Mechanical characterisation and structural design of hybrid timber- glass diaphragms with integrated photovoltaics

Abstract

This study investigates the structural performance of hybrid timber-glass frame walls designed to enhance horizontal stability, the so-called racking resistance in façade structures. An important additional challenge is the integration of photovoltaic solar cells within structural glass elements, while maintaining structural integrity. Lightweight timber structures are an increasingly important field in construction engineering since these structures have a lower environmental impact than traditional steel or concrete structures. In parallel, recent developments in the architectural design of buildings favour open spaces. When such a building has fewer inner structural walls, the stability and stiffness of the façade become more important. The timber buildings’ racking resistance is of particular concern. The racking resistance of a timber frame can be increased by using sheathing panels (creating diaphragm walls), wind bracings or stiff timber connections between studs and rails. However, current calculation methods (Eurocode 5) do not account for the structural contribution of panels with window openings. This raises problems regarding the in-plane stiffness (racking resistance) of buildings with large surfaces of glass windows. One solution is to increase the number of diaphragms in the timber frame building’s façade by structurally activating the glass panels. In order for the glass to contribute structurally to the façade, a structural bond between the timber and the glass can be employed. In this thesis, the mechanical performance of bonded timber-glass connections is examined through experimental tensile and shear tests. Four two-component silicone adhesives and one one-component polyurethane adhesive are evaluated. Special attention is given to the failure behaviour of the adhesives, where both cohesive failure and loss of adhesion are identified. The nonlinear stress-strain behaviour of these adhesives is evaluated and used to assess different hyperelastic material models. Two calibration methods are used to determine the model parameters of the hyperelastic material models. The first method, assuming theoretical stress states, is inadequate for the performed tensile tests, whereas the second method, using an inverse parameter fitting method, yields good results. Additionally, a phase-field damage model is developed to predict adhesive failure loads. These findings can be used to model bonded timber-glass connections in larger structures. The structural performance of structural glass elements with integrated photovoltaics is also investigated using numerical modelling. The model predicts stresses and strains in the glass and solar cells under various loading conditions. To validate these simulations, experimental in- and out-of-plane bending tests are performed on glass/glass photovoltaic (PV) modules. This combined numericalexperimental approach results in reliable models for designing structural timberglass façade elements with integrated photovoltaics. The study further evaluates the effects of shear loads on the system’s components. Eight diaphragm specimens (1.2 m × 1.2 m) are tested using two different structural silicone adhesives, with and without a tie-down anchoring of the leading stud. Various measurement techniques, including displacement sensors (linear variable differential transformers or LVDTs), digital image correlation, fibre Bragg gratings and strain gauges, are used to analyse component behaviour. The specimens primarily failed due to adhesive rupture. Results show that anchoring the wall with a tie-down increases the system’s racking stiffness by 30%. The strains on the glass and solar cells are analysed during experimental in-plane shear tests, confirming that the solar cells remain undamaged. Electroluminescence testing verifies this finding. Furthermore, an analytical design method based on Eurocode 5 (FprEN 1995- 1-1) and spring models is proposed and compared with the experimental results. While this method tends to underestimate the wall elements’ stiffness, it accurately predicts the minimum load-bearing capacity. Finally, a detailed finite element model is developed and calibrated using small-scale connection tests. The model closely matches experimental measurements in terms of strength and stiffness. A parametric study assesses the influence of different aspect ratios, glass thicknesses, adhesive bond dimensions, and other parameters. Overall, this study demonstrates that glass can be structurally activated to increase the stiffness of lightweight timber structures when bonded with adhesives. This challenges the conventional assumption that glass openings do not structurally contribute. Additionally, integrating photovoltaics into the glass enables multifunctional façade elements. The findings confirm that, for the tested specimens, standard solar cells withstand in-plane shear forces in the glass without compromising structural performance. This research advances the field by exploring innovative solutions to overcome current challenges in timber frame design, offering a pathway toward more sustainable architecture and structural engineering, and contributing to the development of next-generation building systems that integrate both structural performance and renewable energy generation.