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

Vietnam's industrial construction sector is undergoing a significant transformation, driven by foreign direct investment (FDI) and the need to localize global supply chains. In this context, the technical legal framework, particularly steel structure design standards, plays a crucial role as a "common language" connecting domestic technical capabilities with stringent international requirements. This article will delve into the foundations of TCVN 5575:2012, its inherent limitations, and the technical leap forward offered by the draft TCVN 5575:2024, while also comparing them with global standards such as AISC 360 (USA) and Eurocode 3 (Europe). For leading general contractors and steel structure manufacturers like Hai Long Construction Joint Stock Company, mastering this shift is not just a matter of compliance, but also a key factor in asserting their position in large-scale projects.

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

TCVN 5575:2012 “Steel Structures – Design Standard” is based on the Limit State Design (LSD) method, which has deep roots in the former Soviet Union's SNiP standards system. This method represents an advancement over the classical 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 Load Capacity (ULS)

The ultimate limit state is the boundary at which a structure can no longer withstand external forces, leading to failure, instability, or irreversible plastic deformation. In TCVN 5575:2012, this is strictly controlled through conditions for strength and stability.

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

For the overall stability problem, TCVN 5575:2012 uses a longitudinal bending coefficient, which depends on the slenderness ratio and design strength of the steel. The determination is based on empirical lookup tables or approximate formulas, which are adjusted from test data on Russian and Eastern European steels. This sometimes poses 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 undertaking.

1.2. Second Limit State: Terms of Use (SLS)

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

One point to note is that TCVN 5575:2012 requires checking deflection under standard load (without multiplying by the overload factor), while strength testing uses design load. 

2. Technical Limitations and the Need for Innovation

Despite having provided the foundation for thousands of projects, TCVN 5575:2012 reveals limitations when faced with modern design requirements:

1. Shortage of high-strength steel grades: Outdated standards do not fully reflect the mechanical properties of increasingly common high-strength steels (such as Q345, Q355, Grade 50, S355). Extrapolating calculation formulas for these types of steel can lead to safety errors or material waste.
2. Stability analysis method: TCVN 5575:2012 is primarily based on the Effective Length Method (ELM). This method assumes that structural elements work independently and are connected by ideal boundary conditions. In practice, for complex, large-space frame systems such as airport terminals or large-span industrial plants, the interaction between elements and the influence of support deformation require more advanced analysis methods such as the Direct Analysis Method (DAM), which AISC has been applying since 2005.
3. System of units and symbols: The lack of consistency and clarity in measurement units (kgf/cm2 versus MPa) and symbols that are not standardized with the international ISO standard make 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 a significant turning point, bringing Vietnamese standards closer to international standards such as ISO and the Russian SP 16.13330.2017 (modernized version).

3.1. Section Classification

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

• Level 1 (Class 1 – Flexible): The cross-section can form plastic hinges and maintain load-bearing capacity after reaching the yield limit, allowing for redistribution of internal forces. This is extremely important for seismic design and material optimization.
• Level 2 (Flexible Piano): The cross-section may reach the yield limit, but the plastic rotation capability is limited by local instability.
• Level 3 (Class 3 – Elasticity): Local instability occurs before the entire cross-section reaches its yield limit.

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

3.2. Supplementing the Concept of Bi-moment 

TCVN 5575:2024 is the first to introduce the concept and calculation formula for Bimoment and fan coordinates, solving the warping torsion problem in open thin-walled structural elements (such as I-shaped, C-shaped, and Z-shaped steel). This is a critical factor when designing large-span girder systems, crane beams, or components subjected to eccentric loads, where normal stresses due to torsion can account for a significant proportion of the total stress. The omission in the old standard was a potential safety flaw that the 2024 version has thoroughly addressed.

3.3. Updating Materials and Safety Factors

The new standard expands the list of permitted steel grades, including high-strength steel and imported steel conforming to foreign standards (Appendix M), creating a clear legal framework for the use of diverse materials from global sources. At the same time, the reliability coefficients are adjusted to more accurately reflect the probability of failure, based on modern statistical data and compatible with TCVN 2737:2023 on load.

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

4. Comparison with International Standards: AISC and Eurocode

Understanding the similarities and differences between Vietnamese National Standards (TCVN) and international standards is a competitive advantage for Hai Long Construction when bidding on international projects.

• AISC 360 (USA): AISC uses the LRFD (Load and Resistance Factor Design) method. The biggest difference is that AISC uses the Direct Analysis Method (DAM), in which the stiffness of the structural element is reduced (usually by 0.8) to account for the influence of residual stress and initial geometric errors right in the internal force analysis step, instead of using the calculated 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): Eurocode is very strong in the theory of cross-section classification and stability analysis. The concept of "Bucklel Curves" in Eurocode is more detailed than TCVN, clearly distinguishing different types of cross-sections (hot-rolled, welded composite, thick-walled, thin-walled) to apply load-bearing reduction factors. TCVN 5575:2024 has adopted this cross-section classification approach, 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 design based on experience and fixed lookup tables, the Vietnamese steel structure industry is moving towards design based on the actual behavior of materials and structures (performance-based design). For Hai Long Construction, early grasp and proficient application of TCVN 5575:2024, combined with a deep understanding of AISC and Eurocode, will be key to conquering highly complex projects, optimizing costs for investors, and ensuring absolute safety for the construction.