Buckling Class of Cross Sections
Buckling is one of the most critical failure modes in compression members such as columns, struts, truss members, and compression flanges of beams. The buckling strength of steel sections depends not only on slenderness ratio but also on the geometry of the cross section, fabrication method, residual stresses, and axis of buckling.
IS 800:2007 classifies steel sections into different buckling classes to determine the design compressive stress and buckling curves for compression members.
What is Buckling?
Buckling is the sudden lateral deflection of a compression member when subjected to compressive loads beyond a critical value. Even if the material stress is below yield stress, slender members may fail due to instability.
Buckling Classes in IS 800:2007
IS 800:2007 categorizes compression members into four buckling classes:
| Buckling Class | Buckling Curve | Imperfection Level | Performance |
|---|---|---|---|
| a | Best Buckling Curve | Lowest Imperfection | Highest Buckling Strength |
| b | Moderate Curve | Low Imperfection | Good Buckling Resistance |
| c | Higher Imperfection Curve | Moderate Imperfection | Moderate Buckling Strength |
| d | Lowest Curve | Highest Imperfection | Lowest Buckling Strength |
Classification of Cross Sections
| Cross Section | Limits | Buckling About Axis | Buckling Class |
|---|---|---|---|
| Rolled I-Section | h/bf > 1.2 and tf ≤ 40 mm | z-z | a |
| y-y | b | ||
| 40 mm < tf ≤ 100 mm | z-z | b | |
| y-y | c | ||
| Rolled I-Section | h/bf ≤ 1.2 and tf ≤ 100 mm | z-z | b |
| y-y | c | ||
| tf > 100 mm | z-z | d | |
| y-y | d | ||
| Welded I-Section | tf ≤ 40 mm | z-z | b |
| y-y | c | ||
| tf > 40 mm | z-z | c | |
| y-y | d | ||
| Hollow Sections | Hot Rolled | Any Axis | a |
| Channel, Angle, Tee and Solid Sections | — | Any Axis | c |
| Built-up Members | — | Any Axis | c |
Understanding Important Terms
1. h/bf Ratio
The ratio of section depth (h) to flange width (bf) influences the buckling behavior and stiffness of I-sections.
2. Flange Thickness (tf)
Thicker flanges generate higher residual stresses during rolling or welding, which affects buckling strength.
3. Buckling Axis
- z-z Axis: Major axis buckling
- y-y Axis: Minor axis buckling
Why Different Buckling Classes are Used?
Different steel sections behave differently under compression because of:
- Residual stresses from fabrication
- Initial geometric imperfections
- Cross-sectional shape
- Welding effects
- Load eccentricity
- Variation in stiffness about different axes
Rolled I-Sections
Rolled I-sections generally provide better buckling performance because rolling produces lower residual stresses compared to welding.
Advantages
- Higher buckling resistance.
- Better material uniformity.
- Lower residual stresses.
- Suitable for heavy compression members.
Welded I-Sections
Welded I-sections are fabricated using plates and welding processes. These sections allow larger custom sizes but are more sensitive to residual stresses.
Characteristics
- Higher residual stresses due to welding.
- Reduced buckling performance.
- Used in large built-up sections.
- Economical for heavy girders.
Hollow Sections
Hot rolled hollow sections belong to buckling class “a” because they provide excellent symmetry and uniform stress distribution.
Advantages
- Excellent buckling resistance.
- Uniform torsional stiffness.
- Better compression behavior.
- Architectural appearance.
Effect of Buckling Class on Design Strength
Buckling class directly affects the design compressive stress obtained from buckling curves in IS 800:2007.
Where:
- Pd = Design compressive strength
- Ae = Effective cross-sectional area
- fcd = Design compressive stress
Comparison of Buckling Classes
| Buckling Class | Buckling Resistance | Residual Stress Level | Typical Sections |
|---|---|---|---|
| a | Very High | Very Low | Hot Rolled Hollow Sections |
| b | High | Low | Rolled I-Sections |
| c | Moderate | Moderate | Angles, Channels, Built-up Members |
| d | Low | High | Heavy Welded Sections |
Applications in Structural Engineering
- Steel columns
- Bridge compression members
- Industrial structures
- Transmission towers
- Roof trusses
- Built-up compression members
Conclusion
Buckling class is a very important concept in steel design because it governs the buckling strength of compression members. Different cross sections and fabrication methods produce different levels of imperfections and residual stresses, which affect column behavior. Understanding buckling classes helps engineers select appropriate sections for safe, stable, and economical structural design according to IS 800:2007.
References
- IS 800:2007 – General Construction in Steel
- Steel Structures by N. Subramanian
- Limit State Design of Steel Structures by S.K. Duggal
- Structural Stability of Steel by Theodore V. Galambos
- Design of Steel Structures by Ramchandra
Disclaimer
This article is intended for educational and informational purposes only. Engineers should refer to the latest IS codes, design manuals, and professional engineering judgment before performing actual structural design calculations.
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