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Africa Rocks – "natural" habitats at San Diego Zoo

Visitors to San Diego Zoo can experience the biodiversity of Africa in a completely new way – like going on a safari on foot. This is not a place where animals spend their lives in grey concrete landscapes behind metal bars: Here the new 68-million dollar complex has different landscapes. The barrier-free view of the animals gives visitors the feeling that they are in the middle of a genuine habitat cleverly copied from its natural counterpart. This is all made possible by a barely perceptible delicate mesh construction which is the only separation between man and animal.

Over 32,500 m², the enclosure, opened at the end of 2017, offers a unique insight into the natural habitats of the different species in six biomes – from the Ethiopian Highlands through Madagascan forests to the coastal regions of South Africa. A 6,500 m² transparent stainless steel mesh construction made of “X-TEND” by Carl Stahl Architecture spans part of the enclosure. Following the natural, gently rising topography, it effectively spans eight individual enclosures lined up one after the other along a long, winding visitor's path. This is where all kinds of different animals find a home inspired by their natural habitat. The Acacia woodland biome, which houses leopards, meerkats and birds, features open grassy areas, wetlands and marsh plants. The Madagascar biome is home to honey badgers (ratels), fossas and lemurs and is dominated by pinnacle rock formations as well as alternating thorny woods and deciduous forests. The minimized roof mesh construction, which apart from nine inner pylons does without any additional sight-impeding supporting elements, forms a three-dimensional mesh structure creating space under the roof which can be used to the full. In this way, zoo enclosures made of prestressed stainless steel mesh promote the natural behaviour of the animals by extending their freedom of movement into the third dimension and allowing them to climb right up to the delicate mesh roof.

At the heart of the matter

Visitors to Africa Rocks are at the heart of the action: something ensured by the stainless steel mesh suspended between the roof mesh and the ground. The mesh is delicate and transparent, opening up visitors' views of the extensive enclosures. The individual sections of the wall mesh are fastened to each other without any visible joints or additional vertical or horizontal cables. This means visitors have an unimpeded view into the compound on their way along the enclosures. The individual enclosures are integrated harmoniously into the topography which, together with the paths, bridges, rock formations and vegetation, presents a virtually natural habitat.

Africa Rocks sets architectural standards

Elegant leopards, lemurs jumping from one tree top to the next, bustling meerkats, colorful birds and fossas that look like a curious mixture of dog, cat and mungo are the stars of Africa Rocks at San Diego Zoo. The only thing that separates them from the visitors is the graceful mesh of stainless steel cables. It is hardly noticeable but acts as a delimitation of the eight expansive enclosures and also spans the area as a transparent, organically shaped roof. The roof mesh is borne by just nine inner supporting pylons with load-distribution rings – an impressive construction at its size and minimization and without doubt a milestone in modern zoo architecture.

Paradigm shift in the relationship between man and animal

The exotic charm of wild animals has been attracting people to zoological gardens for more than 150 years. The reason for going to a zoo has not changed between then and now. What has changed though is the way people feel about the animals they are looking at. Whereas in former times the "wild creatures" used to be an object of curiosity, today they are seen as living beings who are to be treated sensitively and with respect. Animal conservation and preservation has become one of the major tasks zoos serve today. Zoos all over the world are respecting this change in the relationship between man and animal. Lots of zoos are commissioning new enclosures which, down to the very last detail, are created to reflect the natural habitat of the animals inside them. And visitors too benefit from this because modern enclosures are transforming zoos into an exciting world of adventure which not only lets us see lots of different animals but also gives us a glimpse of exotic landscapes live. Last but not least, sustainable zoo architecture is increasing the attractiveness of zoos. And San Diego Zoo, which celebrated its centenary in 2016, is pleased about the constantly growing number of visitors coming through its gates.

Existing trees skillfully integrated

Carl Stahl Architecture faced very special challenges when it came to the landscaping because a large number of the valuable trees, which were taken out of the old compound dating back to the 1930s for the duration of construction, had to be replanted while the mesh was being erected. "A total of around 800 trees were planted in the whole complex, around 150 of which are under the mesh surface," says architect Wolfgang Betzler, Head of the Carl Stahl Architecture technical office. "Since the trees can be anything up to eight meters high, it was not possible to replant them any later." And that meant precise coordination with the landscape gardeners. And the fact that among the plants there were some which were particularly worthy of protection, such as a well-established camel-thorn tree that birds had nested in for years and which under no circumstances should be damaged, did not make things any easier. At the northern end of the mesh construction the planners had to leave plenty of space around an old weeping fig with a crown diameter of more than 40 m so the roof mesh would not hinder its growth.

Ingenious assembly technique in 15 m height

Depending on the particular species, the wall mesh of the individual enclosures has a mesh width from 25 to 50 mm, the main roof mesh of 100 mm. To stop any small rodents or birds getting into the enclosures from the top, Carl Stahl Architecture assembled a second mesh layer on the main roof mesh over every individual enclosure. "We developed this overlapping of the mesh area especially for the Africa Rocks project," says Wolfgang Betzler. And that meant the assembly engineers were faced with a special challenge: At great heights of up to 15 m above the ground, they had to lay out the fine mesh, stretch it and fasten it to the edging cables on the main roof mesh from both a standing position as well as on their knees.

"To make sure the subsections were able to interlock smoothly during construction, we developed a special assembly technology for the project which considerably reduced the otherwise standard use of scaffolding and other assembly aids such as cranes and elevation platforms," says project lead Vito Gualazzini. "This was how we could ensure that for example the landscape gardeners could also carry out their work during the nine-month construction phase so that at the end of the day everything fit together."

Role model: spider's web

"Cable mesh constructions made of X-TEND can be stretched around enclosures and are thus ideally suited to modern zoo architecture," says Gualazzini. The 42-year-old civil engineer was involved in the Africa Rocks project from the very first talks with zoo management and architects more than three years ago to the final approval and hand-over of the operational enclosure to the customer. "They unite safety and aesthetics, are hard-wearing and durable, and offer virtually barrier-free insights and views thanks to their transparent structure," he explains. Quite appropriately, the planners based their ideas on nature: Like a spider's web, the area-covering structural element takes up the natural force path. The connection of sturdy stainless steel cables to a mesh of friction-pressed diamond shapes creates a special, flexible mesh structure which can take strong loads and offers a range of design possibilities. "Ultimately it is all about harmonizing the influential forces," explains Gualazzini. The lightweight formally minimized constructions have their own design language, which is always focused on the requirements of the particular species, topography, usability and appeal for the visitors.

Marlene Thimet, responsible for technical drafts for zoo architecture at Carl Stahl Architecture, explains:

"Together with zoos and architects all over the world, we develop organically shaped mesh constructions, adapted down to the very last detail to suit individual requirements. In the process we always take into consideration the demands of the everyday work and integrate inspection ports, sliding gates, trees and plants, climbing areas and places to sleep as well as of course entries and exits for the zookeepers".

Thimet also designed the mesh supporting structure in San Diego: "The enclosures are the result of our extensive experience and careful technical planning, precise structural analysis and ultimately also know-how in terms of assembling on site." The successful planning and structural analysis of the complex are not just the work of special, state-of-the-art software programs. In fact Carl Stahl Architecture actually makes great use of classical architecture model construction.

Thimet sees that as being particularly advantageous:

"For me the model is both a draft that makes it possible for me to calculate the perfect mesh form, as well as a visualization aid for the customer. I have found that the people responsible at zoos can work better with a model they can see and touch, something they can walk around and view from different angles, because it gives them the perfect idea of the planned enclosure."

For Carl Stahl Architecture, the San Diego Zoo project is the largest the company has realized to date worldwide. Around 17,000 m² of mesh were implemented with a net material weight of around 6.5 t.

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