Post-tensioned glass beams

Abstract

Over the last few decades, the application of glass in building construction has extended from small four-side supported windowpanes to areas traditionally reserved for other materials, such as roofs, floors, staircases and full building envelopes. Glass is nowadays applied as a primary structural element - a beam, column, wall or shell; however, its intrinsic brittleness calls for adequate safety concepts to account for the consequences of glass failure. Although very strong on the inter-atomic level, the flaws which arise on the glass surface during production and service life of glass elements, significantly decrease its effective tensile strength. The concept of post-tensioned glass beams continues on the concrete analogy of the reinforced glass beams, which has been validated over the past years for various loading and environmental conditions. Adding ductile reinforcement in the tensile zone of a glass beam significantly enhances the post-cracking behaviour and redundancy. By additionally prestressing the reinforcement in a post-tensioned system, a compressive pre-stress is applied on the glass beam, enhancing the initial cracking resistance by compensating for rather low tensile strength of glass. Compressive pre-stress may fully counter the tensile stresses induced by permanent loads, diminishing the negative effect of stress corrosion on long term tensile strength of glass. Furthermore, in case of glass cracking, the tendon bridges the cracks, providing ductile failure behaviour, as in the reinforced beam concept. This research aims to increase the understanding of the structural response of post-tensioned glass beams through experimental and analytical investigations and provide calculation tools which will facilitate the application of the developed beam systems in structural glass projects. The chosen beam layouts aim to maintain the visual simplicity of the common glass beams, comprising laminated glass panes and flat stainless steel tendons, incorporated in the glass section. To investigate the structural behaviour of post-tensioned glass beams, three beam systems with varying beam layout, post-tensioning method and level of pre-load are tested in four-point bending. The observed behaviour is interpreted with analytical models, which focus on the failure mechanisms at two stages: during post-tensioning procedure and during tests in bending. For the considered failure modes, which govern the load capacity of explored beam systems, allowable post-tensioning pre-load and flexural resistance can be determined based on the provided analytical expressions. Failure controlled by shear is qualitatively analysed based on the kinematics of a critical shear crack, which shows the potential importance of this failure mechanism and will serve as a basis for future research on the shear capacity of glass beam members. The proposed systems demonstrate the feasibility of post-tensioning glass beams in terms of enhanced resistance at serviceability limit states with a more efficient use of material, as well as increased safety in the event of glass failure achieved through ductile failure behaviour. The complete data of the experiments can be found in a complementary technical report EPFL-REPORT-230071.