Adaptive Structures minimise energy by replacing passive resistance through material with a comparatively small amount of operational energy. To achieve this, the design methodology formulated for Adaptive Structures takes a substantial shift from conventional methods of structural design.
The first graph shows the total energy of the structure against the ‘degree of active design’ (related to how strong the passive part of the structure is). The total ‘whole-life’ energy comprises two parts – (1) energy embodied in the physical components and (2) operational energy in-use.
For a completely passive design (e.g. a conventional steel roof structure), the embodied energy equals the total energy. By contrast, a highly active design has low embodied energy, but the whole-life operational energy necessary to control the structure is high (e.g. a JCB excavator arm is relatively lightweight but is controlled
by energy-intensive hydraulic actuators).
An optimum Adaptive Structure Design has a combination of active and passive systems which result in a minimum total (whole-life) energy.
The loads on buildings are variable (e.g. some days it is windy, other it is calm, and there are occasional storms) so for any particular load – whether it is wind, snow, pedestrians, earthquakes etc. – a probability distribution can be defined, see the second graph above.
The actuators in an efficient Adaptive Structure do not act every day because that would use up a great deal of operational energy. Instead a load Activation Threshold is determined, below which the actuators aren’t needed for controlling loads and movements.
An optimised Adaptive Structure generally has an activation threshold tuned so that the active system is needed for only a small proportion of the structure’s life e.g. to compensate for occasional hurricane winds
on a tower, unusual crowds in a stadium etc.
The Adaptive Structures Design Method
The Adaptive Structures Design Method has been developed to find the optimal load path and corresponding arrangement of material to achieve the minimum possible lifetime energy. To do this it needs to design a structure with the perfect combination of active and passive behaviour.
The design algorithm controls this balance of active and passive by varying a parameter termed the material utilisation factor (MUT – see step 1). If the MUT is high, the structure will have a low activation threshold, be very light (minimal passive structure) and the active system will do lots of work (use lots of energy).
Section sizes of the structural members are then determined (Step 2) so that they have sufficient capacity for the worst expected effect from all possible loads and load combinations. From this, the method finds the optimal number and position of strategically located active elements (Step 4) whose work – in the form of length change – is needed for displacement correction and load redistribution. So rather than increasing the stiffness of the structure using material mass, the actuators change the shape of the structure. In this way, deflections can be kept within desired limits and the walkable deck remains level.
The process is repeated many times to find the Adaptive Structure design which has the minimum whole-life energy.
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