Biological designs and novel fabrication strategies come together yet again in the annual research pavilion from the Institute for Computational Design (ICD) and the Institute of Building Structures and Structural Design (ITKE) at the University of Stuttgart, in Germany. The project is the result of 18 months of interdisciplinary investigation by faculty and students in architecture, engineering, and the natural sciences led by professors Achim Menges and Jan Knippers (the duo behind a lobster-inspired pavilion in 2012). Their goal for the 2014–2015 pavilion? Rethinking the constructed habitat of the water “diving bell” spider as a structural assembly. A closer look at their intriguing construction reveals the potential of biomimetic design.
For their project, the interdisciplinary team emphasized the use of fiber-composite materials, as these form the basis of most lightweight construction in biological structures. Moritz Dörstelmann, the ICD research associate in charge of the project, says such materials offer important lessons for industrial development. “These are relevant for applications in architecture, as industrial production processes for fiber-composite materials are designed for mass production of identical components and thus cannot meet the varying demands of individual buildings,” he says. “Biological processes, however, form customized fiber-reinforced structures in a very material-efficient and functionally integrated way.” To Dörstelmann and his colleagues, the methods that spiders and caterpillars use to build webs and cocoons—including the water spider's underwater air-trapping enclosure (left)—were of particular interest, since these organisms can tailor their constructions to local conditions.
The team investigated two types of materials to turn this particular model of arachnid architecture into a shelter for humans: fiber composites for structure and a light-transmissive film for the skin. For the former, carbon fiber was picked for its superior modulus of elasticity, which would minimize the amount of structural material required. ETFE was chosen for the latter for its stiffness, translucency, and common use in the construction of pneumatically supported domes.
To construct the shelter, the team inflated an ETFE membrane into a tent-like structure and placed an industrial robot inside the resulting air-supported environment. Working from inside, the robot printed lines of carbon-fiber bundles onto the membrane's underside. A composite adhesive, applied by the robot, was used to bond the two materials without affecting the polymer matrix’s mechanical properties. As more fibers were deposited, the soft membrane became a stiff, self-supporting fiber-composite shell.
The resulting structure is remarkably thin and lightweight. The 0.2-millimeter-thick ETFE film, which was pneumatically pre-stressed during construction, spans a maximum of 25 centimeters and is supported by a differentiated grid of carbon-fiber bundles with a structural depth ranging from 1 millimeter to 2 centimeters, says ITKE research associate Valentin Koslowski, who acted as project structural engineer. Using finite element analysis, Koslowski analyzed the structure during the design phase to optimize the arrangement and size of the carbon-fiber bundles to resist buckling while using as little material as possible.
The 4.1-meter-tall (13.4-feet-tall) pavilion spans 7.5 meters (25 feet), covers an area of 40 square meters (430 square feet), and contains 130 cubic meters (4,591 cubic feet) of volume—all with a weight of only 260 kilograms (573 pounds). Moreover, the structure has a safety factor of two to three, and it is designed to resist loads from the region’s wind gusts, which can reach up to 26 meters per second (58 miles per hour).
While the final pavilion is impressive, its real contribution to architectural and engineering research is in its innovative fabrication process. “During the concept development … it became clear that we would have to deal with a far more dynamic fabrication environment than in previous projects,” Dörstelmann says. The team could not find an established method for robotically printing carbon fiber onto an air-supported ETFE membrane while defying gravity, so they developed their own prototypes to do the job. Initial tests relied heavily on the fibers’ tension to support the structure, resulting in the robot pulling fibers off of the membrane. The team then employed an active fiber extrusion device, which syncs with the robot armature’s movements to deliver the right amount of material. Additionally, a spray device integrated within the armature supplied the optimal quantity of adhesive between the carbon fiber and the ETFE skin.
The resulting technology—what the team calls a cyber-physical system—was designed to adapt to field circumstances in real-time, enabling variations in fiber orientation and density during the construction process. “Rather than utilize the agent-based computational model to pre-rationalize a fiber layout, the agent was applied iteratively throughout the fabrication process and as such could react to existing site conditions and previously laid fiber densities,” Dörstelmann says.
However, the responsive design-build process is more reminiscent of the pavilion’s model biology than is the physical result. On the one hand, the pavilion aesthetically resembles a water spider’s bubble, with lines of thin fibers supporting an elegantly shaped translucent membrane. Yet we know that this membrane is a polymer film surrounded by air—as opposed to the surface that forms when air is trapped in water—and, as such, it is simply a visual representation of a natural phenomenon. Still, a construction process that allows for real-time design and fabrication adjustments based on unpredictable local conditions is strikingly similar to the way in which animals (water spiders included) construct their own habitats. Therefore, it has the potential to be adapted to a variety of contexts.
The ICD/ITKE team’s counter-intuitive decision to print the structural material upside-down—not unlike the spider’s delivery of dragline silk above its body—also had beneficial outcomes. “We realized that deformations within the pneumatic formwork were larger when pressure was applied from the outside,” Dörstelmann says. “Fiber placement from within made better use of the ETFE as a tension element and resulted in [fewer] deflections that had to be compensated.”
And when a protective tent accidentally collapsed on the pavilion during construction, he says, the inherently resilient structure “elastically deformed and jumped back into shape without remaining damage," proving the robustness of the design in application.