Optimisation of Network Tied-Arch Bridges

Author: Fabian Killer
Language: English

Abstract

Bridges play a central societal role by providing essential connections across natural and man-made obstacles, enabling efficient mobility. Among the various bridge types, Network Tied-Arch Bridges represent a distinctive class, often used both functionally and as expressive architectural structures. Their design, however, is frequently guided by personal experience rather than systematic optimisation.

Network Tied-Arch Bridges are defined by an arch positioned above the deck and by inclined hangers that intersect at least twice. This configuration creates truss-like behaviour within the arch plane, resulting in a stiff structural system even with slender elements and offering particular advantages under asymmetric loading. By appropriately prestressing the hangers, bending moments in both the arch and the tie girder can be significantly reduced.

This thesis revisits and extends four previous Master’s theses on Network Tied-Arch Bridges completed in 2020 and 2021 at the Chair of Concrete Structures and Bridge Design. Neglected aspects—such as construction costs, buckling behaviour, floor system considerations, and the economics of short-span bridges—are critically examined and incorporated where necessary.

A preliminary design of two 40 m-span Network Tied-Arch Bridges is developed using a SAP2000 3D model. The two designs differ in their floor systems: one employs a conventional concrete deck, while the other uses a steel–concrete composite system. This comparison enables an assessment of how deck-level configurations influence global behaviour and highlights remaining gaps in current design approaches.

The results demonstrate the significant impact of construction costs on optimal arch geometry. Higher costs for work at elevated positions favour lower rise-to-span ratios compared with designs that ignore such effects. The relevance of second-order and buckling influences is also underlined; while they reduce arch resistances, they only partly affect optimal hanger arrangements. Achieving an optimal layout requires balancing the objectives of minimising global forces and limiting the maximum unbraced length of the arch, acknowledging that improvements in one aspect may compromise the other. Overall, previous recommendations on hanger configurations largely remain valid, though certain arrangements become more advantageous under specific boundary conditions.

Finally, the preliminary design highlights the material demands associated with the two floor systems. Composite decks offer clear benefits in terms of reduced dead load—a decisive advantage for wider or longer bridges—allowing for lower internal forces and more slender structural designs.