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Kupla – The Bubble, Korkeasaari Zoo Lookout Tower

Korkeasaari Zoo and Wood Focus Finland organised an architectural competition for the students of architecture at the Helsinki University of Technology, with the assignment of designing a 10 m high viewing tower from timber.

From January to May 2001, students developed the draft design by assembling scale models and exploring structural details. Because the management of the irregular form proved difficult, a plastoline model was moulded. Digital images of the model then functioned as a basis for the AutoCAD drawings. Using the level drawings, a 3D-computer model was constructed, onto which the curved battens that would form the gridshell were ‘taped’. Full-scale test pieces of battens were laminated, which would form the basket-like gridshell. When the laminated timber battens resisted twisting, steaming, a traditional method in boatbuilding, was used to shape them. The effect of surface treatment with linen oil-based wood balm on moisture content was determined by moisture tests and the durability of the joints was proved by tension tests. The tower was erected in August 2002. The hot, sunny summer dried the timber, and the duct pipes that were used for steaming proved useful. It took three months to complete the project and now the bubble is a delicately transparent landmark of Korkeasaari Island. The load-bearing structure consists of 72 long battens, with a section of 60 mm x 60 mm that are bent and twisted on the site from seven pre-bent types. Because the tower has no structural protection against weathering, it is treated with a linen oil-based wood balm with UV-protection.