Column Buckling per Eurocode 3 (EN 1993-1-1)

Beam-columns are structural members subjected to combined axial load and bending moment. Buckling is a phenomenon where a member under compression suddenly deflects laterally. This guide explains the different types of buckling that can occur in beam-columns: flexural, torsional, and flexural-torsional buckling, according to Eurocode 3 (EN 1993-1-1).

Types of buckling

Flexural buckling

Flexural buckling, also known as Euler buckling, is the primary mode of buckling for columns. It involves bending about the axis with the least resistance (i.e., the smallest moment of inertia). The weak axis can be supported with lateral restraints to increase the buckling capacity of the column.

Torsional buckling

Torsional buckling occurs when a member twists about its longitudinal axis. This is more common in members with open cross-sections, such as channels and angles.

Torsional-flexural buckling

Torsional-flexural buckling is a combination of flexural and torsional buckling. It occurs when a member bends and twists simultaneously. This is also more common in members with open cross-sections that have low torsional stiffness. It can be the critical buckling mode if flexural buckling about the weak axis is constrained with lateral restraints.

Critical buckling length

The critical buckling length (L_cr) is the effective length of a column that is used to calculate its buckling resistance. It depends on the actual length of the column and the end conditions.

The critical buckling length can be calculated as:

Where:

• k is the effective length factor, which depends on the end conditions of the column.
• L is the actual length of the column.