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Sun 19 Oct 2008 04:00 AM

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Building bridges

Zaha Hadid Architects’ Zaragoza Bridge Pavilion introduces a new building typology to the City of the Four Cultures.

Zaha Hadid Architects’ Zaragoza Bridge Pavilion introduces a new building typology to the City of the Four Cultures.

‘Building bridges' is a phrase with a variety of meanings. There's the literal interpretation, of creating a crossing between two points. Then, there's the idea of connecting people to create a better understanding.

Zaha Hadid Architects' (ZHA) Zaragoza Bridge Pavilion manages to neatly combine the meanings of the phrase in one remarkable structure.

Expo Zaragoza 2008 was held in the Spanish city between June and September, covering the theme of water and sustainable development.

During 2005, the city authorities held a competition to design a bridge and a pavilion which would illustrate the importance of water as resource. ZHA's unique take on the competition, held in June 2005 proved to be a winner of an idea, according to project architect Manuela Gatto.

"All the other competition entries separated the two elements, probably for reasons of time," explains Gatto. Because of the nature of the Expo hosting process, time was particularly short.

The project needed to be completed by June 2008, leaving less than three years for the complete design and build of the project.

"All the other entries were thinking of placing the pavilion on the riverbank and then constructing the pedestrian bridge separately. What the client appreciated was that our entry didn't go in that direction," she adds.

The combination of exhibition pavilion and crossing structure makes the Zaragoza Bridge project entirely unique within Spain, as it is the only inhabited bridge within the country.The typology of the project also presented an exciting challenge.

"[This kind of bridge] was really used more in the 19th century, so it was really interesting to see how you can actually have a new take on a typology that has disappeared," says Gatto.

Generally, inhabited bridges haven't been built for over 100 years, which means there are few ‘templates' for architects to draw inspiration from. Nevertheless, despite being a somewhat revolutionary typology, Gatto is pragmatic about the idea of creating something contextually new.

"The structure wasn't a total invention, because in a way the basic elements are typical of bridge construction," she explains. "It's a series of diaphragms which collaborate with the deck, and that's typical bridge construction."

The real revolution stems from the fact that the bridge is curved and asymmetrical. "The spine of the whole structure is ingenious. If you narrow it down to the minimum elements, it does have the concept of a bridge, but the level of asymmetry and the variation of sections was the challenge," says Gatto. Despite this added structural challenge, ZHA made it clear they would accept no compromises on the design.

Engineering challenges

The asymmetry of the bridge and the uncertain quality of the ground also meant the bridge had to overcome several structural challenges.

Design:Zaha Hadid and Patrik Schumacher

Project Architect:Manuela Gatto (Associate)

Client:Expoagua 2008

The site was selected by the client because of the presence of a natural island in the river, which helped mitigate some of the potential problems.

"The island in the middle provided a very useful footing for the bridge where we could gain a support point without interfering with the river too much," says Gatto.

The island provides the home for the conical main support, which bears half of the structural load for the entire bridge-roughly 7,000 tonnes. The island also contains the lion's share of the 22 piles used for the bridge-10 of which are located on the central isle.

The bridge also has the deepest piles ever used in Spain, reaching up to 72.5 metres deep. The ground is generally inconsistent and it varies dramatically metre by metre.

"You can do boreholes before doing the foundations and you'll never be entirely sure you're getting an accurate result," says Gatto. "For this reason there were load tests being carried out once the foundations were put in place."

Despite Zaragoza's arid climate, flooding remains a possibility on the River Ebro, so a wedge of land was build on the right bank to support the bridge so that the entrance rests above flood level.

Meanwhile on the left bank, the ground is laid out in wide bands.

Dimensions

Length:270m (185m from the island to the right bank + 85m from the island to the Expo riverbank)

Distance between foundations:155m (from the island to the right bank) and 125m from the island to the Expo riverbank)

Floor area:6,412sqm

Maximum height:30m

Foundation piles:68m

North Section (3 ‘pods') weighs 3,500 tonnes

South Section (1 ‘pod') weighs 2,200 tonnes

One band is slightly raised to cover a small structure containing the bridge's service elements, which need to be located on land.

Unique typology

The practice claims that the idea of incorporating the pavilion within the bridge merges two traditionally distinct and separate building typologies-the ‘infrastructure' element (i.e. the bridge) and the ‘architectural' element (i.e. the pavilion).

"As the Bridge Pavilion is not one particular building typology, this really adds to the richness of the spaces inside," explains Hadid.

"We build the complexity of all our projects in relationship to the inherent complexity of the program, but then clarify the diagram as much as possible to be a logical configuration. All the forces operate at the same time, so that the view of the exhibition is inherently related to nature of the visitors' path through the Bridge Pavilion - but equally, the experience of the path shifts according to what the viewer is seeing at a given moment."

The bridge itself is gladiolus-like in design, with one narrow end resting on the right bank of the river and incorporating three ‘stems' that branch out and rest on the site of the Expo 2008.

Each stem hosted an exhibition on the theme of water being a unique resource, with the stem structure encouraging a natural flow of people through the building-much like the consistent flow of water.

Maintaining a steady flow of people was important as the bridge provided a primary access point to the Expo site.

Chronology

Competition:June-July 2005

Preliminary design:September 2005

Scheme design:October-December 2005

Tender documentation for facades and structures:January-March 2006

Tender documentation for interiors detailing and plants:June-December 2006

As a result, the linear route through the bridge can accommodate 1,300 people an hour over a 650m walk, which takes 21 minutes as a straight walk through, or an estimated 45 minutes when stopping to look at the exhibition elements.

Sustainable by design

For a development that was to be a landmark for an exposition on sustainable design, there was a clear need for the Bridge Pavilion to embody the principles of the Expo.

"The client already had a clear idea of the materials they did not want to see, meaning for instance, PVC wouldn't be allowed, or certain types of insulation. They didn't want materials that would be difficult to recycle," said Gatto.

A façade of concrete reinforced with glass fibre was selected because the material is partially recyclable. The material also has a degree of inertia to temperature change, important for reducing the overall cooling load required for the bridge.

Zaragoza has a surprisingly hot and arid climate for a European city, with temperatures reaching up to 40ºC in summer.

Increasing shade within the building while minimising the loss of light was an important consideration. While façade is generally opaque, openings are distributed across it; some glazed, some left as perforations.

"There's fairly diffused internal illumination, creating interesting patterns on the floor depending on the position of the sun," says Gatto. The patterning of the façade itself is inspired by shark scales.

Foundations:February 2006

Structure:January 2007

Push launch of south section into position:December'07-January'08

Facade:November 2007

Interiors:March 2008

Project completion:13 June 2008

The perforations also allow for a degree of natural ventilation, although ZHA had to balance the loss of flexibility created by this with the need of an exhibition space to be flexible. 1/3 of the bridge is ventilated naturally.

"That dramatically cuts down the cost of the life of the bridge in terms of electric and water consumption," says Gatto.

Beyond the expo

Although the Expo has now finished, the bridge will continue to provide a unique crossing point in Zaragoza.

Despite primarily having been designed to deal with the extremes of the city's summer climate, Gatto indicates that the structure will be modified slightly to make it a true year-round design.

"It was created for the specific climate condition of summer, which is around three months. Whatever needs to be done to make it more suitable for use in winter, for instance, will be done," she says.

"But in general I don't think there will be much drastic variation."

Talks still continue over the future use of the bridge, but it is likely that it will maintain its role as an educational and cultural facility.

As well as building a bridge across the river, ZHA's design is likely to provide a bridge to knowledge for some time to come. "I learnt so much working on this project," says Gatto. It's likely that the end users will too.

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