Development of an Innovative Composite Mullion Made of Aluminium and Timber

Abstract

The facade, as the envelope of a building, provides essential functions such as structural resistance against wind loading, thermal insulation and weather independence. The aluminium-glass curtain wall facade system has become a popular alternative for high-rise buildings due to its aesthetic appeal and efficient unitised installation. In a unitised glass curtain wall facade system, the vertical structural frame, named mullion, is the dominate wind load-bearing element but the critical thermal bridging element. Extruded aluminium mullions have been popularly used due to their high strength to weight ratios, high corrosion resistance and extrudability to allow the facade frames light, durable and flexibly unitised. However, the aluminium mullions might not be so good as some mullions in other aspects, e.g., in thermal performance, aesthetics and sustainability. To address these issues, several attempts to improve their performance, including introducing thermal breaks or structural timber, and hiding the exposed aluminium extrusions, were proposed and evaluated in recent decades. However, none of them could solve all the drawbacks of aluminium mullions. This thesis proposes an innovative aluminium-timber composite mullion, which is durable, unitised, structurally sound, energy-efficient, aesthetically pleasing and sustainable, and aims at its structural and thermal behaviours. In the structural investigation, three series of experimental tests were firstly conducted to investigate and evaluate the structural behaviour of the novel composite mullions: material property tests, connection tests, and four-point bending tests. Material property tests were done to obtain the mechanical properties of aluminium and timber used in the novel composite mullions. Four types of aluminium-timber connections were subjected to shear loading in the connection tests. The results showed that the proposed adhesive bond formed the optimum connection between aluminium and timber. After determining the connection type, a series of four-point bending tests were conducted using a back-to-back restrained test set-up under both positive and negative loading cases. Furthermore, the traditional aluminium mullions were also tested for comparison. The aluminium mullions failed in local buckling under negative loading, while the composite mullions failed due to plywood compressive yielding and aluminium flange local buckling. Moreover, distortional buckling was observed during aluminium mullion tests subjected to positive loading, while the composite mullions failed due to parallel to the grain tensile failure in the plywood flanges. It was found that the composite mullion had greater structural performance subjected to bending. Then, a numerical study using finite element models was conducted to further investigate the bending behaviour of the composite mullion based on the experimental results. Two types of finite element models, experimental models and ideal models, were used and validated with the experimental tests. Parametric studies were also performed using validated ideal models to develop the design rules of the composite mullions. The finite element analysis results were then compared with two current design methods: limiting stress method (LSM) and total moment capacity approach (TMCA). It was found that the LSM was too conservative and that the TMCA was only accurate under positive loading with a mean ratio of 1.07 and a COV of 3% for male mullions, 1.09 and 3% for female mullions. Hence, this thesis proposes a modified TMCA using an elastic-plastic approach under negative loading to improve its accuracy. The modified TMCA to predict the negative loading case reached an adequate accuracy with a mean ratio of 0.99 and a COV of 3% for male mullions, and a mean ratio of 1.00 and a COV of 3% for female mullions. This thesis also evaluates and compares the thermal performance of this novel composite mullion with several current aluminium mullions. It showed that the composite mullion had a much greater thermal performance than the traditional mullion with an improvement of 52%, though it was less energy-efficient than thermally broken aluminium mullion. Furthermore, it was found that the thermal conductance of the composite mullion could be further reduced by four proposed energy-efficient strategies. The optimal combination could reach a reduction of 69% compared with the initial design of the innovative composite mullion. Moreover, a generic study was also performed to investigate the influence of various parameters on the thermal conductance of the composite mullions. The results and evaluation are detailed in this thesis.