BridgesA First: New Bridge Building Technology Successfully Used in Austrian Alps

Published 9 March 2020

There are many different methods for erecting bridges, but the new technique — the balanced lowering method — is quite spectacular: the bridge is not built horizontally, as would normally be case, but erected in a vertical position and then rotated into the horizontal position. The new bridge construction technology has now been successfully used in the construction of the Fürstenfeld Motorway in the Austrian Alps.

There are many different methods for erecting bridges – but the new technique developed by TU Wien, the balanced lowering method, is quite spectacular: the bridge is not built horizontally, as would normally be case, but erected in a vertical position and then rotated into the horizontal position. The first large scale tests were carried out in 2010, since then the method has been refined and tuned until finally finding its first application by Austria’s Motorway and Expressway Financing Joint-Stock Company (Autobahnen- und Schnellstraßen-Finanzierungs-Aktiengesellschaft, or ASFINAG) for two bridges of the Fürstenfeld Motorway S7 in the Tyrol region in the Alps Mountains in west Austria. With the erection process successfully completed for the Lahnbach Bridge, the 116 m long bridge over the Lafnitz was “unfolded” on 27 February 2020. With no scaffolding needed this new bridge construction method not only saves time but also money and resources.

The Umbrella Principle
“Depending on the size and location a variety of bridge building techniques are applied nowadays,” Prof. Johann Kollegger of the Institute of Structural Engineering of TU Wien explains. If the bridge is not too high from the ground it can be erected using scaffolding. Another technique is to erect the bridge pier and work from there in a precisely balanced way in both directions. Sometimes steel girders are constructed and then pushed forward bit by bit in a horizontal position until the span is completed.

The erection method developed by Kollegger is based on a completely different principle: girders are mounted in a vertical position on both sides of a concrete pier and are then unfolded, like an umbrella. “The two girders are connected to each other at the top, directly above the pier,” Kollegger explains. “With hydraulic systems, this joint is then slowly lowered, and the girders unfold to both sides.”

The girders themselves consist of thin-walled prefabricated elements with steel reinforcement and are initially hollow. Once they have reached the final horizontal position they are filled with concrete. “Erecting bridges using scaffolding usually takes months. The elements for the balanced lowering method on the other hand, can be set up in two to three days, and the lowering process takes around three hours,” says Kollegger.

TUWien says that the new bridge construction method not only saves time but also money, and the durability of the bridge is the same if not better than that of bridges built using other methods,