In the context of modern industrialization, the demand for buildings with large usable space, not hindered by intermediate support columns, is increasing. To meet this requirement, the solution large-span steel structures has become the superior technical choice. By definition, this is a steel structure system designed to span distances of 30 meters or more, even up to 60 - 100 meters, often used in special projects such as large-scale factories, sports halls, exhibition centers, and airport terminals.
The core challenge of these structures lies in managing the complex relationship between self-weight, load-bearing capacity and deformation over a large unsupported span. Any errors in design or construction can lead to serious consequences due to the scale of the project. Therefore, strict compliance with national technical standards is not only a recommendation but a mandatory requirement to ensure bearing safety, durability, normal usability and economic efficiency throughout the life of the project. In Vietnam, the TCVN standards, especially TCVN 555 and TCVN 2737, serve as the basic technical legal framework providing a common language and rules for engineers to solve this complex problem.
Design basis: Standard TCVN 5575:2024
TCVN 5575 is a standard that specifies the basic requirements for the design and calculation of steel structures in civil and industrial works. The foundation of this standard is the limit state calculation method, a modern design philosophy to ensure that steel structures do not exceed safety thresholds and are used in all the most unfavorable load combinations.
Limit states are generally divided into two main groups:
- Limit state of bearing capacity (Group 1): These are states associated with major structural failure or damage, including tensile failure (material yielding), global or local instability (member buckling), and brittle failure.
- Limited Service State (Group 2): These are states where the structure no longer ensures normal use conditions, affecting the operating function, equipment or user psychology, including deformation, excessive deflection and excessive vibration.
One of the most important updates of the version TCVN 5575:2024 Compared to the 2012 version is the inclusion of the concept "component classification"Accordingly, the components are divided into three levels based on the stress-strain state of the calculated cross-section:
- Level 1: Elastic state.
- Level 2: Elastic state.
- Level 3: Plastic state (plastic joint formation).
This classification is a big step forward, helping Vietnamese standards get closer to advanced international standards such as Eurocode 3. It allows engineers to perform design calculations more optimally, especially with bending members under static load conditions, including the development of plastic deformation to save materials while still ensuring absolute safety.
In addition, the standard also clearly stipulates material requirements, encourages the use of domestically produced steel, and corresponding standards for welding materials and bolted connections, creating a synchronous reference system for the entire process from design to manufacturing.
Service Limit State Control: Deflection Limit according to TCVN 2737:2023
For large-span steel structures, due to their slenderness and lighter weight than concrete, the serviceability limit state—specifically deflection—is often a design factor. Excessive deflection is not only unsightly, but can also damage non-structural components such as partition walls and glass doors, or affect the operation of sensitive equipment such as bridge cranes.
One important update to note is that since TCVN 2737:2023 “Loads and impacts” was issued, regulations on deflection limits for all types of structures (including steel, reinforced concrete) were unified and centrally regulated at Appendix G of this standard. This replaces Table 1 in the previous TCVN 5575:2012, creating a single reference source, reducing ambiguity and ensuring consistency in design practice.
The table below summarizes typical limit deflection values for steel structural members, quoted from Appendix G of TCVN 2737:2023.
Table 1: Allowable Deflection Limits of Steel Structural Components (According to TCVN 2737:2023 – Appendix G)
Note: is the span of the structure. The above values are for reference only and need to be applied specifically according to the detailed requirements in Appendix G.
