Adaptive Structures  

The Truss Prototype

Extremely Slender

The prototype shown in this exhibition was designed using the Adaptive Structures Design Method. This structure is a 6000mm (length) × 800mm (width) × 160mm (depth) cantilevered truss with a 37.5:1 span-to-depth ratio. This degree of slenderness is simply not possible with conventional structural solutions (a ratio of 12:1 might be expected for conventional trusses and 20:1 for conventional steel beams).

The structure consists of 45 passive steel members and 10 electric linear actuators strategically positioned within the diagonal members which will be under tension. The structure is designed to support its own weight (102 kg including actuators and cladding) plus a live load of 100 kg at the tip of the cantilever (equivalent to a person standing at the end of the deck).

The frame is fully instrumented to monitor the stress in the passive members, the deflected shape, and the operational energy consumed by the active elements. The passive steel members (in the truss) have been sized to prevent collapse, but instead of adding more material, a state-of-the-art control system governs the more onerous requirements of deflection and movement. Due to the fail-safe nature of the actuators, if the power is cut, the actuators simply stop moving with no compromise of load carrying capacity.

Infinite stiffness

A major objective of the Adaptive Truss Prototype was to test the applicability of the Adaptive Structures Design Method to a real structure and investigate the feasibility of controlling its displacements in real-time without any assumption on direction, position and magnitude of the external load. 

Extensive load tests (including asymmetric loading causing overall torsion) showed that the displacements were reduced to as close to zero as our instruments could measure, thus effectively achieving an infinite stiffness structure (zero deflection under loading). 

The displacements were measured using a probe and a self-levelling laser which has an accuracy of 2mm over 30m. The difference in the vertical position between two consecutive nodes was
within ±1 mm and between the supports and the free end nodes within ± 2mm.

The figure shows an example of the difference between the uncontrolled/deformed shape and the controlled shape respectively. Similar zero-deflection results were recorded when a person walked on the deck.

Material Mass and Whole-Life Energy Comparison

Less Material and Minimal Whole Life-Energy

The Adaptive Truss is designed to withstand its own self-weight (dead load) as well as a person walking along the deck (live load). The most onerous scenario for the Ultimate and Serviceability Limit State design is when the person stands at the tip of the cantilever structure. 

In order to validate the performance of the prototype, load tests were carried out by placing weights ranging from 10kg to 100kg (the design load) at the cantilever tip. The operational energy use was measured by monitoring the power being consumed by the actuators and the control hardware during load / displacement control.  

The total energy (embodied + operational) of the Adaptive Truss Prototype was then benchmarked against that of two equivalent passive structures. The first structure is made of two steel I-beams, the second is a truss designed using state-of-the-art optimisation methods. The Adaptive Truss achieves 70% total ‘whole-life’ energy savings compared to the I-beams and 40% compared to the passive optimised truss.

These experimental results confirmed that the Adaptive Structures Design Method, and adaptive structures in general, can save substantial material mass and total energy compared to equivalent passive structures.

 

       

Generously funded by: