Message in a bottle: An exploration in truss automation

In design and construction, a truss is defined as a framework of elements that, when combined, act as a single component providing support for a larger structure such as a bridge or a roof. Typically the design of a structural truss is dependant upon numerous calculations accounting for the effects of physical forces such as weight, tension, compression, material limitation, etc. These calculations are usually daunting and time consuming... until now.

A research team at Hasso Plattner Institute, led by Robert Kovacs, has been working to find a way to automate those calculations. These efforts have culminated in the creation of TrussFab; a SketchUp Extension that populates 3D printable truss elements for SketchUp users. TrussFab allows users to turn SketchUp geometry into a truss network that can easily be assembled in real life -- with real life recycled materials -- giving the user the ability to assemble trusses for their own projects easily, quickly, and reliably. Robert and the Truss Fab Team sit down with us to talk about how they built such powerful solution to a common problem.

Where did the idea for TrussFab come from?

My first project at the Hasso Plattner Institute was the Protopiper, which focused on prototyping room-sized objects at actual scale. We next wanted to push this idea of large-scale personal fabrication towards more functional objects. Our first thought was to complement small 3D printed parts with ready-made objects, which became, in this case, plastic bottles. While playing around with this idea, we quickly realized that the key design principle is to always form closed triangular contours -- trusses -- otherwise the connectors will break or the bottles will buckle. When we built our first chair capable of holding a person’s weight, we knew this was the way to go.

Can you briefly explain the underlying physics of the forces that are acting on these assemblies?

Truss structures essentially consist of triangles. In such an arrangement, the structure prevents deformation, rather than just the individual bottle. The main strength of trusses is that they turn lateral forces (aka bending moments) into tension and compression forces along the length of the edges, or "members". Bottles make great members: while they buckle easily when pushed from the side, they are very strong when pushed or pulled along their main axis.

Tell me about these 3D printed hubs you’ve integrated into the extension.

During development we evaluated many hub designs capable of holding bottles together. Initially we used a threaded-snap connector pair for each edge. We later settled on a design that can accommodate different bottle types and is easier to 3D print. With this design the connector has only a simple flange, which is held to the bottle with a clip-on c-cuff (see picture below). The main advantage of this design is that the cuffs can be oriented in beneficial printing directions, allowing for maximum strength.

How did you go about incorporating engineering functions into the application?

With TrussFab, our main goal was to allow novice users to build structurally stable large-scale objects in just a couple of minutes. The underlying design principle behind TrussFab is very well known in architecture, but typically less understood in everyday life. With the emergence of 3D printers we can fabricate far more objects with exceedingly better quality. However, when going larger, eyeballing the material and wall thickness often fails because the forces are growing exponentially with the size of the object. To overcome these difficulties, we have implemented truss-specific functions that help users create sturdy structures. TrussFab’s editor offers base shape primitives in the form of elementary trusses (tetrahedra and octahedra), tools that create large beams in the form of trusses, and tools for tweaking the shape of the structure while maintaining rigidity.

What’s the most inspiring structure you’ve built using this app?

The largest structure we’ve built is a 5m high pavilion for the CHI’17 conference. The whole installation used more than 1,500 bottles and 191 3D printed hubs. The pavilion was designed by the architect Oanh Lisa Nguyen-Xuan exclusively using TrussFab’s editor.

Does the extension self-validate larger structures for structural integrity? How does that work?

TrussFab computes the effect of weight placed onto the structure. First, the software looks for flaws in the truss structure; it searches for parts that are not completely locked in place by other members and are subject to shearing or bending forces. If found, the software would suggest placing additional stabilizing members. Second, the software checks whether the structure will hold the specified weight. Using finite element analysis, the software calculates the forces that apply to every member of the structure. As show in the figure below, TrussFab shades all members accordingly. The six vertical members of the octahedron now appear in shades of red, suggesting that these are experiencing compression. So does the chair’s “backbone”. All other members are tinted blue, suggesting that these are subject to tension.

Besides SketchUp, what other digital tools were used in the creation of TrussFab?

Besides SketchUp we use some external tools; Karamba3D for calculating the forces inside the structure and OpenSCAD to generate the precise hub geometries for 3D printing.

That's a lot of soda pop. Tell us about those bottles.

We source them from the local stores, which are the collection points for these bottles in Germany. For higher structural stability, we used refillable bottles, which are somewhat sturdier. Alternatively, softer bottles can be inflated to achieve similar strength.

Why was SketchUp chosen for this project?

We wanted to use a widely available platform for our project that would also allow us to easily develop our own 3D editor functionalities. We also had some previous experience with SketchUp through our LaserStacker project.