Zaha Hadid Architects

Zaha Hadid Architects Three structural shapes of bridge come together as they approach right bank

Fast-track construction of an unusual bridge and building at Spain’s Expoagua Zaragoza 2008 will not be fast enough to provide a cushion of time ahead of next month’s exhibition opening.

Its flowing, organic profile will make the “Pavilion Bridge” Expo’s landmark project, and its trickiest. The official inauguration will occur immediately before the June 14 start. “It is the [last] building that’s going to be ready,” says Expo spokeswoman Livia Alvarez. “It’s [been] the most difficult since the beginning.”

Pavilion Bridge will be the Expo’s main access and forms one of two crossings over the Ebro River to the site. It will provide 4,000-sq-meters of exhibition space and a 2,500-sq-m walkway.

Curved in two planes, the 275-m-long structure crosses the river in two spans separated by an island. From the left bank, the structure starts as three conduits with intersecting diamond-shaped sections. Toward the right bank, the conduits merge.

Bridge supports are neither parallel to each other nor aligned. The shortest distance between supports is 94 m in the left bank span and 146 m in the right. Its width ranges from 12 m to 29 m.

Slideshow Pavilion Bridge, Zaragoza, Spain

The flowing profile is characteristic of London-based Zaha Hadid Architects, which led the winning team of a design competition in 2005. Arup Group, London, is handling all engineering design.

Pavilion Bridge’s design difficulties stem partially from its hybrid nature, says Carlos Merino, Arup’s design coordinator and the site director. It includes the finishes, equipment and the vibration limitations of a building. It also must satisfy bridge codes.

Geometry added complexity, says Merino. His team faced a “3-D idea in the air,” he says. To allow the engineers and fabricators to understand it in 2-D, designers established 76 parallel ribs, 3.6-m apart and linked by plane surfaces.

The structure is effectively a tube, adds Kevin Acosta, a senior Arup designer. It includes a hexagonal box beam about 1-m wide and deep at the crown. Sides above the deck are “diagrids.” Below the deck, they are steel plates.

Arup’s original plan included a triangular truss below-deck structure. That would have been covered in mesh and sprayed concrete. But the construction joint venture of Dragados S.A. and URSSA S. COOP secured approval for an alternative using plate assemblies in a “shipbuilding style,” says Acosta.

Zaha Hadid Architects Steel grids comprise the above-deck parts of the hexagonal bridge beams.

“It was easier to build,” says the contractor’s head of engineering, Alejandro Mendoza. Since the contractor planned to install one span by sliding, “We weren’t sure about the first design by Arup.”

The contractor won the $51-million contract in August 2006. An 80% hike is said to provide the backdrop to ongoing negotiations. The original 13-month schedule proved inadequate. “A lot more details came up during the site stage,” says Merino, citing that as just one cause of delay. “There were fundamental fabrication issues that did affect the design.”

The contractor’s plate-structure design initially trimmed steel weight to substantially below the roughly 5,000 tonnes of Arup’s scheme. But the contractor’s detailed design pushed it to 5,800 tonnes. “There was a lot of reinforcement,” says Mendoza.

Kevin Acosta Crossing over Ebro also provides exhibition space for Spanish exposition to be held this summer.

Located on unreliable karstic ground, the bridge is founded on 2-m-dia piles, some exceeding 70 m in length. With piles done, the contractor filled in the river for the shorter span, allowing erection on falsework. The other span was fully assembled on the right bank and pulled into place by twin cables from 40-m-tall temporary pylons on the island.

Because of space constraints, the 2,400-tonne assembly was erected 9 m off alignment, then slid sideways and forward, says Pedro Garcia, project engineer with the sliding subcontractor, ALE Heavylift Iberica S.A. This was trickier than expected, taking 80 rather than 53 days, says Mendoza. The contractor did not opt for Arup’s method of installing a temporary river pier because it would have meant waiting to complete the other span and clearing the temporary fill first, says Mendoza.

Structural completion, originally due last September, was substantially achieved this January. Now working triple shifts on cladding, finishes, services and exhibits, the project team is in “the final dash,” says Merino.