Optimization of network tied-arch bridges
Author: Riccardo Cavegn
Language: English
Abstract
Nowadays structures in general, but bridges in particular have to meet a large number of requirements. For example, a bridge must be able to guarantee the safety of its users and the surrounding people as well as the intended use for its purpose, and this accordingly throughout its lifespan. This places strict requirements regarding structural safety, serviceability and durability. In addition, bridges must also be able to be built economically and should adapt well into the environment. The former results in the requirement that the structures should be as slender as possible so that the material consumption and accordingly the material cost can be reduced.
This Master’s Thesis tries to give a contribution for the optimization of network tied-arch bridges. Network tied-arch bridges are tied-arch bridges, in which the hangers are not arranged vertically but with an inclination so that they are crossed by each other. In their current form, they represent a rather new system. Therefore, not all aspects of the structural behaviour are yet known for this bridge typology.
The subject of this work is the development of an optimization process for such a network tied-arch bridge and its application to a reference bridge. The developed process considers the parameters arch geometry, self-equilibrium stress state and the hanger pattern (arrangement). With the self-equilibrium stress state, the hangers of such a bridge are prestressed so that a favourable structural behaviour for the tie girder and the hangers can be achieved. Regarding the arch geometry parameters, the arch geometry is set so that it matches the thrust line for permanent loads. With respect to the parameters of the hanger pattern, a suitable pattern is sought, which optimizes the stresses of all structural elements so that the lowest possible material costs result. Both symmetrical and asymmetrical load cases are therefore considered.
The parametric study carried out for the reference bridge showed that with an appropriate choice of the self-equilibrium stress state and the arch geometry, a considerable reduction of the bending moments in the arch rib and the tie girder can be achieved. It was also shown that the corresponding thrust lines of the examined variants can deviate very much from each other as well as from a parabolic or a circular arch and is therefore strongly dependent on the specific hanger pattern. Last but not least, the parametric study, in which a total of 89 hanger arrangements were examined, showed that the material costs of an arch structure can be minimized when using hangers arranged as a fan where the fan opens towards the arch.
With the knowledge gained in this Master’s Thesis, it can be said that when designing a network tied-arch bridge, the hanger pattern with a fan, which opens towards the arches should be chosen. Both the self-equilibrium stress state and the arch geometry should be optimized for the selected hanger pattern by prestressing the hangers and adjusting the thrust line. Considering these aspects in the design, a favourable structural behaviour and thus a reduction of the material cost can be achieved.