Optimised design of slender reinforced concrete columns subjected to compression and biaxial bending : A comparison between Eurocode 2 and Numerical analysis

Sammanfattning

This thesis investigated the potential of a Python-based automated design software for slender reinforced concrete columns subjected to axial compression and biaxial bending. The main aim was to improve the structural efficiency compared with the design using simplified criteria in Eurocode 2. Conventional design workflows for bridge substructures often rely on such simplified criteria or detailed finite element analyses, where concerns regarding design conservativeness and time-consumption may arise. To address this, a computational tool was developed in Python using the open-source library StructuralCodes, which enabled numerical cross-section analysis in combination with the design procedures from Eurocode 2 for nonlinear concrete material, geometric imperfections and second order effects using the nominal stiffness and nominal curvature methods. The software was verified through hand calculations using interaction diagrams and by comparison with the commercial software Strusoft Concrete Column 6.5. The results showed excellent agreement in the uniaxial bending capacity of the cross-section for a given axial compressive force, with deviations of less than 2 % for the studied methods. Furthermore, when the simplified biaxial bending interaction criterion in Eurocode 2 was applied, all three methods produced similar utilisation ratios. However, when the biaxial bending algorithm from StructuralCodes was used, a significantly lower utilisation ratio was observed, compared with the Eurocode 2 criterion. Indicating that the simplified Eurocode 2 criterion was conservative for the studied cross-section and load effects. Consequently, the biaxial bending utilisation ratio was reduced by 20 %, in line with findings from other research on this topic for rectangular cross-sections. Practically enabling a reduction in reinforcement content by about 50 % or a decrease in concrete cross-sectional area by about 23 %. A case study of the 25 m tall columns of the bridge Verkeb¨acksbron demonstrated that the numerical biaxial bending capacity was identical to the simplified Eurocode 2 interaction criterion for circular cross-sections. Indicating that the Eurocode 2 criterion is well-suited for this type of analysis, and not enabling any material savings to be made by adopting a numerical analysis. A comprehensive parametric study was performed for circular, square, rectangular and elliptical column cross-sections with varying slenderness ratios, reinforcement contents, concrete strengths, and load cases. All calculations were made using both the simplified Eurocode 2 interaction criterion and the numerical analysis based on StructuralCodes to highlight their differences and similarities. The results showed that the simplified Eurocode 2 biaxial bending interaction criterion can be significantly conservative for rectangular cross-sections, particularly for low relative axial forces, which is common for slender columns to avoid buckling and large second order effects. Due to the different definitions for the exponents in the Eurocode 2 criterion based on cross-section geometry, circular and elliptical cross-sections with the constant exponent of aN = 2 showed little to no difference compared to the numerical analysis. Overall, this study demonstrates that a Python-based automated numerical design can substantially reduce the material consumption of concrete and steel reinforcement by increasing the biaxial bending capacity for rectangular cross-sections, compared with the simplified design criterion offered by Eurocode 2. The proposed method offers a time and computationally efficient alternative to advanced finite element modelling, which enables a more optimal design process for practising structural engineers.