A 1.2-km-long viaduct featuring a novel, 4.5-m-deep composite deck box girder with steel tube webs designed to enhance prestressing efficiency and save weight is nearing completion in France. The $50-million Meaux Viaduct 40 km east of Paris is part of the government’s bridge innovation program.

WEB OF STEEL French viaduct features steel tube webs that enhance prestressing and weigh less than concrete box girders. (Photo above and top courtesy of Razel S.A.)

As Paris-based contractor Razel S.A. advances the deck by a span every two weeks, bright paintwork highlights the unusual steelwork. Green deck box sides are comprised of roughly 51-cm-dia vertical tubes, set just over 1.5 m apart and linked by welded plates up to 2.5 cm thick. The tubes are embedded near the edges of the 13-m-wide concrete base and into the 31-m-wide top slab. Outside the box, blue tubes lean out from the bottom corners to prop up edges of the 22-cm-thick top slab.

"It’s a very interesting idea," says Daniel Lecointre, technical director for large bridges at the government highway design agency, Service d’Etudes Techniques des Routes et Autoroutes (SETRA), which checked Razel’s design. "You don’t transfer the forces of the prestressing into the web [because] the tubes can distort and all the compression goes into the two slabs."

Curving across the Marne Valley, the viaduct lies on a new 20-km stretch of the A140 highway being built to carry traffic around the city of Meaux. Its 22 spans rise 30 m above ground in lengths from 49 m to 59 m, rising to 93 m over the Marne River. Because of its size and prominence, government engineers sought innovative engineering with an attractive design.

Rather than bidding conventionally, the regional transportation ministry, Direction Departmental de l’Equipement, called for contractor ideas four years ago. This procurement approach aims to encourage new developments from within the industry. But "it is very difficult to find an owner who is able and wants to take the responsibility for innovation," says Lecointre, who was on the government panel that selected the winning project.

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Out of eight French bidders, the team of Razel and Paris-based architect Berdj Mikaelian won in October 2000, taking on design responsibility. Razel’s price was 4% higher than the lowest bid, which offered a standard concrete box girder. But "the main criterion was an innovative solution," says Placidi.

Weighing some 10% less than an all-concrete design, the deck needs less prestressing, says Placidi. The tubes also allow webs to offer little resistance to pre-

stressing. And the webs themselves further enhance prestressing by concentrating deck masses near the top and bottom extremities.

Additionally, the web’s wide tubes form robust moment connections with the slabs, increasing the box’s torsional resistance, says Placidi. Internally, inverted Vs of 35-cm-sq concrete elements brace the box at roughly 3.1 m intervals along the bridge, aligning with the blue external struts.

As DSD-Socometal S.A., Sainte Ave, supplies and installs some 7,000 tonnes of steelwork, Razel is jacking the entire deck over piers from behind the east abutment. Deck erection got off to a slow start in Dec. 2002, and Razel is about a month late. Jacking a six-lane viaduct took getting used to, says Lecointre. Progressively adding 29-m lengths at the rear, Razel has pushed the deck as far as a temporary pier in the middle of the Marne River. With jacking forces growing to 55,000 tonnes, the deck is due to reach the west abutment this May.

As a prototype, the viaduct is marginally more costly than a conventional alternative. But with repeated use, "it will be less expensive...because each material works in the best way," says Placidi.