines.

Stability Check

Stability check is the type of the buckling check. There are 3 available options:

Eurocode 3:

  • LTB - “Lateral Torsional Buckling acc. to EN 1993-1-1, 6.3.3

  • FLEX - “Flexural/Torsional Buckling acc. to EN 1993-1-1, 6.3.1

  • LAT - “Lateral Buckling” acc. to EN 1993-1-1, 6.3.2

DIN 18800/OEN 4300

  • LTB - “Lateral Torsional Buckling”


Loadcase number for storing the results.

In the LCST column, you can add more than one LCST value (“,” character is the separator, “-” will take into account all loadcase number “from - to”). Below you will find few input examples.

To save the results to loadcase 951 input:


To save the results to loadcases 951 and 953 input:


The results will be saved to loadcases 951, 952, 955, 956, 957, 958:



Start/End Supports

The boundary conditions for START and END of the design element are here explicit defined. Default is “Fork-Fork” (Fork: uy = fix, uz = fix, ϕy = fix).

CVL (Lateral Restraints)

The lateral restraints (Continous and Point) along the design-element are defined in this column.

  • ‘’Reference’’ column:

    • START: The station will be referenced to START of the design element.

    • MID: The station will be referenced to MIDDLE of the design element.

    • END: The station will be referenced to END of the design element.

    • XI: This represents a percentual length of the design element, min. value is = 0, max. value is = 1

      e.g. ‘’Reference’’ = XI and ‘’Station’’ = 0.3 represents a lateral restraint at position 30% of the design element length

    • HDIV: Will create lateral restraints at equal distance e.g. Reference = HDIV and Station = 3, will generate 3 lateral restraints at equal distance.

  • CY/CZ Type column: Spring constant for lateral displacement in local y or z direction (FIX, FREE or <SPRING> value)

  • DX/DZ Type: Rotational spring constat (Torsion) (FIX, FREE or <SPRING> value)

CVM (Intermediate Supports)

Define intermediate supports at position of structural points. The table will show all structural points along the design element. For intermediate supports you can define CB, CY, CZ, DX, DY, DZ boundary conditions (see BDK manual for more details).

β (y-y) and β (z-z)

Buckling factors.

  • If defined, BDK will use the Euler formula for the Ncr calculation (for the calculation of Mcr eigenvalue analysis is always used).

  • If nothing is specified the buckling lengths will be determined automatically (default).


    The boundary conditions should be defined explicitly from the user.

Cross-Section Class

If automatic is defined then the cross-section class will be determined automatically. It is also possible to input the cross-section class manually.

Lateral Torsional Buckling curves

If automatic is defined then the buckling curves will be determined automatically. It is also possible to input the LTB buckling curves (a,b,c,d) manually.

Buckling Factor

Two options are possible (Eurocode 3 only):

  • General Case acc. EN 1993-1-1,

  • LTB Curve for rolled section or equivalent welded sections acc. EN 1993-1-1,

Sway (y-y), Sway (z-z)

If the sway checkboxes is checked, BDK will set the equivalent uniform moments Cmy = 0.90 or Cmz = 0.90 respectively (EN 1993-1-1, Method 2, Table B.3.). If the sway mode is un-checked, then BDK will determine the Cmy and Cmz values according to the Eurocode formulas.

Torsional susceptible

If automatic is selected BDK will try to find automatically if the cross-section is torsional susceptible. The type of the cross-section is checked. Also the moment of inertia It is compared with Iy and Iz. If you want to modify this value explicitly, then checkbox_checked_small (Yes) or checkbox_unchecked_small (No) options are available (acc. to EN 1993-1-1, Method 2 (Annex B), Table B.1 or B.2)