The steel-rod hangers on the Czech capital's new Vltava River crossing—one of Europe's most slender, bow-string arch bridges—are crisscrossed, not vertical. This design saved up to 40% in materials, the designers claim.

Prague’s Troja Bridge, which stood on its own once crews decommissioned temporary river piers in March, is slated to open next spring.

With a steelwork arch rising only 20 meters over its 200.4-m-long deck, the bridge’s height-to-span ratio is about half what is typical. That results in shorter hangers, reduced wind resistance, less steelwork and lighter foundations, claims the bridge's lead designer, Ladislav Šašek, in the Prague office of London-based Mott MacDonald Group. Stiffness of the network of solid-steel rod hangers plays a major role, he adds.

The estimated $55-million bridge in northern Prague will carry two central tram tracks and four road lanes on its 30-m-wide steel-and-concrete deck. Steelwork walkways cantilever from its sides.

Forming part of Prague's new inner-city ring road, the bridge was included in a construction package awarded some nine years ago by the city to Metrostav A.S., Prague. The project also includes the 5.5-kilometer Blanka series of road tunnels, says Jan Vítek, a senior specialist with Metrostav.

Stressful Beginning
At first, the bridge was envisioned as a cable-stayed structure, says Mott's Šašek. But the city rejected that option and ordered a design competition. Mott, with local architect Roman Koucky, won the competition in 2006. Rather than call for new bids, the city negotiated new terms with Metrostav.

The contractor struggled to persuade the architect to accept design changes that would ease construction, says Vítek. However, the rules of the competition effectively gave the architect veto power over modifications, he explains.

Negotiations lasted three years. By then, the bridge had moved off the ring road’s critical path, says Vítek. The tunnel project was running late, partly because of at least three collapses through July 2010.

In the final design, the bridge’s steel-box arch is a slender structure, measuring 6.2 m wide and 0.8 m deep at its crown. The box widens and then splits into two 4.5-m-deep, 1.1-m-wide legs. At the deck level, two longitudinal prestressed concrete beams tie together the opposite arch legs. The beams also enclose steelwork girders, which provide anchorages for the 200 rod hangers and the underslung crossbeams of the deck.

On either side of the bridge, two sets of 50 hangers are inclined in opposite directions to increase the structure’s stiffness. To minimize hanger stretch, steel rods up to 10.5 centimeters in diameter are used instead of cables.