Kaeng Krachan Elephant Park Shell

The Kaeng Krachan Elephant Park compound at the Zurich Zoo is capable of withstanding the 15-ton force generated by a charging occupant, but that didn’t stop local firm Markus Schietsch Architekten (MSA) from imparting an aesthetic delicacy to the structure. Completed in 2014, the 90,900-square-foot structure currently hosts eight Asian elephants, each weighing between 2 tons and 5.5 tons, in a nature-inspired habitat topped by a 73,200-square-foot shell roof made primarily from wood.

From a distance, the undulating, freeform shell looks like a giant tortoise shell with its intricate patterning and silver-hued patina. Spanning 260 feet, the complex assembly comprises 550 uniquely shaped, cross-laminated spruce panels stacked in three layers connected by more than 600,000 21-centimeter-long nails. Each panel layer is offset at a rotation of 60 degrees to divide the force distribution along the wood grain. The panels were pre-scored to guide the on-site assembly process in which the Aichach, Germany–office of Metsä Wood CNC-milled the panels, and carpenters from the Swiss construction firms Implenia Holzbau and Strabag Holzbau gradually bent them over scaffolding-pipe structures to achieve a bending radius between 40 meters and 80 meters (131 feet and 262 feet) over the entire roof system.

The shallow roof also features 271 apertures, creating the dappling effect of a tree canopy. The openings were precisely pre-cut in the top panel layer, but hand cut in the bottom two layers in situ. Overall, 30 percent of the shell is open. ETFE cushions form raised skylights in each aperture, while the edges of the skylights are clamped into an integrated aluminum gutter that keeps the membrane profile above the timber shell and facilitates water drainage. Rainwater and snow runoff are harvested from the roof to irrigate plants and fill the elephants' pools.

Andreas Buschmann These timber shell panels and protective layer were left unfinished to showcase their natural patina.

An outermost layer of laminated veneer lumber (LVL) protects the entire roof assembly. These panel faces were left unfinished to showcase the wood’s natural patina. “When the roof is dry from the sun, it’s almost a reflective silver, and when it’s wet from rain or dew, it’s almost black,” says MSA partner Philipp Heidemann, the project architect. “We like that the material is almost like a living thing.”

MSA didn’t want the compound’s façade to look clunky or overwhelmed by the load of the roof, Heidemann says, so the firm undulated the roof edge to “make it look more natural and … to correspond to program underneath.” Working in Rhino with Grasshopper plug-ins, MSA modeled the rippling surface to experiment with different variables, such as the size and location of the skylights, to improve structural efficiency. A reinforced-concrete tension ring, supported by concrete slab walls clad in glulam spruce panels at the low points of the roof along the building’s perimeter, prevents the shell from splaying. At these points, it transfers loads into a concrete slab foundation, which in turn is supported by a series of subgrade, steel-reinforced, prestressed concrete piers.

Dominique Marc Wehrli Where the shell dips down, it transfers loads into a concrete slab foundation that is supported by a series of sub-grade concrete piers.

The lamella-like façade of the Elephant Park appears to dissolve into the roof, thanks to the weight distribution of a double-hinge structural system. One hinge occurs in the timber fins, or columns, and the other in the beams that connect the shell to the façade; as a result, the façade can adjust slightly in response to thermal movement and to roof loads.

At one side of the center, an arch spans over a mezzanine level that serves as event space. The programmed visitor's area along the periphery also restricts the elephants’ access to the delicate façade, manufactured by German steel-construction company Züblin Stahlbau, which cannot withstand their strength.

“These elephants can push up to 6 tons with their head plates and pull up to 3 tons [from a standstill],” Heidemann says. “The sphincter muscle in their trunks can open screws. If something is loose, they will not only play with it, but they will also teach the others so groups of elephants will begin unscrewing their habitat.” As a result, the magnificent mammals have direct access only to the house’s northwest-facing, steel-reinforced concrete wall.