In-Depth Analysis and Application Guide for TCVN 9386:2012 in Seismic Design in Vietnam

Part 1: Executive Summary: Decoding TCVN 9386:2012 for Engineering Practice

National Standard TCVN 9386:2012, “Design of structures for earthquake resistance,” is the foundational technical and legal document governing all seismic design activities for construction projects in Vietnam. Developed based on the adoption of the European Standard Eurocode 8, TCVN 9386:2012 embodies a modern, performance-based design methodology focused on the performance and deformation capacity of structures. However, the very complexity and high scientific nature of this standard have posed significant challenges to its practical application in Vietnam.

This report identifies a considerable gap between the standard’s rigorous theoretical requirements and common design practices. Core issues include the incorrect selection and application of structural ductility classes, particularly the trend of overusing Ductility Class Low (DCL) for projects in high seismic risk zones to avoid complex detailing requirements, leading to potential safety risks. Furthermore, the assessment and calculation of seismic loads in low seismicity cases are often overlooked or misinterpreted.

These challenges are further complicated by the fact that TCVN 9386:2012 cannot be applied in isolation. It is part of a tightly integrated technical-legal ecosystem, requiring designers to cross-reference and simultaneously comply with regulations in the National Technical Regulation QCVN 02:2022/BXD (providing ground acceleration data) and Circular 06/2021/TT-BXD (classifying building importance to determine the importance factor). A lack of connection or incomplete understanding of this relationship is a common source of design errors.

Against this backdrop, this report offers key recommendations, emphasizing the need for a comprehensive and systematic approach. The report’s centerpiece is the development and proposal of a detailed “Integrated Application Model,” presented in Part 8. This model provides a step-by-step, logical workflow to help structural engineers accurately pinpoint requirements, correctly look up parameters from relevant legal documents, and make compliant and safe design decisions. The adoption of this model is expected to minimize risks, enhance consistency, and improve the quality of seismic design work.

Finally, the report also provides a glimpse into the future of seismic design in Vietnam by analyzing the expected content of the TCVN 9386:2023 revision. This new version promises to address some of the current standard’s limitations, notably by introducing a more appropriate response spectrum curve for low seismic hazard zones, thereby optimizing design requirements.

Part 2: The Foundation of TCVN 9386:2012: Origin, Objectives, and Scope

2.1. History and Adaptation

The National Standard TCVN 9386:2012 is the result of a direct transition from the previous industry standard, TCXDVN 375:2006. In essence, the technical content of these two standards is identical. The core foundation of TCVN 9386:2012 is built upon the selective adoption of the European Standard Eurocode 8: “Design of structures for earthquake resistance,” one of the most advanced seismic design standards in the world. Specifically, the Vietnamese standard adopted the two most critical parts of Eurocode 8:

• Part 1 of TCVN 9386:2012 corresponds to EN 1998-1: “General rules, seismic actions and rules for buildings.”
• Part 2 of TCVN 9386:2012 corresponds to EN 1998-5: “Foundations, retaining structures and geotechnical aspects.”

Although the main methodology was inherited from Eurocode 8, TCVN 9386:2012 was adapted to fit Vietnam’s specific context through national annexes. These annexes replace or supplement locally specific content, including:

• Annex E: Provisions for Importance Classes and importance factors of buildings.
• Annex F: Guidance on the classification of construction projects.
• Annexes G and H: Provided seismic zoning maps and tables by administrative district (now superseded by QCVN 02:2022/BXD).

The combination of an international theoretical framework and local input data created a highly scientific standard. However, its application reveals an underlying challenge. TCVN 9386:2012, with its Eurocode origins, was designed to operate within a synchronized Eurocode system (e.g., Eurocode 2 for concrete, Eurocode 3 for steel). Meanwhile, Vietnam’s corresponding material standards, such as TCVN 5574 (Concrete and Reinforced Concrete Structures), originate from the former Soviet standards system, which has a different design philosophy and detailed provisions. This creates a “philosophical incompatibility,” where engineers must apply the modern seismic principles of TCVN 9386 while adhering to detailing and material rules from standards that are not fully compatible. This situation demands flexible interpretation and application from designers but also carries the potential for inconsistent application and design risks.

2.2. Fundamental Performance Requirements

The design philosophy of TCVN 9386:2012 is built on two fundamental performance requirements, ensuring the structure behaves appropriately under different levels of earthquakes:

• No-collapse requirement (Ultimate Limit State): This is the most critical requirement, aimed at protecting human life. The standard mandates that the structure must be designed and detailed to withstand the design seismic action without local or global collapse, maintaining its structural integrity and residual load-bearing capacity after the seismic event. The design seismic action for this requirement corresponds to a 10% probability of exceedance in 50 years, equivalent to a 475-year return period.
• Damage limitation requirement (Serviceability Limit State): This requirement aims to limit economic losses and ensure the continuous operation of critical facilities. The structure must be designed to withstand a more frequent (and thus less intense) seismic action without incurring damage that affects its use or repair costs that are disproportionate to the structure’s value. This seismic action has a 10% probability of exceedance in 10 years, equivalent to a 95-year return period.

2.3. Scope and Applicability

TCVN 9386:2012 applies to the design of new buildings and civil engineering works in seismic regions.7 The standard’s scope is comprehensive, covering everything from the superstructure to geotechnical elements, including:

• Building structures: Detailed rules for common structural systems such as reinforced concrete, steel, composite structures, timber, and masonry.
• Foundations and geotechnical aspects: Includes design requirements for shallow foundations, pile foundations, retaining walls, and soil-structure interaction analysis.

An important point to emphasize is that TCVN 9386:2012 does not replace other material design standards (like TCVN 5574 for reinforced concrete). Instead, it serves as a supplementary standard, providing specific rules and requirements for seismic aspects that other standards do not cover.

Part 3: Core Principles of Seismic Design: An In-Depth Analysis of Principles in TCVN 9386:2012

3.1. Conceptual Design Principles

Before diving into quantitative calculations, TCVN 9386:2012 emphasizes the importance of conceptual design. A rational structural scheme, conceived from the earliest stages, provides far superior and more economical seismic performance than merely increasing member sizes and reinforcement percentages. These principles include:

• Structural simplicity: Prioritizing structural systems with clear and direct load paths from the point of inertia load application down to the foundation. Simplicity minimizes uncertainties in modeling and predicting the structure’s behavior during an earthquake.
• Regularity, symmetry, and redundancy:
  • Regularity: A uniform distribution of mass, stiffness, and resistance in both plan and elevation helps avoid the formation of “weak stories” or “soft stories” and minimizes stress concentrations.
  • Symmetry: A symmetrical layout of load-bearing elements helps the centers of stiffness and mass on each floor to nearly coincide, thereby limiting adverse torsional effects.
  • Redundancy (Hyperstaticity): Increasing the structure’s degree of redundancy creates multiple alternative load paths, allowing for force redistribution and enhancing energy dissipation capacity as some members begin to yield.
• Bi-directional resistance and stiffness: An earthquake is a spatial event; therefore, the structural system must have sufficient resistance and stiffness in both orthogonal directions in plan.
• Torsional resistance: In addition to a symmetrical layout, placing the main seismic-resisting elements (like shear walls or cores) near the building’s perimeter significantly increases torsional stiffness and overall seismic efficiency.
• Diaphragm behavior of floors: Floors and roofs act as rigid horizontal diaphragms, responsible for collecting inertia forces at each level and distributing them to the vertical lateral load-resisting system (frames, walls).
• Adequate foundation: The foundation system must be designed to ensure it can uniformly transmit the actions from the superstructure to the ground. Individual footings must be interconnected by tie-beams in both directions.

3.2. Ductility and Energy Dissipation: Ductility Classes

Modern seismic design philosophy does not aim to keep the structure fully elastic during a strong earthquake, as that would be extremely costly. Instead, the standard permits the structure to deform beyond its elastic limit (plastic deformation) in a controlled manner to dissipate the earthquake’s energy. This plastic deformation capacity is known as “ductility.” TCVN 9386:2012 classifies structures into three ductility classes, which determine the design strategy and the complexity of detailing requirements:

• DCL (Ductility Class Low): Applied to structures with limited energy dissipation capacity. The design is primarily strength-based (elastic behavior) and does not require special seismic detailing.
• DCM (Ductility Class Medium): Applied to structures designed to have significant energy dissipation capacity through the formation of plastic hinges at predetermined locations. Design in this class requires adherence to “capacity design” principles and strict reinforcement detailing rules to ensure the plastic hinge zones can perform as intended. This is the most common ductility class for ordinary buildings in seismic zones.
• DCH (Ductility Class High): Applied to structures requiring very high energy dissipation capacity. This class demands the most stringent analysis and detailing rules.

The choice of ductility class is a strategic decision that profoundly impacts the entire design process, construction cost, and the structure’s safety level. However, a worrying trend has emerged in Vietnamese practice: engineers intentionally selecting DCL even for buildings in high seismic zones (a_g ≥ 0.08g), where the standard would require DCM or DCH.4 The purpose of this is to evade the complex requirements of capacity design and the strict seismic detailing rules of DCM. This practice creates a severe potential hazard. The structure is calculated for a large seismic force (due to DCL’s low behavior factor, q), but it lacks the necessary ductility to survive a real earthquake. The result is a stiff but brittle structure, which is at risk of sudden failure rather than deforming and dissipating energy as the standard’s philosophy intends.

3.3. Behavior Factor (q)

The behavior factor, denoted as q, is a core parameter in linear elastic analysis. In essence, it is a force reduction factor that allows the calculated elastic seismic force to be reduced to a lower level for design.18 The q-factor reflects the structure’s capacity to dissipate energy through non-linear behaviors, such as plastic deformation and other damping mechanisms.

The value of q is directly dependent on the chosen ductility class and the structural system type. A higher q-factor (e.g., q = 3.9 for a DCM reinforced concrete frame system) implies the structure has high ductility, permitting a significant reduction in design forces. However, to justify using a high q-factor, the designer must demonstrate that the structure possesses sufficient ductility by strictly adhering to the analysis and detailing requirements corresponding to that ductility class. Conversely, for DCL, the assumed ductility is very low, so the q-factor is limited to a small value (e.g., q = 1.5).

3.4. Primary and Secondary Seismic Members

TCVN 9386:2012 allows structural members to be classified into two groups to simplify the analysis model 2:

• Primary seismic members: These are the members designed and detailed to participate in the seismic load-resisting system.
• Secondary seismic members: These are members not considered part of the seismic-resisting system. They are still designed to carry gravity loads, but their stiffness is neglected in the seismic analysis model.

This classification is only permitted if two strict conditions are met:

• The total stiffness of all secondary seismic members does not exceed 15% of the total stiffness of all primary seismic members.
• The secondary seismic members and their connections must be designed to maintain their capacity to support gravity loads when subjected to the displacements caused by the design seismic action.

This approach helps focus on the most critical elements of the seismic system but requires careful verification to ensure the assumptions are valid.

Part 4: Quantitative Analysis: A Comprehensive Guide to Seismic Load Calculation and Structural Modeling

4.1. Step 1: Defining the Seismic Action

This is the first and most crucial step in any seismic calculation process. The seismic action is characterized by the design ground acceleration, denoted as a_g. This parameter is determined through a multi-step process that requires referencing several different regulatory documents.

Governing Formula:

The design ground acceleration a_g is determined by formula 19:

a_g = γ_I × a_gR

Where:

• a_gR is the reference peak ground acceleration on Type A ground.
• γ_I is the importance factor of the building.

Procedure for Determining Parameters:

• 1. Determine the Reference Peak Ground Acceleration (a_gR):
  • Source: The value of a_gR is found in Table 6.1 of the National Technical Regulation QCVN 02:2022/BXD.19 This regulation provides a_gR values (usually expressed as a fraction of gravity, g) for every district-level administrative area in Vietnam. This is the mandatory legal document that supersedes the maps in the annexes of TCVN 9386:2012.
• 2. Determine the Importance Factor (γ_I):
  • Step 2a: Classify the Building: First, the engineer must use Circular 06/2021/TT-BXD to determine the “Building Grade” (e.g., Special Grade, Grade I, Grade II, Grade III) based on criteria like scale, height, function, and project impact.
  • Step 2b: Look up γ_I: After obtaining the “Building Grade,” the engineer refers to Annex E of TCVN 9386:2012 to find the corresponding Importance Class and its associated γ_I value. For example, a Grade II building typically corresponds to Importance Class II and has γ_I = 1.0.
• 3. Classify the Ground (Soil) Type:
  • Based on the geotechnical investigation report, the soil at the construction site must be classified into one of the ground types A, B, C, D, or E according to Table 3.1 of TCVN 9386:2012. This classification is based on parameters such as the average shear wave velocity in the top 30m (V_s,30), SPT-N value, or undrained shear strength (c_u).22
  • The ground type determines the soil factor S and the corner periods (T_B, T_C, T_D), which in turn define the shape of the design response spectrum.

4.2. Structural Analysis Methods

TCVN 9386:2012 provides a range of analysis methods with varying levels of complexity and accuracy, allowing the engineer to choose the most appropriate one for the structure.

• Method 1: Equivalent Lateral Force (ELF) Analysis: This is the simplest method, where the seismic action is converted into a set of static forces distributed along the building’s height. This method is only applicable to buildings with limited height and regular shapes in both plan and elevation.
• Method 2: Modal Response Spectrum Analysis (RSA): This is the most common linear-elastic analysis method and is widely applied to most buildings in Vietnam. This method accounts for the contribution of multiple modes of vibration. The total response is combined from the responses of individual modes using rules like the Square Root of the Sum of Squares (SRSS) or the Complete Quadratic Combination (CQC).
• Method 3: Nonlinear Static (Pushover) Analysis: This is a nonlinear static analysis method where a lateral force pattern is incrementally applied to a nonlinear structural model until a target displacement is reached or the structure fails. This method allows for the assessment of the structure’s actual capacity and ductility, identifies the sequence of plastic hinge formation, and determines the failure mechanism.
• Method 4: Nonlinear Time-History Analysis (NLTHA): This is the most advanced and accurate analysis method, directly simulating the response of the nonlinear structure over time under the action of one (or a set of) actual ground motion records. However, this method requires high-quality input data and significant computational power, making it rarely used in standard design practice in Vietnam.

Below is a detailed comparison table to assist engineers in selecting an analysis method.

Table 4.1: Comparison of Seismic Analysis Methods in TCVN 9386:2012

Method Name	TCVN Reference	Application Conditions	Advantages	Limitations	Typical Use Case in Vietnam
Equivalent Lateral Force (ELF)	4.3.3.2	Buildings regular in elevation with fundamental period T1 ≤ min(4 × TC; 2.0 s).	Simple, intuitive, easy to perform and verify by hand.	Not suitable for tall, irregular, or dynamically complex structures.	Low-rise, regular buildings; preliminary design.
Modal Response Spectrum Analysis (RSA)	4.3.3.3	Applicable to most building types, including irregular ones.	More accurate than ELF, considers multiple modes, the standard method in practice.	Still a linear-elastic analysis, does not directly show nonlinear failure mechanisms.	Most high-rise buildings and public structures.
Nonlinear Static (Pushover)	4.3.3.4.2	Performance assessment, checking existing structures, or when higher accuracy than elastic analysis is needed.	Reflects nonlinear behavior, plastic hinge formation sequence, and deformation capacity.	It is a static analysis; cannot capture dynamic effects over time.	Assessment and retrofit of existing structures; performance-based design for critical projects.
Nonlinear Time-History Analysis (NLTHA)	4.3.3.4.3	Critically important structures, complex structures, or when suitable ground motion records are available.	The most accurate and comprehensive, simulates the structure’s actual behavior over time.	Requires high-quality ground motion records (rare in Vietnam), demands very high analytical and computational capability.	Very rare, mainly in scientific research or for special projects (nuclear power plants, large cable-stayed bridges).

4.3. Key Calculation and Modeling Issues

• Directional Combination: Because an earthquake acts simultaneously in multiple directions, the standard requires combining the response components. The most common rule specified in TCVN 9386:2012 (and Eurocode 8) is the “100% + 30%” rule. Accordingly, the design response is taken as the maximum of two combinations: (1) 100% of the action in the X-direction plus 30% of the action in the Y-direction, and (2) 30% of the action in the X-direction plus 100% of the action in the Y-direction. This method is generally more conservative than the built-in SRSS combination option in software like ETABS.
• Displacement Calculation: Linear elastic analysis yields the elastic displacement (d_e). However, the actual displacement of the structure as it yields (d_s) will be much larger. The standard provides a simple formula to estimate the nonlinear displacement from the elastic analysis result: d_s = q_d × d_e, where q_d is the displacement behavior factor, often taken as equal to the behavior factor q.26 Checking this displacement (especially the inter-story drift) is extremely important to control damage to non-structural components and to avoid pounding against adjacent buildings.
• Stiffness Modeling: Reinforced concrete is a material that cracks under load, causing the actual stiffness of members to be lower than their gross section stiffness. TCVN 9386:2012 requires this effect to be considered in the analysis model. Unless a more refined analysis is performed, the standard permits taking the flexural and shear stiffness of concrete members as 50% of the uncracked gross section stiffness. Neglecting this stiffness reduction will lead to a shorter calculated building period, which can result in incorrect seismic forces and an inaccurate assessment of displacements.

Part 5: Comparative Analysis: TCVN 9386:2012 in the Context of International Seismic Standards

5.1. TCVN 9386:2012 and Eurocode 8

The relationship between TCVN 9386:2012 and Eurocode 8 is very close, as the Vietnamese standard is essentially an adoption and localization of Eurocode 8. Therefore, the design philosophy, analysis methods, fundamental formulas, and ductility requirements are nearly identical.

The main difference lies in the National Annexes. While Eurocode 8 provides a general framework, each European member state issues its own National Annex to define locally-dependent parameters. Similarly, Vietnam has replaced these parameters with its own provisions, primarily:

• Seismic Hazard Data: The zoning maps and tables for reference ground acceleration (a_gR) are specific to Vietnam, developed by the Institute of Geophysics and promulgated in QCVN 02:2022/BXD.
• Importance Factor (γ_I): The classification of buildings and the assignment of corresponding importance factors are specified separately to align with Vietnam’s construction management system.

5.2. TCVN 9386:2012 and ASCE 7 (American Standard)

Comparing TCVN 9386:2012 with ASCE 7 (specifically ASCE 7-10/16) is extremely important, as ASCE 7 is often used by international investors and consultants for large projects, especially high-rise buildings, in Vietnam. Although both are modern standards, they have fundamental differences in philosophy and methodology.

• Seismic Hazard Level: This is the most fundamental difference. TCVN 9386:2012 defines the design seismic action (for the no-collapse limit state) based on a 475-year return period.15 In contrast, ASCE 7 uses the concept of the “Risk-Targeted Maximum Considered Earthquake” (MCE_R), which corresponds to a much rarer event with an approximate 2500-year return period and a 1% probability of collapse in 50 years.30 This means the input seismic hazard level in ASCE 7 is fundamentally higher and more stringent.
• Displacement Response Spectrum: This is the most practically significant difference, especially for structures with long periods, like high-rise buildings.
  • The displacement spectrum in TCVN 9386 (inherited from Eurocode 8) is constructed to become constant (flat) once the period T exceeds a corner period of T_D = 2.0 seconds.
  • In contrast, the displacement spectrum in ASCE 7 continues to increase almost linearly up to a much longer transition period, T_L, which can range from 4 to 16 seconds depending on the geographic location.
• The consequence is that for a high-rise building with a fundamental period greater than 2.0 seconds, ASCE 7 will predict a significantly larger peak displacement than TCVN 9386:2012. Many studies have shown that capping the period at T_D = 2.0 seconds in Eurocode 8 (and thus in TCVN 9386) may be non-conservative for long-period structures. This creates a potential systemic risk for high-rise buildings being designed in Vietnam if they only mechanically comply with TCVN 9386:2012 without additional consideration.
• Site Classification: While the goal is similar, the specific criteria for classifying soil types and the corresponding site amplification factors differ. For example, a soil profile classified as Type C under TCVN 9386:2012 might be equivalent to Site Class D under ASCE 7, leading to different design requirements.

Table 5.1: Comparison of Key Differences between TCVN 9386:2012 and ASCE 7-10/16

Parameter	TCVN 9386:2012	ASCE 7-10/16	Implication for Design Practice
Seismic Hazard Definition	Based on 10% probability of exceedance in 50 years (475-year return period).	Based on MCER, approx. 2500-year return period and 1% probability of collapse in 50 years.	ASCE 7’s input hazard level is higher, generally leading to larger design seismic forces.
Behavior Factor	Behavior factor q, reduces elastic forces to get design forces.	Response modification coefficient R, serves a similar function to q.	Conceptually similar, but specific values differ for various systems and are tied to each code’s philosophy.
Displacement Response Spectrum	Becomes constant for periods T > TD = 2.0 seconds.	Continues to increase up to a long-period transition TL (4-16 seconds).	TCVN 9386:2012 can significantly underestimate the displacement of high-rise buildings, affecting P-Delta checks, separation gaps, and non-structural damage.
Importance Factor	Factor γI, multiplies the reference ground acceleration agR.	Factor Ie, generally applied to the seismic forces after the spectrum is determined.	Different approaches, but the final goal is similar: to increase the safety requirements for important structures.
Site Classification	Type A, B, C, D, E. Based on Vs,30, SPT, cu.	Site Class A, B, C, D, E, F. Classification criteria may differ.	Careful mapping is needed when comparing or converting between systems, as the same soil profile may be classified differently.

The difference in the displacement spectrum for high-rise buildings is a particularly serious issue. It suggests that, for high-rise projects (e.g., over 40m), the minimum standard of care should include performing an additional check on the building’s lateral displacement using the ASCE 7 response spectrum. This does not mean the entire structure must be designed to ASCE 7, but this check provides a critical safety measure, helping to identify early on cases where the design might be non-conservative in terms of deformation, even if it fully complies with the strength requirements of TCVN 9386:2012.

Part 6: The Regulatory Ecosystem: Navigating the Legal and Code Landscape

Applying TCVN 9386:2012 cannot be done in isolation. This standard is one link in a chain of mandatory technical regulations, and understanding the connections between them is a prerequisite for a correct and legal design.

6.1. QCVN 02:2022/BXD – The Mandatory Source for Seismic Data

National Technical Regulation QCVN 02:2022/BXD, “Natural physical and climatic data for construction,” is the document that provides mandatory input data for load calculations, including wind and seismic loads. For seismic design, its role is paramount because it:

• Supersedes Old Annexes: This regulation completely replaces the seismic zoning maps in Annexes G and H of TCVN 9386:2012. All references to ground acceleration must be taken from this latest version of the QCVN.
• Provides a_gR: Table 6.1 in QCVN 02:2022/BXD provides the reference peak ground acceleration (a_gR) for every district-level administrative unit in the country. This is the starting point for all seismic calculations.

Therefore, it can be said that TCVN 9386:2012 is the manual that explains “how to calculate,” while QCVN 02:2022/BXD is the legal document that provides the data “what to calculate with.”

6.2. Circular 06/2021/TT-BXD – Classifying the Building

Circular 06/2021/TT-BXD from the Ministry of Construction regulates the classification of construction projects and guides its application in managing construction investment activities.37 This circular’s role in the seismic design process is an indispensable intermediate step:

• The engineer uses the criteria in Circular 06/2021/TT-BXD (such as height, scale, function) to determine the “Building Grade” for their project (e.g., Grade I, Grade II, etc.).39
• After obtaining the “Building Grade,” the engineer returns to TCVN 9386:2012, looks up Annex E to determine the corresponding “Importance Class,” and from that, obtains the importance factor γ_I.

This process ensures that the required safety level for the building (represented by γ_I) is determined consistently and in alignment with the national general building classification system.

6.3. Interaction with Other Standards

As mentioned, TCVN 9386:2012 is a specialized seismic standard. A complete design process requires combining it with other standards, such as:

• TCVN 5574:2018: “Concrete and Reinforced Concrete Structures – Design Standard.” This standard provides the basic rules for the design and detailing of reinforced concrete members.
• TCVN 2737:2023: “Loads and Actions – Design Standard.” This standard specifies other types of loads (dead, live, wind loads) and the load combinations.

This interaction requires caution, especially since the seismic detailing requirements in TCVN 9386:2012 (for DCM and DCH) can be more stringent and must take precedence over the ordinary detailing requirements in TCVN 5574.

Part 7: The Future of Seismic Design in Vietnam: Transitioning from TCVN 9386:2012 to the 2023 Revision

7.1. The Impetus for Revision

After years of application, TCVN 9386:2012 has revealed several limitations and shortcomings, creating the impetus for its review and revision. The main reasons include:

• Challenges in Practical Application: The standard’s complexity and the lack of detailed guidance have caused many difficulties and inconsistent applications within the engineering community.
• Outdated Base Standard: TCVN 9386:2012 is based on the 2004 version of Eurocode 8. Since then, Eurocode 8 itself has undergone significant revisions and updates.
• Changes in the Related Regulatory System: The issuance of new versions of key regulations and standards, such as QCVN 02:2022/BXD and TCVN 2737:2023, requires TCVN 9386 to be updated to ensure synchronization.

7.2. Key Changes in the Draft TCVN 9386:2023

The draft revision of TCVN 9386:2023 introduces several important changes aimed at resolving the above issues and improving the standard’s usability.

• Introduction of Type 2 Response Spectrum: This is the most impactful change. The 2012 version uses only one type of response spectrum (Type 1), which was developed for regions of moderate to high seismic hazard. The 2023 draft will introduce a Type 2 Response Spectrum, recommended for low seismic hazard regions where the governing earthquakes have a surface-wave magnitude of M_s ≤ 5.5. Based on studies by the Institute of Geophysics, the South-Central and Southern regions of Vietnam are suited for the application of the Type 2 spectrum. This will help optimize designs, making the requirements more rational and economical for these regions.
• Updated References: The new version will remove the annexes containing seismic maps and will reference QCVN 02:2022/BXD directly, ensuring the input data is always the latest and most legally binding version.6 References to other standards will also be updated.
• Restructuring and Reorganization: The standard will be split into separate parts (e.g., Part 1 for buildings, Part 5 for geotechnical aspects), similar to the structure of Eurocode. Unnecessary annexes will be removed, and the content on importance classification will be integrated into the main body of the standard for easier reference.

Table 7.1: Summary of Key Revisions in Draft TCVN 9386:2023

Item	TCVN 9386:2012	Draft TCVN 9386:2023	Practical Implication for Engineers
Response Spectrum	Uses only Type 1 Spectrum, suitable for strong earthquakes.	Introduces Type 2 Spectrum for low-seismicity regions (Ms ≤ 5.5).	Designs will be more economical and rational for projects in Southern and South-Central Vietnam.
Seismic Zoning Data	Provided in Annexes G and H (outdated).	Removes Annexes G, H; references QCVN 02:2022/BXD directly.	Ensures the legality and currency of ground acceleration data; the lookup process becomes clearer.
Importance Classification	Defined in Annex E, based on an older building grading system.	Integrated into the main body, classification system updated to better align with international practice.	Easier to look up and apply, increasing consistency with international standards.
Standard Structure	Combines Eurocode 8 Parts 1 and 5 into one document.	Separated into distinct parts (Part 1, Part 5, etc.) similar to Eurocode.	Clearer, more coherent structure, easier for reference and future expansion.
Cited Standards	References older versions of TCVN 2737, QCVN 03, etc.	Updated to reference the latest versions (TCVN 2737:2023, QCVN 03:2022, etc.).	Ensures synchronization and consistency within Vietnam’s entire system of construction standards and regulations.

Part 8: The Integrated Application Model: A Strategic Workflow for Efficient and Accurate Implementation

To address the challenges in applying TCVN 9386:2012, a systematic workflow is essential. The model below presents a logical sequence of steps, connecting the relevant regulations and engineering decisions, to help ensure the design process is conducted quickly, accurately, and in full compliance. This model is illustrated through a case study: the design of a 24-story residential building in District 1, Ho Chi Minh City.

Phase 1: Project Definition and Seismic Parameter Determination

This is the input data collection phase, the foundation for all subsequent analysis.

Step 1.1: Define Project Location and Scale:

• Example: A 24-story residential building with 3 basement levels, total GFA 15,000 m2, located in District 1, Ho Chi Minh City.

Step 1.2: Look up Reference Peak Ground Acceleration (a_gR):

• Open QCVN 02:2022/BXD.
• Go to Table 6.1: “Reference peak ground acceleration a_gR on type A ground.”
• Look up the location “District 1, Ho Chi Minh City.”
• Example Result: a_gR = 0.06g.

Step 1.3: Determine “Building Grade”:

• Open Circular 06/2021/TT-BXD.
• Go to Appendix I: “Classification of civil works.”
• Based on the scale (24 stories), determine the building grade.
• Example Result: The building is Grade II.

Step 1.4: Determine Importance Factor (γ_I):

• Open TCVN 9386:2012.
• Go to Annex E: “Importance classes and importance factors.”
• For a Grade II building, look up the corresponding Importance Class and γ_I factor.
• Example Result: Importance Class II, γ_I = 1.0.

Step 1.5: Calculate Design Ground Acceleration (a_g):

• Apply the formula: a_g = γ_I × a_gR.
• Example Result: a_g = 1.0 × 0.06g = 0.06g.

Step 1.6: Classify Seismicity Level:

• Compare the a_g value against the thresholds in TCVN 9386:2012.
• Example Result: Since 0.04g ≤ 0.06g < 0.08g, the project is in a “Low Seismicity” zone.19

Step 1.7: Classify Ground Type:

• Based on the geotechnical report, determine V_s,30, SPT, or c_u values.
• Open TCVN 9386:2012, go to Table 3.1 to classify the ground type (e.g., Type C).
• From the ground type, look up the response spectrum parameters (soil factor S, periods T_B, T_C, T_D).

Phase 2: Conceptual Design and Strategy Selection

This phase translates the input data into strategic decisions about the structural system.

Step 2.1: Select Ductility Class:

• Based on the seismicity level classified in Step 1.6.
• Example Result: Being in a “Low Seismicity” zone, the standard permits the use of Ductility Class Low (DCL).

Step 2.2: Select Behavior Factor (q):

• Based on the chosen Ductility Class.
• Example Result: For DCL, the behavior factor is taken as q = 1.5.

Step 2.3: Develop Structural Scheme:

• Design a structural layout that adheres to the principles of regularity, symmetry, and adequate stiffness in both directions to optimize seismic performance.

Phase 3: Modeling and Computational Analysis

This is the phase of performing the analysis in specialized software.

Step 3.1: Select Analysis Method:

• Based on the building’s characteristics.
• Example Result: For a 24-story building, Modal Response Spectrum Analysis (RSA) is appropriate and mandatory (as it does not meet the conditions for the ELF method).

Step 3.2: Build the Analysis Model:

• Model the structure in software (e.g., ETABS, SAP2000).
• Apply stiffness reduction factors for reinforced concrete members to account for cracking (e.g., 0.5 for beams, 0.7 for columns and walls).

Step 3.3: Define the Design Response Spectrum:

• Use the parameters determined in Phase 1 (a_g, ground type, S, T_B, T_C, T_D) to define the response spectrum function in the software.

Step 3.4: Combine Loads:

• Perform the seismic analysis in two orthogonal directions separately.
• Create design load combinations where the seismic actions are combined using the “100% + 30%” rule.

Step 3.5 (Critical Cross-Check):

• As the building is high-rise (24 stories), perform a supplementary analysis to check the inter-story drift using the ASCE 7 displacement spectrum. Compare this result with the displacement calculated from TCVN 9386:2012 to assess the safety margin for deformation.

Phase 4: Detailed Design and Detailing

This phase translates the analysis force results into reinforcement design.

Step 4.1: Extract Design Forces:

• Obtain the member forces (moments, shears, axial forces) from the worst-case load combinations.

Step 4.2: Design Reinforcement:

• Use the extracted forces to calculate and design the reinforcement for all members (beams, columns, walls, foundations) according to the provisions of TCVN 5574:2018.

Step 4.3: Apply Detailing Requirements:

• Example Result: Because DCL was selected, there is no requirement to follow the special seismic detailing provisions in TCVN 9386:2012. The reinforcement details only need to comply with the ordinary provisions of TCVN 5574:2018. The foundation also does not require seismic design.4 (Note: If the building were in a high seismic zone and DCM was chosen, this step would be far more complex, requiring capacity design and special detailing).

Phase 5: Verification and Documentation

Step 5.1: Self-Check using a Checklist:

• Use a checklist to review the entire process: Were the input parameters looked up from the correct and latest regulations? Was the chosen ductility class appropriate for the seismicity level? Was the analysis method valid?

Step 5.2: Prepare Calculation Report:

• Prepare a clear, transparent design report that fully documents all steps in the process, from parameter determination, strategy selection, and analysis results, to the final detailing.

Part 9: Conclusion: Key Takeaways and Strategic Recommendations for Practicing Engineers

This report has conducted a comprehensive, in-depth analysis of Standard TCVN 9386:2012, from its theoretical underpinnings, technical provisions, and legal context to its comparison with international standards and future development trends. From this, the following conclusions and strategic recommendations can be drawn for the engineering community in Vietnam.

Summary of Key Findings:

• A Modern but Complex Standard: TCVN 9386:2012, with its Eurocode 8 origins, is a modern seismic design standard. However, its complexity and demand for specialized knowledge are significant barriers, leading to difficulties and errors in practical application.
• Dependence on a Regulatory Ecosystem: The standard cannot be used in isolation. Correct calculation requires properly referencing and integrating data and rules from QCVN 02:2022/BXD and Circular 06/2021/TT-BXD.
• Potential Risks in Practice: The report has identified two serious risks in current design practice:
  • Misuse of Ductility Class Low (DCL): The intentional selection of DCL for projects in high seismic zones to avoid complex detailing is a dangerous practice, creating brittle structures with a high risk of collapse.
  • Underestimation of High-Rise Displacements: The displacement response spectrum in TCVN 9386:2012 is potentially non-conservative for long-period structures, leading to a risk of underestimating the actual displacements of high-rise buildings.

Actionable Recommendations:

Based on the above analysis, this report proposes the following recommendations to improve the quality and safety of seismic design work in Vietnam:

• Adopt an Integrated Workflow: Engineers and design firms should establish and adhere to a systematic workflow, similar to the “Integrated Application Model” presented in Part 8. This ensures all legal and technical factors are considered completely and in the correct sequence, minimizing errors from omissions or incorrect data sources.
• Prioritize the Ductility Class Selection: The decision on Ductility Class must be made consciously, transparently, and with a defensible basis, founded on the calculated design ground acceleration a_g. Never select DCL for projects where a_g ≥ 0.08g.
• Perform Displacement Cross-Checks for High-Rise Buildings: For buildings of significant height (recommended 15 stories and up), performing a supplementary check on displacement using the ASCE 7 response spectrum should be considered part of the standard of care. This provides an important layer of protection against risks related to excessive deformation.
• Focus on Detailing: For projects requiring DCM or DCH, it must be understood that strict adherence to reinforcement detailing requirements is not optional; it is a mandatory condition to ensure the structure can achieve the ductility assumed in the analysis. Special attention must be paid to clearly showing these details on the drawings and enhancing site supervision during construction.
• Stay Current and Prepare for the Future: Engineers must proactively learn about and prepare for the transition to the TCVN 9386:2023 version. Grasping the changes early, especially how to apply the Type 2 Response Spectrum for low seismic hazard zones, will provide a competitive advantage and help optimize design solutions in the future.