Innovative structural concepts for motorway flyovers
Author: Jean-Philip Zindel
Language: English
Abstract
Planned motorway widenings in Switzerland, such as the extension of the A1 to six lanes, have renewed interest in efficient and durable flyover concepts. A first step toward developing improved solutions is to analyse existing overpass types, compare older and more recent designs, and identify the requirements that define modern flyover construction. This assessment highlights the importance of selecting structural systems that can accommodate longer spans and tight clearance constraints.
This thesis investigates the potential of composite girders as an innovative option for future motorway flyovers. Because concrete deck slabs remain essential for load distribution, their geometry and material must be optimised to achieve sufficient resistance while minimising weight. Deck and girder design are interdependent: the slab thickness influences the steel girder’s effective static depth, while the girder system constrains feasible slab configurations.
Innovation must therefore combine structural optimisation with material development. Different steel grades for the girder and various concrete types for the deck were evaluated. Particular focus was placed on UHPFRC, whose high compressive strength suggests the possibility of substantially reducing slab thickness. Structural analyses using Rüsch tables, FEM simulations, and fatigue checks show, however, that weight reductions are significantly smaller than often assumed. Compared with a 200-mm C50/60 slab, a full UHPFRC slab achieves only a 20% reduction in self-weight. Meeting fatigue requirements also demands unusually high reinforcement ratios—up to 3.7% for full slabs and 3.2% for ribbed slabs—which are more typical of columns than of slabs.
Material costs and environmental impacts were also assessed. A conventional 200-mm concrete slab costs about 50 CHF/m, whereas UHPFRC full and ribbed slabs cost roughly 320 CHF/m and 270 CHF/m, respectively. CO₂ emissions are 120 kg for C50/60, compared with 208 kg for a full UHPFRC slab and 176 kg for a ribbed UHPFRC slab—an increase of at least 45%. Considering deck replacement cycles of about 20 years, a full UHPFRC slab remains around 22% more expensive over the service life. Ribbed UHPFRC slabs, however, reduce this difference to about 7.6%, making them a more promising option.
