STEEL STRUCTURE DESIGN STANDARDS – TCVN 5575:2012; TCVN 5575:2024 AND STRATEGIC IMPACT

The Vietnamese industrial construction sector is undergoing a strong transformation, driven by foreign direct investment (FDI) inflows and the demand for localizing global supply chains. In this context, the technical legal framework, especially steel structure design standards, acts as a “common language” connecting domestic technical capabilities with rigorous international requirements. This article will delve into the foundation of TCVN 5575:2012, its inherent limitations, and the technical leap provided by the draft TCVN 5575:2024, while placing them in a comparative correlation with global standards such as AISC 360 (USA) and Eurocode 3 (Europe). For general contractors and leading steel structure manufacturers like Hai Long Construction JSC, mastering this transition is not just a matter of compliance but a key factor in asserting their position in large-scale projects.

1. Limit State Design (LSD) in TCVN 5575:2012

TCVN 5575:2012 “Steel Structures – Design Standard” was built based on the Limit State Design (LSD) method, which has deep roots in the SNiP standard system of the former Soviet Union. This method represents a step forward from the classic Allowable Stress Design (ASD) method by considering the randomness of loads and the load-bearing capacity of materials through separate safety factors.

1.1. First Limit State: Ultimate Limit State (ULS)

The Ultimate Limit State is the boundary at which the structure no longer has the ability to resist external impacts, leading to failure, instability, or irreversible plastic deformation. In TCVN 5575:2012, this is strictly controlled through conditions regarding strength and stability.

One of the outstanding features of TCVN 5575:2012 is the use of design strength (𝑓) instead of standard strength directly in the verification formulas. Design strength is determined by dividing standard strength by the material reliability factor.

For the overall stability problem, TCVN 5575:2012 uses a buckling coefficient, which depends on the slenderness ratio and the design strength of the steel. Determination is based on empirical lookup tables or approximate formulas, which were calibrated from test data on Russian and Eastern European steels. This sometimes causes difficulties when applied to steels originating from Japan (JIS), the USA (ASTM), or China (GB), which are common in FDI projects in Vietnam that Hai Long Construction is executing.

1.2. Second Limit State: Serviceability Limit State (SLS)

The second limit state focuses on the normal operation of the structure (Serviceability Limit State), ensuring comfort for users and smooth operation of equipment. Key criteria include deflection and displacement.

A point to note is that TCVN 5575:2012 requires checking deflection under standard loads (without multiplying by the overload factor), while strength checks use design loads.

2. Technical Limitations and the Need for Innovation

Although it has provided a foundation for thousands of projects, TCVN 5575:2012 reveals limitations when facing modern design requirements:

1. Lack of high-strength steel grades: The old standard has not fully updated the mechanical properties of high-strength steels (such as Q345, Q355, Grade 50, S355) which are becoming increasingly popular. Extrapolating calculation formulas for these steels can lead to safety errors or material waste.
2. Stability analysis method: TCVN 5575:2012 is mainly based on the Effective Length Method (ELM). This method assumes that members work independently and are connected by ideal boundary conditions. In reality, with complex frame systems and large spaces like airport terminals or large-span industrial factories, the interaction between elements and the influence of support deformation require more advanced analysis methods such as the Direct Analysis Method (DAM) that AISC has applied since 2005.
3. Units and symbols: The lack of consistency and clarity in units of measurement (kgf/cm2 vs. MPa) and symbols that are not synchronized with the international ISO standard makes technical communication with foreign partners difficult.

3. TCVN 5575:2024 – A Turning Point for International Integration

The introduction of TCVN 5575:2024 (replacing TCVN 5575:2012) marks an important turning point, bringing the Vietnamese standard closer to international benchmarks such as ISO and Russia's SP 16.13330.2017 (modernized version).

3.1. Section Classification

One of the revolutionary new points of TCVN 5575:2024 is the classification of sections into 3 classes (Class 1, 2, 3), similar to Eurocode 3.6

• Class 1 (Plastic): Sections can form plastic hinges and maintain load-bearing capacity after reaching the yield limit, allowing for internal force redistribution. This is extremely important for seismic design and material optimization.
• Class 2 (Plastic-elastic): The cross-section can reach the yield limit, but plastic rotation capacity is limited by local buckling.
• Class 3 (Elastic): Local buckling occurs before the entire cross-section reaches the yield limit.

This change allows Hai Long engineers to maximize the material's performance. For example, with built-up beams commonly used in pre-engineered steel buildings, designing sections to Class 1 or 2 allows for the use of plastic moments instead of elastic moments, increasing the calculated load-bearing capacity by approximately 10-15% without increasing steel weight.

3.2. Addition of the Bimoment Concept

TCVN 5575:2024 introduces, for the first time, the concept and calculation formulas for Bimoment and sectorial coordinates, addressing the issue of warping torsion in open thin-walled members (such as I, C, and Z-shaped steel). This is a critical factor when designing purlin systems, large-span crane beams, or members subjected to eccentric loads, where normal stress due to warping torsion can account for a significant portion of the total stress. Omitting this in the old standard was a potential safety loophole that the 2024 version has thoroughly addressed.

3.3. Updates to Materials and Safety Factors

The new standard expands the list of permitted steel grades, including high-strength steel and imported steel according to international standards (Appendix M), creating a clear legal framework for using diverse materials from global supply sources. Simultaneously, reliability factors have been adjusted to more accurately reflect failure probabilities, based on modern statistical data and compatibility with TCVN 2737:2023 regarding loads.

Table 1: Comparison of Technical Parameters between TCVN 5575:2012 and TCVN 5575:2024

4. Comparison with International Standards: AISC and Eurocode

Understanding the similarities and differences between TCVN and international standards is a competitive advantage for Hai Long Construction when bidding for international projects.

• AISC 360 (USA): Uses the LRFD (Load and Resistance Factor Design) method. The biggest difference is that AISC uses the Direct Analysis Method (DAM), where member stiffness is reduced (usually to 0.8) to account for the effects of residual stress and initial geometric imperfections directly in the internal force analysis step, rather than using the effective length factor as in TCVN.3 TCVN 5575:2024 has begun to approach this method but still retains the foundation of the effective length method for simple cases.
• Eurocode 3 (Europe): Very strong in the theory of section classification and stability analysis. The concept of “Buckling Curves” in Eurocode is more detailed than in TCVN, clearly distinguishing between different types of sections (hot-rolled, welded built-up, thick-walled, thin-walled) to apply reduction factors for load-bearing capacity. TCVN 5575:2024 has adopted this section classification mindset, making it easier for engineers familiar with Eurocode to adapt to the new Vietnamese standard.

5. Conclusion

The transition from TCVN 5575:2012 to TCVN 5575:2024 is not just a change in numbers but a revolution in design thinking. From designing based on experience and fixed lookup tables, the Vietnamese steel structure industry is moving towards performance-based design. For Hai Long Construction, early mastery and proficient application of TCVN 5575:2024, combined with deep knowledge of AISC and Eurocode, will be the key to conquering highly complex projects, optimizing costs for investors, and ensuring absolute safety for structures.