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Admiring monkeys through a remarkable netted bow structure Iv-Bouw

A new outdoor enclosure with maximum visibility

In Rotterdam Zoo (Diergaarde Blijdorp), the building formally used as an enclosure for carnivores (a national monument designed by Architect Sybold van Ravesteyn), has been completely restored and renovated into a new enclosure for Geladas. A Gelada is a unique type of primate from Ethiopia which closely resembles a baboon. Rotterdam Zoo is the only zoo in the Netherlands where these monkeys can be admired. All the more reason to design the new outdoor enclosure in a way that provides maximum visibility of the monkeys, and in a setting that imitates their natural habitat. The solution was found in the form of a remarkable structure consisting of striking steel bows spanned with huge transparent nets. This solution provides maximum visibility for viewing both the monkeys as well as the monumental building, plus the monkeys have the freedom to do as they please without being able to escape. The design of the outdoor enclosure was realised together with Rotterdam Zoo, fmarchitecten, CAE (part of Iv-Bouw) and Walraad Architecten.

A complex design process

The original enclosure for predators, constructed in 1939, was built in a functional style in combination with Neo-Baroque elements. As the building is listed as a national monument, it has been restored as much as possible to its original state. An important requirement was that the monument was to be adapted to satisfy new insights concerning animal welfare and health and safety requirements. Due to past modifications and the overgrowth of plants, a significant backlog of necessary maintenance and damage to the building had accumulated. This accumulation had to be taken care of to preserve the monument. Increasing the aesthetic value of the building and the realisation of a tunnel for visitors were important aspirations for this project. The design challenge, therefore, lay in uniting these amended requirements and objectives while optimally preserving the national monument.

The main supporting structure

The main supporting structure of the new outdoor enclosure consists of two large steel bows reaching up to 12.5 metres high, and five smaller bows. The bows are held in place by steel cables with diameters of
12 millimetres and 16 millimetres. The tension forces in the steel cables are transferred to the new roof of the monumental building, which balances both sides of the bow structure.

Huge transparent nets have been applied over the bows. The steel bow trusses are circular hollow sections, ranging from 355/20 for the largest span to 244.5/10 for the smaller spans. The larger bows span a distance of no less than 61 metres and 40 metres and, without the restrictions of columns, provide an open outdoor enclosure. Thus, the structure offers a perfect space to imitate the natural habitat of the Geladas with artificial rocks and boulders, hardy shrubs and trees that resemble the natural vegetation of the Ethiopian lowlands. The five smaller bows span approximately 22 metres and are situated at the outer edges of the outdoor enclosure. These, together with the stainless steel cables and netting, enclose the outdoor area. This structure offers the possibility to view the Geladas in the outdoor enclosure at a short distance from any angle, and you can almost stand face-to-face with these extraordinary monkeys.

Anchoring and distribution of the bow trusses

Much attention has been given to the design concerning the anchoring and distribution of the steel bow trusses. The bow trusses transfer the forces to the foundations via twisted flare-shaped components in a cast steel base. The forces and the required rigidity, combined with the small area available in the supporting points for guiding the forces, were such critical factors that a solid steel twisted flare-shaped component provided the best solution.

The twelve cast steel base pieces have been made in six different variations, each with a unique position and angle of rotation. The casting of the steel base pieces was extremely precise work as this greatly determined the fit of the steel bow trusses.

The bow has been divided into sections as it was too large to transport in its entirety by road. Of course, from a constructive point of view, this was not desirable. The division has been achieved within the profile and finished with thin-rolled steel sheeting. The sleek appearance of an entire bow has been retained through this method of division.

Due to the complex shape and cohesion between the bows, it was necessary to temporarily support the bows with secondary steel, tensioning straps and three telescopic cranes.

Snow load; a determining feasibility factor

The effect of snow load on the structure has been accounted for in the design in a unique way. The Dutch load standard does not include a specific calculation method for snow load on netted structures. Given the large netted surface areas of the structure, this type of load occurrence was decisive for the feasibility of the design and prompted much discussion within the construction team.

Based on experience with countries such as Germany and Austria, the net and cable supplier had a totally different perception than what is usual in the Netherlands regarding the snow load. Nets are often used in these countries to prevent avalanches, whereby as much snow load as possible has to be calculated. According to the Austrian approach, the load would be three to four times higher than that allowed for in the original design. On paper, the existing building would have been torn apart in the event of heavy snow.

Combined with a partly closed area of the netted surface, the average snow load has been determined in accordance with the Dutch load standard. For increased safety, a load situation of only one part of the enclosure was calculated, which creates an upper limit for the load on the structural stability in the existing building.

Sustainable adjustments to the national monument

Sustainability was an important starting point in the design process. The existing structure of the monumental building has been preserved as much as possible. Adjustments have been made to the existing structure to accommodate the bow trusses which apply forces through the stainless steel cables on the existing structure of the building. In addition to extending the existing girders, steel braces (for stability) have also been applied to the roof area.

Not only has the bow structure been made sustainable, but also the monumental building itself, for which the latest sustainability technologies in the field of energy management have been applied. For example, the monumental building has a green roof, a heat pump has been installed instead of a central heating system, and solar panels have been placed to generate heat and electricity.

All in all, the design process has been challenging from the beginning through to the actual execution of the construction. A process that has resulted in a beautiful steel structure that has retained the original open and monumental character of the enclosure.
Would you like to know more about this project? Etienne will be pleased to tell you more. Contact him via 088 943 3500 or send Etienne an email.
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