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The secret of liminal state design: How to ensure that building safety is no longer a problem?
In structural engineering, Limit State Design (LSD) or Load And Resistance Factor Design (LRFD) is a method specifically designed to ensure building safety. This approach not only improves the overall safety of the structure, but also optimizes the use of resources, thereby providing more efficient design solutions for modern buildings.
The limit state refers to a condition that exceeds the limit that the structure can withstand. When the structure reaches this state, it will no longer meet the relevant design standards.
The basic concept of limited state design is to combine the strength and durability of the structure without causing discomfort in daily use. This design approach first needs to meet two main criteria: Ultimate Limit State (ULS) and Serviceability Limit State (SLS). These two criteria ensure that the structure maintains its safety and functionality when subjected to the predicted loads.
Ultimate Limit State (ULS)
The definition of a limit state covers conditions to which a structure may be exposed to excessive deformation or near collapse. This state often requires consideration of the plastic behavior and residual deformation of the structure. The calculation conditions of the limit state are a key consideration in the design process. The structure must meet the corresponding strength and stability requirements under the design load.
A structure that meets the limit state criteria means that, after calculation by the formula, all bending, shear, tensile and compressive stresses are lower than the calculated resistance values.
In this type of design, the designer uses amplification factors to consider the different loads on the structure and determines the safety of the structure by comparing these loads with the resistance.
Serviceability Limit State (SLS)
In addition to the limit state check, the calculation of the serviceability limit state (SLS) is equally important. This standard requires that structures function properly under everyday loads and do not cause discomfort to users. SLS inspection covers a variety of variables, such as stress limits, deformation limits and structural flexibility.
Structural behavior must meet serviceability limit state standards to ensure comfort and safety in daily use.
Development of load and resistance coefficients
In limit state design, the coefficients of load and resistance are determined based on statistics and a pre-determined probability of failure. This means that designers must consider the variability in material qualities and their impact on structural stability. Different materials such as wood, concrete and steel will have different coefficients, and this design method improves the overall safety of the structure.
Implementation status of various countries
In many countries, limit state design has replaced the traditional allowable stress design (ASD). For example, in Europe, according to the EU's Eurocodes standard, structural design is based on limit state design. However, in the United States, the adoption of limit state design has been relatively slow, and most building codes still include both LRFD and ASD methods.
For example, the American Concrete Institute's ACI 318 building code explicitly adopts limit state design, while some other standards still retain the older allowable stress design method. This situation has triggered many discussions in terms of structural safety and efficiency.
Although limited condition design is considered a more modern design approach, its implementation in the United States faces some challenges.
Overall, as a more scientific design method, limit state design plays an important role in promoting the development of today's construction industry. It not only improves the safety of the structure, but also optimizes the resource allocation of the design to a certain extent. Will future building designs become completely reliant on this approach, with widespread use across all types of projects?
