What are buckling failure modes?

In our previous articles on beams, we discussed various types of deflection and how we go about calculating it in various structures.  We’re going to pivot into buckling and how it can affect structures.

What is buckling?

Buckling occurs when a structure, such as a column or strut, is under compression and where a ‘critical’ load is exceeded.  The leads to buckling of the column.  Buckling is a failure limit-state for columns.

Buckling has been recognised over several years in certain design areas, notably cold-formed steel sections and aluminium. There are occasions where similar behaviour is observed in hot-finished sections, particularly with thin flanges and webs where the d/t ratio is high. In these cases, the designer must allow for the reduction in strength.

Buckling in Structural Engineering

In structural engineering, buckling is the sudden change in shape of a strut or column under load, for example, bowing under. An example of column buckling can be seen in the water reservoir shown below:

Because of the characteristics of the materials, the causes of failure of steel and reinforced concrete columns are often different.  Steel columns normally fail due to buckling as they are mostly slender with a small area, which, due to the good mechanical characteristics of steel and the ability to shape the cross section, can withstand higher loads with a small consumption of material, but as a result, slender elements are sensitive to buckling.

Reinforced concrete columns are often rectangular or circular, which makes them less susceptible to buckling. Considering the production technology, the possibility of imperfections in reinforced concrete columns is greater than in steel columns, and because of that the most common type of failure of these columns is due to the combined action of axial force and bending moment. Some examples of buckling in structures are seen in the images below:

BS 5950

The rules in BS 5950 that surround buckling are set out so that an engineer can assess the carrying capacity in various situations to determine where local buckling could occur.

They have been grouped into four classes:

• Class 1: plastic cross-sections where all elements subject to compression have plastic hinge rotations. A plastic hinge can be developed with sufficient rotation capacity to allow the redistribution of moments within the structure.

• Class 2: Compact cross-sections. The full plastic moment capacity can be developed, but local buckling may prevent the development of a plastic hinge with sufficient rotation capacity to permit plastic analysis and design.

• Class 3: Semi-compact sections. The stress at the extreme fibres can reach the design strength, but local buckling may prevent the development of the full plastic moment.

• Class 4: Slender sections are those that contain slender elements subject to compression due to moment or axial load. Local buckling may prevent the stress in a slender section from reaching the design strength.

We’re going to go into more detail about buckling and what influences it in further articles, so make sure you keep an eye out.

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