Numerical calculation of members and joints at elevated temperature
Authors:   , Der Batuhan 
Published: May 2024

The failure of steel joints may result in the collapse of the entire building at elevated temperatures due to their influences on internal forces distribution and overall deformation. Hence, the resistance of steel joints at elevated temperatures should be accurately predicted to understand the structural fire performance. In the fire, stiffness and strength of steel members have significant reductions at elevated temperatures, leading to a decrease in the ultimate load capacity. EN 1993-1-2 suggests that simplified analytical methods may be used to design of isolated steel members subjected to fire assuming a uniform temperature in the cross-section considering the mechanical properties of steel at elevated temperatures can be analysed using. The experimental investigation is the most accurate way to calculate the fire resistance of steel members and joints. Due to the high cost of full-scale fire tests and the size limitations of existing furnaces, these valuable fire tests are not easy to conduct frequently. The prEN 1993-1-14 describes two design methods for finite element models: numerical simulation (NS) and numerical design calculation. prEN-1993-14 [2] defines numerical simulation as an extension of physical experiments to determine the direct resistance of a structure. NS method generally uses an advanced numerical model including the solid elements, the measured material properties, geometrical imperfections, and residual stresses in order to simulate the response of structural components. Advanced solid finite element models have significant advantages in accurately predicting the fire behaviour of steel joints; however, they also come with certain disadvantages. Advanced solid finite element models utilize higher-order elements, and it leads to high computational costs. Another challenge is generating a high-quality mesh for solid models, particularly for irregular or complex geometries. A solid model and a shell model are developed using Abaqus to study the behavior of bolted lap joints and steel beam at elevated temperatures, respectively. The proposed models are validated against experimental results to verify the CBFEM models. The main drawback of analytical models for steel joint design is that it does not accurately reflect the nonlinear behavior of steel joints. They may not be sufficient to accurately evaluate the stress and strain values of steel joints. Therefore, NDC can be used for the static design check of steel joints which may be used to calculate the resistance as stated in prEN1993-1-14. At the design level, analytical models can be replaced by numerical design calculations for predicting the fire resistance of steel joints. This study proposes a numerical design calculation using the CBFEM model to predict the mechanical response of steel joints and members at elevated temperatures. To assess the accuracy and reliability of the proposed model, verification and validation studies are carried out. The model is verified against the results from analytical models or other finite element models. The experimental results are used to validate the steel joint models in terms of load-deformation curves, fire resistance, and failure modes at high temperatures. To predict the resistance of the steel joints and members at ambient and elevated temperatures, the 5% plastic limit strain recommended in EN 1993-1-5 for steel plate elements and the analytical model for the bolts and welds proposed in EN 1993-1-8 were used in the CBFEM model. The influence of temperature on the resistance is considered using the reduction factors for carbon steel and bolts presented in EN 1993-1-2. It can be concluded that CBFEM is a good alternative to analytical models for the design of joints and members at elevated temperatures because it uses advantages of FEM which are not considered by analytical models.