Steel Column Design – Compression Resistance (Eurocode 3)

Dr Adewale Abimbola, FHEA, GMICE

www.edulibrary.co.uk

Aim and Objectives

• Aim: To determine compression resistance of a column section using the Eurocode 3.
• Objectives: At the end of the lesson, the students should be able to:
• Describe the concepts of slenderness ratio and effective length.
• Determine the critical buckling length of steel columns.
• Calculate the maximum design axial load of steel columns with respect to flexural buckling.

Slenderness and Critical/Effective Buckling Length

Classification of Column Based on Slenderness

• Compressive elements, such as columns, can be sub-divided into three groups: short, long/slender, intermediate.

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Short Column

• The slenderness ratio (ratio of effective length to the least lateral dimension) is less than 30.
• Critical stress is high.
• Short columns fail due to crushing or yielding of the steel bars.
• The loads that a short column may support depend on the dimension of cross-section and the strength of materials. Short columns show a little flexibility.

Figure 1. Average stress in columns versus slenderness ratio (Ghosh, 2014).

Critical stress is the maximum compressive stress a column can carry before it suddenly buckles sideways instead of being crushed.

Classification of Column Based on Slenderness

Long/Slender Column

• The slenderness ratio exceeds 100 in long columns. This type of column is also known as the slender column.

• The critical stress is low

• As the slenderness increases, bending deformation increases.

• Long column fails due to buckling effect which reduces load-bearing capacity.

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Intermediate Column

• The slenderness ratio is in between 35 and 100.

• Critical stress is similar to material strength. The stress that causes the column to buckle is about the same as the stress that would cause the material itself to start failing.

• The failure of this element is neither short nor slender. It occurs by a combination of buckling and yielding/crushing.

Figure 1. Average stress in columns versus slenderness ratio (Ghosh, 2014).

Critical stress is the maximum compressive stress a column can carry before it suddenly buckles sideways instead of being crushed.

Steel Column Design – Compression Resistance

• Step 1: Determine the section classification by using Table 5.2 of Eurocode 1993-1-1 OR use the Blue Book – Steel Design Data

Table 1. Maximum width-to-thickness ratios for compression parts

Table 2. Maximum width-to-thickness ratios for compression parts

• Step 2: Calculate the non-dimensional slenderness term.

In Eurocode, Universal Columns (UC) are placed into Classes 1 to 4 depending on how thick or thin their plates are and how easily they buckle under load.�

Class 1 – Plastic UC

Thick flanges and web.

Can carry full load and deform safely without buckling.

Very strong and forgiving.

👉 Typical UC: Heavy UC sections used in building frames.

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Class 2 – Compact UC

Slightly thinner than Class 1.

Can still carry full strength, but with less bending capacity.

Safe but less flexible.

👉 Typical UC: Standard UC sizes in multi-storey buildings.

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Class 3 – Semi-Compact UC

Thinner plates.

Can only be designed to elastic strength.

Buckling starts before full strength is reached.

👉 Typical UC: Lighter UC sections with moderate loads.

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Class 4 – Slender UC

Very thin web or flanges.

Buckles early, even at lower loads.

Engineers must reduce the effective section size in design.

👉 Typical UC: Rare for UC sections, more common in light steel sections.

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Easy Way to Remember

Class 1 → Bend a lot. Safest and strongest.

Class 2 → Bend a bit. Strong, less flexible.

Class 3 → Little bending. Must stay elastic.

Class 4 → Buckles early. Reduced strength.

• Step 3: Select buckling curve type from Figure 2 by using Table 3 and non-dimensional term from step 2.

Table 3. Selection of buckling curve for a cross-section ((Eurocode 1993- 1-1)

Figure 2. Buckling curves

• Step 4: Use maximum non-dimensional slenderness term from Step 2 and buckling curve/mode from Table 3 to determine the reduction factor.

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• Step 5: Use reduction factor to calculate the maximum design axial load.

Worked Example

Table 4. Section properties. Data obtained from the Blue Book (Pages B-8 & B-9)

Solution

Table 3. Selection of buckling curve for a cross-section ((Eurocode 1993- 1-1)

Figure 2. Buckling curves

Self-assessment Task 1

Table 5. Section properties. Data obtained from the Blue Book (Pages B-8 & B-9)

Self-assessment Task 2

Table 6. Section properties. Data obtained from the Blue Book (Pages B-8 & B-9)

Symbols

References /

Bibliography

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• Draycott, T. and Bullman, P. (2009) Structural elements design manual – working with eurocodes. 2^nd edn. Oxford: Elsevier Ltd.

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• Ghosh, B. (2014) 2E4: solids & structures lecture 15. Available at: https://www.slideserve.com/teddy/2e4-solids-structures-lecture-15 (Accessed: 03 February 2024).
• McKenzie, W. M. C. (2013) Design of structural elements to eurocodes. 2^nd edn. London: Red Globe Press.
• The Steel Construction Institute and The British Constructional Steelwork Association Ltd (2015). Steel building design: design data. Available at: https://www.steelconstruction.info/images/b/b7/SCI_P363.pdf (Accessed: 11 January 2024)

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