The Architecture and Art Invitational, Design After Design, at the XXIst Triennale di Milano International Exhibition (2016) challenged designers to build full scale installations re-contextualizing established architectural terms in exciting new ways. Polycentric Pavilion was conceived by New York firm nARCHITECTS and built in conjunction with CRAFT | Engineering Studio and Extravega Fabrication Milano. The skewed barrel vault, traditionally an architectural element expressing the solidity and inertia of heavy masonry construction, was reborn in the Polycentric Pavilion as thin, reflective aluminum sheets, The 5 mm thick sheets vault around a triangular participant base, used for shaded seating and musical performances.
Searching for the optimal geometry for the vaults, nARCHITECTS collaborated with CRAFT using 3D modeling in Rhino, Grasshopper plug-ins Kangaroo and Karamba, and physical models. Parametric studies of design forms gained structural feedback in the space of Grasshopper and Karamba, beginning with an exploration to optimize the rotation of the bases in combination with the twist of the tops. This created a stiff, doubly-curved surface. From the array of results in four wind directions, a twist between 90 and 110 degrees resisted the wind with the least stress on the material.
After establishing that a twisted form could sustain lateral forces over greater spans, modal analysis was performed on arrangements of arches around the triangular base. While the performance of the smaller arches was satisfactory, the single-layer aluminum construction proved to have very low natural frequency at large spans, subjecting the most dramatic arches to potential excitation by wind of participant interaction.
To dampen the natural frequency, Kangaroo (a physics engine within Grasshopper) was used to generate catenary arcs more stable than the radial onces that referenced their Roman antiquity.
Two catenary “chains” of the same length were tied at the skewed bases and hung to form the edges of the vault. Connecting the equidistant points between the two edges produced a doubly-curved developable surface. The result was a 1.5 Hz increase in natural frequency.
After selecting the form with the strongest performance, additional fabrication considerations were taken into account. Fabrication constraints required each arch to be constructed out of three sheets of aluminum and a moment-resisting base connected with four lap seams. The lap seams were added to the Karamba model, and were found to provide additional stiffness in a critical region of the arch. With the language determined, the assembly was tested for its limits in height, span and modal frequency in Karamba. The final design from nARCHITECTS sized the largest span at 5.2 m with a height of 3.9 m, easily within the structural ability of the assembly.
The Pavilion was on display from April to September 2016.