Traffic is now flowing on Prague's new Troja Bridge following its much-delayed recent opening. Straddling the Vltata River, the steel-arch structure incorporates crisscrossing steel-rod hangers, giving it one of Europe's most slender bridge profiles.
Despite the lower-than-normal arch height, the structure is "very stable concerning the dynamic response," says Vladimír Janata, chief technical officer and chairman of the bridge's steelwork design subcontractor Excon A.S.
Opened in early October, the bridge was delayed by some six months by a review of costs following the 2013 election of a new city mayor, reports Janata. The bridge carries two central tram tracks and four road lanes on its 30-meter-wide steel-and-concrete deck.
With the arch rising 20 m over its 200.4-m-long deck, the bridge's height-to-span ratio is approximately half the norm, according to lead designer Ladislav Šašek, at the Prague office of Mott MacDonald Group London.
Using steel-rod hangers, rather than cables, made the low profile possible, reducing wind resistance and total weight, adds Šašek. The hanger system comprises two planes, each with 100 crisscrossing stressed rods up to 10.5 centimeters in dia.
The arch scheme emerged after a 2006 design competition. The arch scheme replaced a cable-stayed design that formed the basis of the construction contract with locally based Metrostav A.S., says Šašek.
Despite the profound change, the bridge remained part of Metrostav's contract, which also included 5.5 kilo- meters of tunnels to form part of the new ring road. Negotiating the design change delayed construction by about three years, according to the contractor.
During the schedule delay, the general contractor experienced several tunnel collapses and had an ongoing legal dispute with the city; that dispute was recently resolved. The tunnels are due to open to traffic this December, many months behind schedule and, at $1.7 billion, reportedly 75% above budget.