Troja bridge design saves 40% on materials

Without the cable network, the crown of the arch would have to stand more than 50m above the deckHow we’re adding value: We’ve developed a bridge type – the network arch – that is dramatically stronger than its structural relative, the bowstring arch, significantly reducing construction cost.

Simply the stiffness of the hanger network makes it possible to use a lot less steelwork. Efficiencies in the way the bridge works are driving efficiencies in the way it’s built

Prague’s Troja Bridge represents a major forward leap in structural efficiency. We have dramatically improved the performance of the classic bowstring arch by reconfiguring the hangers, which conventionally are vertical. On the Troja Bridge the hangers are arranged diagonally, creating a criss-cross ‘network’. This stiffens the bridge and spreads loads from the deck over a larger area of the arch, allowing major design efficiencies. The innovation enables materials savings of 40% compared to a conventional bowstring arch. Construction cost is £29 million.

Part of Prague Ring Road, Troja Bridge will carry trams, a dual carriageway, pedestrians and cyclists across the River Vltava. It will be 200m long and 34m wide, with a welded steel box section arch and post-tensioned, reinforced concrete tie beams.

Precast concrete transverse beams will be bolted to the longitudinal beams. These beams will support a post-tensioned concrete deck, with steel cycle/pedestrian decks cantilevered from this. Construction began in autumn 2010 and is due for completion at the end of 2012.

Innovative network arch

Mott MacDonald lead designer Ladislav Sasek describes the Troja Bridge as a “network arch”. While bridge designers have known the potential advantages of such structures for decades, their complexity has made them impossible to analyse. “It’s only with the advent of very fast computers that we’ve been able to move the design of network arches from theory into reality,” Ladislav says.

Bowstring arches are structurally elegant. Outward spread of the arch is resisted by tension in the deck. This makes the design of abutments relatively simple as they do not have to contend with lateral forces. However, the strength of the bridge requires perfect curvature of the arch. “Depending on the forces applied, arches must have a particular height to span ratio,” explains Ladislav. “That is dictated by the amount of deformation that can be tolerated. When load is applied to an arch it gets pulled out of shape. If it deforms too much the flow of forces through the arch is destroyed and it will collapse.” The longer and lower the arch, the more crucial it becomes to keep the arch true.

Quantum leap in performance

With a conventional bowstring arch, vertical hangers transmit traffic load from the bridge deck to the section of arch directly above. To provide the necessary structural resilience most bowstring arch bridges are designed with a height to span ratio of 1:4. By contrast, Troja Bridge has a height to span ratio of 1:10. “The cable network spreads traffic load across a wide area of the arch. That reduces local strain in the arch and deck,” says Ladislav. “The network enables us to use the arch far more efficiently.”

Driving efficiencies further

Without the cable network, the crown of the arch would have to stand more than 50m above the deck. With the network, that height reduces to 20m. “Simply, the stiffness of the hanger network makes it possible to use a lot less steelwork,” Ladislav summarises. “We’ve got a shorter arch and, as a result, shorter hangers. There’s less dead weight and, because the superstructure has a lower profile, wind resistance will be reduced too.” This has enabled reductions in both the cross section of the arch and the thickness of steel plates used to construct it. “Efficiencies in the way the bridge works are driving further efficiencies in the way it’s built,” he says.

Did you know

Other landmark bridges we’ve designed include Hong Kong’s Lantau Link – the world’s longest suspension bridge carrying both road and rail – and Bangkok’s twin cable stay bridges across the Chao Phraya River.