Abstract of: Residual Load-bearing Capacity of Spannglass-Beams - Effect of Post-tensioned Reinforcement

Abstract

Reinforcement and pre-compression of concrete beams results in a ductile response and allows for the design of structures that are more reliable and efficient. Therefore, this approach has been adapted to structural glass producing reinforced and post-tensioned glass beams. Unbonded tendons in Spannglass Beams are one  structural example for realising this idea and are the subject of the presented study. Two individual laminated safety glass packages connected locally by metal dividers characterise these beams. A gap between the  packages was used to guide tendons in a either three- or a four-point bending configuration. This eccentric layout allowed for an initial uplift of the cross section and a mechanical pre-compression of the vulnerable glass edge. This was the primary objective during the initial design stage of the concept of Spannglass Beams. However, proving sufficient residual load-bearing capacity is a further requirement to ensure the safety of glass structures and requires additional study. During this stage, a common approach is to evaluate the effect of broken layers in the glass section, which may result in eccentric loading by the tendon and introduce additional bending about the minor axis. Thus, the novel structural design may cause an early (lateral) failure of the structure even during a service load condition. We examined a set of 16 Spannglass Beams with 5.0 mm; 8.1 mm and 10.1 mm post-tensioned cables in an experimental study during four-point bending. Additionally, four un-reinforced specimens and four beams with untensioned tendons were tested. The paper includes a testing method to evaluate the
residual load-bearing behaviour. First, the specimens were loaded in bending, after which a single glass layer was damaged manually by a hammer and a chisel. Finally, the assembly was left for 24 hours before taking a next step to damage a consecutive layer. In this manner, it was possible to evaluate the residual load-bearing behaviour in terms of residual service-life as a function of damage scenario and load. Additionally, the crack pattern after each step, the evolvement of the deflection and the change in cable load were analysed before the final failure modes were characterised. This report aims to describe the effects of post-tensioning on the residual load-bearing capacity of glass beams. It was possible to influence the structural response considerably by reinforcing and post-tensioning glass beams. Due to excessive lateral deflection, an additional cable load  reduced the residual service-life. However, the number of connectors in Spannglass Beams  determined the shape during bending with a special focus on the buckling length and thus contributed to increasing the service-life as a major parameter during design. Finally, we derived structural recommendations for a future design of  effective glass beams in facades, roofs or even “floating” glass bridges.