An imaginative new approach to bridge-building was demonstrated for the first time in Austria last week, where engineers have connected the opposing sides of two rivers with a novel construction method likened to opening an umbrella. These unfolding bridges promise a number of benefits over traditional techniques, saving considerable time, money and impact on the local landscape.
Generally speaking, bridges are constructed piece by piece through the use of scaffolding, or through piers which act as vertical load-bearing structures as the bridge is carefully built outwards in either direction, bit by bit to keep things balanced.
For more than 15 years, researchers at the Vienna University of Technology (TU Wien) have been working on a different approach, and actually patented the idea way back in 2006. It involves hollow girders mounted to a pier in an upright position, which are joined at the top and and are gently unfolded downwards.
“The two girders are connected to each other at the top, directly above the pier,” Johann Kollegger of the Institute of Structural Engineering of TU Wien explains. “With hydraulic systems, this joint is then slowly lowered, and the girders unfold to both sides.”
Once these hollow girders have been lowered all the way and are horizontal, they can then be filled with concrete to complete the key structural components of the bridge. According to the team, this approach leads to bridges of the same durability as those constructed through traditional methods, while also saving a whole lot of time.
“Erecting bridges using scaffolding usually takes months,” says Kollegger. “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.”
By extension, this also saves greatly on the costs of constructing a bridge, while the method could prove particularly useful in areas where uneven terrain poses problems or is better left undisturbed, such as a nature reserve.
The team carried out large-scale testing of the method back in 2010, and has since been fine-tuning it ahead of its first real-world debut. This took place as part of construction of a motorway passing through the Austrian town of Fürstenfeld, that will span the Lahnbach and Lafnitz rivers.
To build these connections, hollow girders of 36 m (120 ft) were lowered on either side of a pier, creating a structure measuring 72 m (240 ft) long in total. Gaps between these horizontal girders and abutments were filled with suspended girders to complete a bridge of 100 m (330 ft) in length over the Lahnbach river and 116 m (380 ft) over the Lafnitz.
“Now that we have proven that the method is well-engineered and works perfectly, we hope that it will prevail and will soon become one of the common bridge building methods that are used around the globe and that the S7 Motorway will become an international pioneer,” says Kollegger.
Unfolding highway bridge unfurls like an umbrella [New Atlas]