Structural design of the Birssteg in Basel-Stadt

(original title: Tragwerksentwurf des Birsstegs, Basel-Stadt)

Author: Roman Gisler
Language: German

Abstract

The existing footbridge over the river Birs in Basel is in a bad condition. Therefore, Tiefbauamt Basel-Stadt plans to replace the pedestrian bridge, which is an important connection between the district Breite (Basel-Stadt) and the municipality of Birsfelden (Basel-Land). The requirements are summarised in a service criteria agreement. The decisive boundary conditions are the deck width of 4.5 m, the low height in the shore area, the flood level, the limited space at the abutments and the maximum allowed gradient line of 4%.

Various typologies for the structural concept and their feasibility were examined, especially considering the structural and economic efficiency, the construction process and the aesthetic quality. The best variant in the evaluation was a Vierendeel arch bridge. The structure follows the principle of a tied-arch bridge where the deck is acting as the lower tension chord. The hangers have rigid connections to the arch and the girder. The increased stiffness reduces deflections especially for asymmetric load cases and stabilises the arch against buckling. The main structural elements consist of hollow cross sections made out of construction steel S355. The Vierendeel arch frames are inclined by 20°, which generates a more open space for the user. The static modelling was done in the software AxisVM as a beam model. The static and dynamic examinations led to the design of the decisive dimensions and proofed the general feasibility of the chosen solution.