Classification of UB Section as per BS 5950
The use of structural steel becoming more and more popular with the development of technology in this field and due to the ease of the construction.
Construction time is one of the major factors that people are looking into when selecting the type of material to be used. As a common understanding, concreting buildings takes much more time for completion than the steel building.
The availability of pre-fabrication methods and ease of installations have made the steel construction more popularized.
Classification of the Steel Beam section is discussed in this article. A section can be classified under four categories based on stage where it fails. Mainly, this selection is done based on the aspect ratios.
- Class 01 – Plastic Section
- Class 02 – Compact Section
- Class 03 – Semi Compact Section
- Class 04 – Slender Section
Variation of the stress in a section can be represented as shown in Figure 1.
Based on section classification, the equation to be used to calculate the bending capacity is vary. Mainly, the section modulus changes in each class.
| Class | Bending Moment Capacity |
| Class 4 Slender | M = σ Zxx |
| Class 3 Semi-Compact | M = py Zxx |
| Class 1 & 2 Plastic and Compact | M = Py Sxx |
The behavior of the section can be illustrated as shown in Figure 2. Variation of the rotation of the section with the bending moment is indicated in Figure 2.
As discussed previously, the variation of the moment rotation of the section clearly indicates the behaviour of section and its development of bending moment with the rotation of the section.
Plastic Section
Sections that reaches the plastic moment (Mp) and retain the bending the moment with the formation of the plastic hinge as shown in Figure 2. These types of sections are classified uner Class 1.
Compact Section
Sections that reaches the plastic moment (Mp) and unable to retain the plastic moment due to the local buckling are classified under Class 2 – Compact Section.
Semi-Compact Section
Sections do not reach the plastic moment (Mp) due to the local buckling of one or more of the element in the cross-section are called semi-compact sections, and they are classified under Class 3. Further, extreme fibre could reach the design strength before local buckling occurs.
Slender Section
Sections do not reach the design strength due to the local buckling as a result of higher aspect ratios are classified under Class 4 – Slender Sections
SECTION CLASSIFICATION
When
b/T < 9ε – Flange is Plastic
d/t < 80ε – Web is Plastic
When
b/T < 10ε – Flange is Compact
d/t < 100ε – Web is Compact
When
b/T < 15ε – Flange is Semi-Compact
d/t < 120ε – Web is Semi-Compact
where all the notation are as per the code and
ε = (275/Py)0.5
Based on the section data, classification of universal beam section can be done. Similarly, other sections having axial loads, etc. also can be classified as given in Table 11 of BS 5950: 2000.
