Optimisation of Network tied-arch bridges

with focus on rise-to-span ratio

Author: Moritz Studer
Language: English

Abstract

Network tied-arch bridges are defined as tied-arch bridges with inclined hangers that cross each other at least twice. First used in 1963 by its pioneer Per Tveit, the concept of network tied-arch bridges was avoided for years due to its complexity. Thanks to new computer-based calculating possibilities, it seems to have made its breakthrough only within the last decades. For this reason, this bridge type is still only researched to a limited extent compared to others. However, in order to meet requirements regarding structural safety, serviceability and durability, as well as economic, ecological and aesthetic aspects, further research is necessary.

This master’s thesis is a contribution to a larger research project at the chair of concrete structures and bridge design at ETH Zurich on Network tied-arch bridges. In the framework of this thesis, a parametric study on the rise-to-span ratio is carried out. For this reason, an optimisation process was developed for which the rise of the network tied-arch bridge, as well as the arch shape should be easily and quickly adjustable to gain as much information as possible on their static influence. For this geometry is generated using Grasshopper, a visual programming language and environment that runs in the computer-aided 3D design application Rhinoceros. With this tool, the model can be generated and modified only by changing the desired parameter. The subsequent structural analysis for the different geometries is done in SOFiSTiK.

Thus, the aim of the thesis is to gain an understanding of the optimum range for the investigated parameter and getting it as isolated as possible from other parameters. For the cases which the rise-to-span ratio was investigated, a structural optimisation was carried out, which included an optimisation of the arch shape, as well as an optimisation of the self-equilibrium stress state induced by the hanger pretension.

The reference case Blennerhassett Island bridge, a network tied-arch bridge with a rise-to-span ratio of 1/5, serves as the basis for the parametric study. For this, only the geometry of the arch is adjusted for a varying rise. Other parameters such as the hanger network arrangement, the lateral arch bracing system, the static system of the floating deck as well as the width of the bridge were not changed. It has been shown that the selected optimization criteria of the self-equilibrium stress state can have a decisive influence on the utilization of the various cross-sections of the structural elements.. The evaluation subsequently showed that by increasing the rise, the costs trend decreases considerably up to a rise-to-span ratio of 1/5, after which the costs reduction decreases. Taking into account the additional costs for the construction of a higher arch, which are not included in the cost evaluation, an optimum for the rise-to-span ratio of around 1/5 can be expected. It has also been shown that the rise-to-span ratio may also be strongly dependent on the bridge width. Consequently, no general optimum of the rise-to-span ratio can be given without knowing the width of the bridge. However, it can be observed that the rise-to-span ratio of the reference case lies within the range of the optimum, but there is nevertheless potential for optimisation for the chosen arch shape.