Conversion and vertical extension
Basel, Switzerland
Competition 2022
Planning 2022-2024
Construction 2024-2026
Administration Basel Zoo.
Overview
Client
Zoologischer Garten Basel AG, Switzerland
Architecture
Salathé Architekten Basel, Switzerland
Team ZPF
Patrick Gressler, Katsiaryna Laikova, Tobias Huber, Franck Mahler
The existing operational building at the rear of Basel Zoo has been transformed into a high-quality and representative administrative headquarters. The existing operational building, the adjacent residential house and the three-storey vertical extension have been combined into an 'urban conglomerate'. The project treats sustainability as an integral component of all planning decisions.
The timber-clay hybrid construction of the vertical extension reduces CO₂-intensive concrete to a structurally necessary minimum. A trellis-like structure of round bars surrounds the open and transparent interior, provides shading to the facades and offers support for biodiverse planting. The seasonal greening of the facades and roof surfaces contributes to habitat connectivity, rainwater management and summer thermal protection.
Inside, a cascade staircase connects the three office floors. The building core thus becomes a communicative, experiential zone. The working areas extend across the full depth of the building. The top floor accommodates a generous, column-free lounge and event space with a terrace overlooking the zoo.
The existing load-bearing structure dates essentially from the construction phases of 1952 and 1971. It consists of reinforced concrete floor slabs supported on mass concrete walls, masonry and reinforced concrete columns, which have been partially replaced by steel structures over the decades. Intervention in the existing fabric is limited to what is strictly necessary; however, the existing structural system and the existing foundations were unable to accommodate the additional loads of the vertical extension without strengthening measures. For this reason, the existing core area was completely demolished and replaced from the ground slab to the roof by a new reinforced concrete core that provides lateral stability to the building. The vertical load transfer of the vertical extension is achieved entirely via a structural system that is decoupled from the existing structure and has its own foundations.
The vertical extension is supported on a transfer table consisting of solid steel beams and a composite deck. The transfer structure bears on new external columns. On the south side, the vertical loads are transferred via a load-distribution beam to micro-piles. On the north side, a new reinforced concrete wall in the basement takes over the force transfer. The vertical load transfer within the vertical extension is achieved via timber columns down to the transfer level. Lateral stability against horizontal actions (earthquake, wind) is provided in the longitudinal direction by the new reinforced concrete core, which is fixed at the basement levels. In the transverse direction, a solid reinforced concrete wall (from the ground to the transfer structure) and timber diagonal bracing (from the transfer structure to the roof) complement the lateral bracing system.
Timber-clay floor slabs have been installed above the 2nd, 3rd and 4th upper floors. The clay is rammed between the timber beams as a vault and acts as non-load-bearing thermal mass. The elements, measuring approximately 2.3 m by 4.2 m and weighing approximately 3 t, are designed for a high degree of prefabrication and are ideally suited to spans of approximately 4 m.