Glass is a fascinating material which has been used for load bearing structures in the past decades. The development of structural glass has been moving towards more transparent structures. The thermal properties of glass can be used to create heat bonds. Heat bonds can be produced in two ways: welding and fusing. With welding, the regions that are to be connected are locally heated. With kiln formed fusing, the elements to be bonded will be globally heated in an oven. This results in the research question: What is the best method, among glass welding and kiln formed glass fusing, to produce heat bonded systems for applications in glass structures? In theory, safety measures consist of three basic strategies: creating secondary load transfer mechanisms, protectioning and overdimensioning. These strategies can be used on the scale of the system and the structure. The heat bonded system could theoretically be laminated, reinforced and tempered. On a structural level, safety can be ensured by a combination of the strategies. To produce glass welds, the glass is preheated in an oven. A small opening in the oven is made and a line burner is used to heat the to be welded area from both sides. One element is pressed down on the other to make the connection. The oven is closed and the specimen is annealed. Finite element models have been used to analyze thermal shock failure during the weld production process. The recommendations for production were to use an additional flame on the bottom of the specimen or to maintain the surroundings at elevated temperature. The latter option is applied in the experiments. The results were three transparent, all-glass, soda lime T-shaped objects. Little residual stress remained in the products. The fused objects were globally heated in an oven. The glass was fused in moulds, made of a gypsum plaster. The products were translucent, all-glass, soda lime, T-shaped objects. The objects deformed significantly during production. Little residual stress remained in the products. Throughout the experiments, factors have changed. An aspect with no direct influence on the product was quenching after maximum temperature. Adapting the mould and polishing the glass edges prior to fusing had an influence on the objects' shape or texture. None of the factors influenced the order of magnitude of the deformation or the seam between the elements. A structural test has been performed to compare the specimens with a glass T-shape bonded with transparent UV-curing adhesive. The connections were loaded in shear. The results of the failure loads and stress at failure are of the same order of magnitude. The largest spread was among the fused specimens, the smallest among the welded. The cause of failure is most probably a combination of elevated stress due to the stiffness and support of the test set-up and peak stress due to irregularities on the glass surface. Other causes could have contributed to failure. The welded glass product requires most time, energy, skill and money. The adhesive product requires least time and energy. The welded and fused objects had neither acceptable dimensional accuracy, nor a smooth fillet in the joint. In the current state of development, glass fusion results in less residual stress, requires less skill and is cheaper to produce. Glass welding results in a transparent glass to glass connection and its structural performance is promising. Further research is encouraged into the heat bonded and adhesive connections to create fully transparent connections for structural glass.

Despite the increased popularity of the application of load bearing glass in the built environment, its design does rarely address sustainable or circular aspects. This Master’s Thesis analyses what the influence of connections is on the sustainable performance of structural glass and informs about the limitations and possibilities of sustainable glass design. To answer this question, sustainability has been defined from literature which resulted in ten sustainable, circular design methods. The role of load bearing glass and its connections is discussed in these methods, and it is concluded that there are two main possible design strategies for a sustainable design. First, the biggest impact in design can be made when focus is on reuse as an afterlife application to increase the lifetime of glass elements. Secondly, impact can be made by limiting the environmental footprint of the structure in the initial design by material minimization and selection. Recycling is left out of the scope since recycling on a world-wide scale barely occurs. Even in a country with a well developed recycling network like the Netherlands, just 5-10% of discarded flat glass waste ends up back in the float glass industries and the rest is downcycled to container glass or glass fibres. Besides, the sustainable impact from recycling compared to reuse is far smaller, and measures to limit contamination are out of control of the designer. At first, in order to design for reuse as an end-of-life application, demountable connections are essential and adhesive based connections should be avoided. Although demountability is a key to the circular economy, this study concludes that there are various technical barriers to overcome to make glass elements actually reusable. One important barrier is the lack of standardization in glass structures. As is shown in the redesign of "Kasteel Ruurlo", implementing standardization leads to a modular design and is integrated by using minimum size deviation and the use of a single type of connection. However, as a result of standardization, the design freedom of the architect is taken away and the mechanical connections lead to a less elegant design. Therefore, it is recommended to further development these modules and discuss elegance and design freedom in standardization. Another technical barrier to overcome is the performance and quality of glass elements which makes current reuse impossible: insulated glass units will loose their insulating performance due to failure of the edge sealant after 20 to 25 years, and laminated glass is prone to delamination over time. There is a lack of legislation which sets requirements to the quality and performance of these elements. Only for laminated glass, there is a potential reuse strategy which has been used in the redesign of "Kasteel Ruurlo". It is proposed to use a high quality interlayer as SentryGlas or Trofisol. For both interlayers, delamination problems should be further researched in order to assure a long technical lifetime. Secondly, in order to minimize the environmental impact of the structure, the impact has been quantified using Life Cycle Analysis data and Environmental Product Declarations. With the method by "Stichting Bouwkwaliteit", this data is converted to shadow costs which is used to calculate the environmental footprint of a connection. Different structural connections are considered: various facade connections, different moment rigid frame connections and various ways to connect a panel. From this calculation, the different connections are compared and it is concluded that adhesive based connections have both a smaller environmental footprint than mechanical connections, and that these result in less material use in the overall structure and thereby limiting the environmental footprint. This is also reflected in the redesign of "Kasteel Ruurlo", where due to the mechanical demountable connections, the environmental impact increases compared to the current design. With current possibilities, the relation between the connection and a sustainable design depends on the type of connection: an adhesive based connection will result in an overall low environmental impact but makes reuse unlikely. A mechanical connection could be an outcome here, but with current possibilities, reuse cannot be guaranteed either. The application of current demountable connections on the reuse possibilities can therefore be argued. In order to succeed with structural glass in the circular economy, this research emphasizes the need for legislation, standardization and to solve problems like delamination and leaking insulated glass units which now determine the lifetime of glass structures. It also shows the need for demountable connections which increase the reuse potential and which ideally do not increase the environmental footprint.