The Topology Optimised Glass Bridge

Abstract

This research investigates how to design a glass pedestrian bridge that is structurally optimised for multiple load scenario’s and feasible in terms of fabrication and construction. Linear glass elements such as panes have limited freedom of shape. On the other hand, cast glass can be produced into any shape possible. When glass is used in large quantities with large cross-sections, the annealing process can take a long time and require a significant amount of energy. The annealing time must be kept to a minimum to make cast glass a viable option. This is where topology optimisation can play its part. During topology optimisation, the layout of an element is optimised on the given constraints for the specified design domain. A compliance-based optimisation is found to be the most suitable objective function. The software package Ansys and various plugins for McNeel Rhinoceros/Grasshopper are compared on their topology optimisation tool. During this comparison, Ansys is found to be the most suitable software. This is mainly because of the possibility of implementing a cross-sectional constraint during the optimisation, which is needed for implementing a maximum annealing time. The most suitable production method for topology optimised cast glass elements is found to be kiln casting with a 3D printed sand mould. Glass is a brittle material with a low tensile strength compared to its compressive strength. The only type of bridge that can be made entirely in compression, without pre-tensioning, is an arch bridge. This bridge type is used for the final design. The loads considered during structural analysis are the vertical traffic load (distributed and point) and the vertical wind load. The bridge is made from borosilicate glass. This type of glass is used to reduce the effect of temperature on the cast structure of the bridge. Additionally, the supports and connections are designed so that they do not restrain deformation from temperature. Furthermore, the shape and place of the connections and supports ensure the possibility of unequal settlement without the formation of unacceptable stresses. Multiple variant studies are carried out. In the first variant study, optimisation is done on several 2D models to determine the best shape for the bridge's final design. The second variant study focuses on the 3D shape of the bridge and how the bridge is split into manageable segments. The final bridge comprises four cast outer elements, one cast keystone, and a float glass bridge deck. The bridge is simply supported on both sides of the canal. The connections and supports are based on compression and friction and use a polyurethane interlayer. The weight of the final optimised bridge is reduced from 32t to 11.2t with a maximum cross-sectional thickness of 210 mm to get an annealing time of approximately one month. Due to symmetry in the design, only one-fourth of the bridge is optimised. During the optimisation, seven different distributed load scenarios are implemented. For the point load, a region is excluded from optimisation. After optimisation, finite element analysis and manual verifications are performed.