Some years ago, a group of us here at Expedition began reflecting on some of the fundamental ideas behind structural engineering. Firstly, it’s pretty obvious that we need to design our buildings, bridges and other structures so that they are strong enough and won’t collapse…safe enough to assume that this has been an underlying tenet of structural engineering at least since the first pyramids were built in ancient Egypt!
A second fundamental principle, and one which specifically appears in all modern design codes, is to design structures that are stiff, so that they don’t move or deform too much under some statistically-calculated worst-case loads. Think: high winds, heavy snow, large crowds of people. This is not really a question of safety, rather it is about usability and comfort (or serviceability, to give it its collective engineering-ish name). In many cases, these conditions are rather onerous on the design, to the point where we were finding what we took out (whilst still maintaining appropriate strength), had to then go back on to make it code-compliantly stiff. Simply put, we were putting more material (concrete, steel, timber etc., but all with a financial and carbon cost) in the building than was required for strength.
However, when it comes to movements:
- How much is ‘too much’?
- A question of perception?
- A question of legacy of historic codes and even more historic building practices?
- How often do these ‘expected worst-case loads’ really happen?
For example, football stadiums may only have a full crowd load 20 times per year, for 90 minutes on a Saturday afternoon. Do we as designers really need to put more and more steel and concrete into our buildings for situations which are not safety critical and which occur rather infrequently? This got us thinking about biological organisms, automotive and aerospace designs, and using conventional structural materials to meet strength/safety needs but using very different methods and technologies to tackle stiffness and movement.
In a block of flats, engineers would always need to use enough reinforced concrete to prevent collapse…but why not detail the façade brackets differently so that if the floors and beams move a bit more the windows will still not crack? In an office building, raised floors are often used to hide cables….but couldn’t they also be used to hide/compensate for deformation of the floor structure itself?
More radically we thought, rather than accommodating or compensating, why not control the structure itself? If you hold an empty glass in your hand and fill it with water then your hand doesn’t move. As the weight of water in the glass increases, your brain senses this and via nerves it tells your arm muscles to compensate – thereby holding the glass in position. This really sounded promising, so we decided to fund an Engineering Doctorate (EngD) at UCL with the aim of researching active structures, natural systems and adaptive technology. Through a close collaboration with UCL Civil engineering department, the aim was not just top-notch theoretical studies; additionally the whole research team from both UCL and Expedition wanted this R&D to be useful and inspirational for designers, engineers and architects. Gennaro Senatore, the EngD researcher, did an amazing job developing new algorithms for the optimal design of adaptive structures which, uniquely, were then physically tested and proven by building a triangulated steel truss, installing pistons/actuators, sensors and a control system….with some similarities to a human arm – an adaptive truss!
Of course, the issue with the human arm is that you expend energy all the time, just to hold its own weight (not a problem if you have plentiful food). However, the steelwork can be designed so that in normal ‘day-to-day’ conditions (a light breeze or a handful of people walking on a bridge) the actuators do not have to do any work or expend any energy. When rarer, but higher loads occur (perhaps stronger winds a few times per year, or larger crowds of people) the steelwork is still perfectly strong enough (safe, no collapse) but the actuators and control system start to kick-in to prevent and control movements and deflections.
What does this mean in practice? This research collaboration between Expedition, UCL and Gennaro Senatore has culminated in the creation of a 6 m long cantilever adaptive truss which people can walk on to test out!
- 80% lighter than a conventional equivalent structure
- 6 times more slender (40:1 span/depth ratio)
On a normal use ‘day’, the electricity consumption is < 1 Watt because the actuators and control system only activate for infrequent high loads, so whole-life-energy (embodied + operational) is 60-70% less.
And since it uses linear-electric actuators containing a worm-drive system, if there was a power-cut they automatically lock in-place i.e. there is a fail-safe.
In the broadest terms, the type of projects where adaptive-style structures would have real potential are those where:
- highest levels of deflection/movement control are desired (we can limit to <1mm)
- extreme slenderness could bring delight, awe and a new aesthetic
- low weight is needed either for sustainability drivers or perhaps logistics/crane lift/transportation.
Take a look at Gennaro’s webpage for more more about the science, innovative algorithms, technical drawings and some great images of the truss in-use. The research team have designed it specifically for a person to walk on, rather like a Paris catwalk!
So if you’ve got a project in mind for an active/adaptive-style structure, or perhaps you’ve got some further R&D ideas to collaborate on, do get in touch and come try it out for real!