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September 8, 2020

The What, Why, and Where of Tensegrity Structures

Tensegrity structures

Look at the image. Not a cursory glance, actually look at it.

Doesn’t it look like the long chains are in compression? Clearly they’re not, and magic is unlikely the reason it’s structurally stable.

I’m fascinated by structures (not surprising since I’ve been a practising structural engineer for over 30 years!). Clearly, I haven’t been practicing enough coz’ I’ve actually not heard of these structures – tensegrity structures – before!

In 1948 an artist Kenneth Snelson produced an innovative sculpture called “X-Piece”.

Tensegrity - Snelson

In the 1950s, Buckminster (Bucky) Fuller – the architect who invented the geodesic dome – took controversial ownership of this structural system and created the term “tensegrity” to describe “self-tensioning structures composed of rigid structures and cables, with forces of traction and compression, which form an integrated whole”. If that’s too much of a mouthful, perhaps look at the simplest form of a tensegrity structure, the 3-rod prism.

3 Rod Prism

The ‘rigid structures’ are the wooden dowels. They are held in place by the tensional forces of the string. Tensegrity = “tension” and “integrity”. So long as there’s tension in the strings, there’s compression in the dowels, and structure integrity. You can easily build one of these to prove Fuller’s work!

The use of “tensegrity” in a major structure has been something of a holy grail to architects and engineers around the world for decades. Then, in 2009, the successful completion of the Kurilpa Bridge in Brisbane represented a genuine world first.

The Queensland Government established a design brief and an extremely challenging “not to be exceeded” budget for a new bridge in Brisbane to “provide a link from the Queensland Gallery of Modern Art (GoMA) in the South Bank Precinct to Tank Street in the Central Business District”. The objective was to deliver a landmark pedestrian and cycle bridge – “architecturally striking”, and sympathetic and complementary to its prominent location.

Kurilpa Bridge

Contractor Baulderstone, engineer Arup, and architect Cox Rayner produced the 430m long tensegrity Kurilpa Bridge, a bold fusion of art and science featuring a striking array of masts, cables, and flying steel spars, winning the design competition.

          Key statistics

• Bridge deck 430m long x 6.5m clear width between handrails

• 500 m3 of concrete and 500 tonnes of steel

• Nearly 7 km of high strength steel cables

It wasn’t “just because” that a tensegrity structure was adopted. The potential solutions identified were cable-stayed, arch, suspension, tube, and tensegrity mast-and-cable. The large masts required for the cable-stayed were too visually dominant, the ground conditions too poor for arch and suspension options and the tube option was potentially difficult to construct over the deep and fast-flowing river.

The Arup design satisfied the need for a “visually light, shallow, and buildable form that would sit comfortably beside the GoMA while being innovative, whimsical, apparently random, and radically different from the norm.”

How much did it cost, I hear you ask. $63.3M was the official figure, making it cheaper, and longer, than our own 370 m Matagarup pedestrian bridge. That ended up costing you and I over $90M 😲 

Kurilpa Bridge Brisbane

 

What do you think? I love it!

Designing Intricate Structures

We use computer software nowadays to design intricate structures based on the tensegrity concept. 3D models have been around for a while, and now 3D printing makes the previously impossible to construct within our grasp.

You can expect to see an increasing number of structures that challenge the conventional structural systems in the future of construction. These sure are exciting times.

Have a project that you want to discuss?

Book a meeting with us and we will assist you with the structural design and analysis of your structure to help you produce a structure that is fit-for-purpose and capable of resisting all applied loads without failure during its intended life.

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