Aortic Arc: a Digitally Fabricated Tensile Canopy

Overview

A new canopy for a student lounge at the California College of the Arts (CCA) hangs within a double-height space and functions as a light scope, spatial definer, and viewing portal. The minimum surface structure is made up of 546 unique HDPE panels linked to one another by over 4000 pop-rivets. The name of the piece comes from its resemblance in form to a portion of the human heart and the fact that it leaps over an existing structural beam. The surface is suspended from three upper stainless steel rings (two circular, one elliptical) that are held by and hold each other in tension. A singular large parabolic ring functions as a "hoop skirt" below. The technical and artistic challenges are unique and did not allow for a conventional approach. Collaborating closely with the designers, the engineers employed non-linear analysis tools and parametric BIM technology to model and predict the final minimal energy form of the piece that structurally behaves as a hybrid between a cable-net and membrane structure. A panelized system was developed using Generative Components and a customized Rhino script that took the raw data and turned it into a drawing file to drive a CNC milling machine that generated all the parts. HDPE plastic, the same material used to make milk jugs, was selected for the panels due to its low cost, resistance to solar degradation, recyclability, low embodied energy, and high tensile capability.

Step 1: Initial Digital Model

Need:

Student polls at an art and design college yielded a specific request to create a focal point for student life on campus. A series of workshops with students determined what was needed was a flexible space that could be both a setting for large group meetings as well as a place for individual relaxation. The seemingly contradictory need for a space that could accommodate large gatherings as well as downtime for individuals was compounded when an existing two-story atrium space was chosen as the site for this new student lounge.

Problem:

The first floor was surrounded on three sides with walls that needed to continue to function as a gallery for exhibits of undergraduate work. The second floor was ringed with circulation and studio spaces that overlook the gallery space below. The problem with the existing atrium was that it was more of a place to walk through than a place to feel comfortable sitting in. People could peer down into the space from above as they walked by giving the people below the uncomfortable sense of being watched. The first floor gathering area had neither a sense of definition for large group activities, nor a sense of protection and intimacy for individual relaxation. A visual connection was still desired between the two floors but a degree of control would be required. The two-story atrium was also topped with clearstory windows that lent a generous amount of natural light. The question became how to control the view without blocking the light.

Solution:

The design team developed a canopy that solves the programmatic requirements while pushing the boundary of computer modeling, structural analysis, and digital fabrication techniques. It hangs within a double-height space and functions as a light scoop, spatial definer and viewing portal.

The minimum surface structure is made up of 546 unique HDPE panels linked to one another by over 4000 pop-rivets. The name of the piece comes from its resemblance in form to a portion of the human heart and the fact that it leaps over an existing structural beam. The surface is suspended from three upper stainless steel rings (two circular, one elliptical) that are held and hold each other in tension. A singular large parabolic ring functions as a ‘hoop skirt’ below.

The technical and artistic challenges are unique and did not allow for a conventional approach. Collaborating closely with the designers, structural engineers employed non-linear analysis tools and parametric BIM technology to model and predict the final minimal energy form of the piece that, structurally, behaves as a hybrid between a cable-net and membrane structure.

A panelized system was developed using Generative Components and a customized Rhino script that took the raw data and turned it into a drawing file to drive a CNC milling machine that generated all the parts. HDPE plastic, the same material used to make milk jugs, was selected for the panels due to its low cost, resistance to solar degradation, recyclability, low embodied energy, and high tensile capability.

Process:

The Aortic Arc is the result of a linked set of developments at two different scales: the macro scale of the overall form and the micro scale of the component. The workflow followed a process of speculation and testing through physical and digital analysis in specific stages: form finding, component design, non-linear structural analysis, design optimization, and fabrication.

Step 2: Form Investigation and Component Design

An overall shape which fit the constraints of the site was developed using AutoCAD for the existing context, Form-Z nurb surfaces to work out the canopy design, and Pepakura to make physical paper models to test fit the concept. The next step in the workflow was to analyze the individual components and identify materials and joinery for fabrication. This step started with physical paper models that were then turned into digitally scripted manipulated surfaces using Rhino and Generative Components. A Rhino script was used to turn the 3D forms into 2D AutoCAD drawings that could then drive a laser cutter to make the parts.

Step 3: Non-Linear Analysis: Structural Engineering

From the engineering side of the project, CATIA was used to model the building structures and approximate the surface. CATIA’s parametric drivers allowed visualizing and quickly adjusting the surface and rings locations to avoid interference with the building structure.

Once the geometry was established in CATIA, the boundary conditions were imported (the rings) and a form-finding exercise in Tensyl begun. This model and methodology resulted in minimal surface for these boundary conditions, and allowed the upper rings to shift into the proper location to create a catenary-like. Next, the geometries were exported from Tensyl into Robot, and the system was analyzed as a cable net model. The lower hoop was released, and the entire surface was allowed to deflect and shear as necessary. The surface deflected slightly, but overall experienced little change from the initial Tensyl model. Finally, the surface was analyzed with Robot using panel elements to model the HDPE surface. The stresses were estimated in the surface and were determined if there would be any unacceptable concentrations of planar or shear stresses in the relatively rigid surface. In these final models the surface showed very minimal bending and a smooth distribution of stresses well below the limits of the material.

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Step 4: Design Optimization

The final design included optimization of the component’s shape to allow for more or less light and views from above. Generative Components allowed us to create a gradient in the size of the aperture of each component. It also allowed to vary the height and direction of the scooped portion of each component.

Step 5: Fabrication

The last step in the workflow was turning the final design into a set of parts that could be manufactured. A Rhino script was run on the entire assembly to label each component uniquely and locate the attachment points between each piece. A final Rhino script was run to unfold each piece into a 2D pattern nesting the pieces into panels. The 2D drawing was used as the final approved shop drawing that drove the CNC milling machine used by the steel fabricator.