Support Conditions for Lateral-Torsional Buckling

Technical Article on the Topic Structural Analysis Using Dlubal Software

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Technical Article

A member's boundary conditions decisively influence the elastic critical moment for lateral-torsional buckling Mcr. The program uses a planar model with four degrees of freedom for its determination. The corresponding coefficients kz and kw can be defined individually for standard-compliant cross-sections. This allows you to describe the degrees of freedom available at both member ends due to the support conditions.

The effective length coefficient kz controls the lateral displacement uy and the rotation φz at the member ends. The following options are available in the list of the table column:

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

The warping length factor kw controls the torsion about the member's longitudinal axis ϕx and the warping ω. The list offers the following options:

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

The abbreviations "li" and "ri" refer to the left and right member end. The abbreviation "le" always describes the support conditions at the start of the member.

Cantilever Example

A cantilever is subjected to a moment and an axial force.

In design case 1, the support conditions are defined like for a single-span beam with end fork conditions: kz = 1.0 and kw = 1.0. This results in an elastic critical moment for lateral-torsional buckling of 761.14 kNm.

The mode shape shows the lateral-torsional buckling behavior of a single-span beam.

In design case 2, the cantilever's support conditions are defined correctly: kz = 2.0le and kw = 2.0le. The program determines a significantly smaller critical moment of 371.72 kNm.

The mode shape corresponds to that of a cantilever.

Author

Dipl.-Ing. (FH) Robert Vogl

Dipl.-Ing. (FH) Robert Vogl

Technical Editor, Product Engineering & Customer Support

Mr. Vogl creates and maintains the technical documentation. In addition, he is involved in the development of the SHAPE-THIN program and provides customer support.

Keywords

Buckling length coefficient Warping length factor Boundary condition Degree of freedom Elastic critical moment for lateral-torsional buckling Warping Critical moment

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  • Updated 13 July 2021

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