Investigation of composite façade mullions

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Modern curtain wall systems are typically designed with extruded aluminium members. As a load bearing vertical element of the curtain wall system, mullions are also made of aluminium extrusions with glass fibre reinforced polyamide acting as a thermal break joining the external and internal extrusions together. This research is focused on the behaviour of this type of thermal break composite façade mullions under quasi-static loadings. Literature survey was carried out. Past research works on of the thermal break façade mullions was studied, as well as the current European standard specifying the performance requirement, proof and test for the thermal break profiles. Sandwich theory was studied and laid as a foundation of this research. Literature regarding material properties of aluminium and polyamide; interfacial action between aluminium and polyamide and composite beam bending were investigated and appropriate methodologies were adopted. To investigate the behaviour of the thermal break façade mullions, a typical mullion section was studied. This is a symmetrical composite section made of external and internal aluminium extrusions and joined by a glass fibre reinforced polyamide core. Experimental investigations were carried out to find the section shear and tensile capacity as well as the connectivity constant. The section capacity tests were performed at various temperatures under quasi-static loadings to investigate the temperature effect. Experiments under high strain rate loading have been performed at room temperature to find the relationship between section shear and tensile capacity and loading rates. As the mullions usually work as a simply supported beam under wind, temperature and earthquake loads, bending behaviour is necessary to be investigated. Experiments of four-point bending were performed on this façade section. Specimens of three for four sets of span length each were tested at room temperature under quasi-static loadings. Numerical simulations for the section shear and tensile tests, as well as four-point bending tests were carried out. Interfacial actions between aluminium and polyamide were modelled based on Coulomb’s friction theory. Two new failure models – “Proposed progressive failure model” and “Proposed partitioned multi-phase beam failure model” were developed and applied to section shear capacity model and beam bending models to simulate the interface failure. ABAQUS software was chosen to perform the simulations. The FE modelling results were compared with the experimental results in detail. The results of experimental investigations on section capacities at various temperatures concluded that the section shear and tensile capacity as well as connectivity constant increased with decreased temperature. Experiments under high strain rate loads showed the section shear and tensile capacity was not sensitive to strain rate. However, the connectivity constant showed a clear trend of strain rate sensitivity. Comparisons between experimental results and the numerical results were made. Failure modes observed from the shear and tensile experiments were repeated by the FE shear and tensile models. Load vs slippage graph obtained from shear model matched the experimental one very well. The load-displacement graph generated by the FE tensile model with equivalent material properties agreed well with the experimental one. Results obtained from the FE beam models correlated to the experimental results very well. Load vs mid-span displacement graphs produced from both experiments and the FE models showed consistent peak loading capacity. The three-stage progressive failure mode observed from the experiments was reproduced by the FE models. Mid-span strain distribution diagrams at elastic range, generated by the FE models, were compared with the experimental ones as well. It was found that the FE model results were relatively consistent with the experimental ones. However, further improvements can be made in future studies. The relationship between moment and curvature at mid-span bottom extreme fibre obtained from the FE models confirmed consistency with experimental results. A proposed frame work for an analytical solution of four-point bending of this type of composite thermal break façade profile in the elastic range was presented in this thesis. Based on the sandwich theory and superposition approach, formulations were derived to work out deflection and stresses, including peeling stresses between aluminium skins and polyamide core. Due to limited time and scope, the analytical solution has not been verified by experimental and numerical works in this research. It is recommended that experimental and numerical investigations be carried out to verify the analytical solutions and apply them to the industry applications in future studies. Another asymmetrical thermal break profile was also investigated numerically. Finite element models of the section shear and tensile capacity were established by ABAQUS software. The proposed progressive failure model was successfully applied to simulate the failure mechanism in the shear model. A four-point bending beam model was built in ABAQUS software with the proposed partitioned multi-phase beam failure model, effectively simulating the interface failure mechanism. The FE models generated similar trends as the typical section models, especially shear and tensile capacity models. However, variations in the beam model were observed. Further experimental investigations are required to confirm the phenomenon revealed by the numerical investigation in future studies. Further research on the thermal break façade mullions can be extended to further investigation of strain rate sensitivity of section shear and tensile strength by performing large quantities of experiments and numerical simulations under high strain rate loadings. Future studies to carry out experimental and numerical investigations to verify the analytical solution and extended into industrial applications are highly recommended as well. Future studies involving experimental investigation of the asymmetrical thermal break sections to confirm the behaviour shown by the FE modelling is also valuable to provide further insight.
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