Designing structures with minimal environmental impact is now a common concern in the construction sector. Conventional structural design practice usually involves ensuring that the strength and deformation of the structure (e.g. towers, bridges, stadia) meet the required limits to cope with the worst load cases.
However, most of the time such structures experience loading much lower than the design load meaning that they are effectively over designed for most of their working life.
This fact leads to consider that building structures could be adaptive rather than relying only on passive load-bearing capacity.
Adaptive Structures, a new design philosophy in structural engineering
A new Design Methodology for Adaptive Structures
Active control has been used in engineering structures for a variety of purposes. The most widespread application in civil engineering has been in vibration control. A few have investigated the potential of using adaptation to save material, but whether the energy saved by using less material makes up for the energy consumed through control and actuation has so far received little attention. During his doctorate, a research project collaboration between the University College London and Expedition Engineering, Gennaro Senatore formulated a new methodology to design adaptive structures. Adaptive structures are structures capable of counteracting loads actively by means of sensors, control intelligence and actuators.
This method is illustrated diagrammatically in figure 1 which shows the total energy of the structure as a function of some notional degree of active control of the structure. The total energy is made of two components: embodied and operational. In this method, embodied energy refers to the energy needed for extraction and manufacturing. For a completely passive design the embodied energy dominates the total energy: members are designed to bear 100% of the design loads to meet strength and serviceability requirements. By contrast for a highly active design, the embodied energy will be small but the operational energy necessary to control the structure throughout its lifetime will be high. The active-passive system that corresponds to the minimum of the total energy is the optimum sought.
The adaptive structure is designed to cope with ordinary loading events using only passive load bearing capacity whilst relying on both passive resistance and active control to deal with events that have a smaller probability of occurrence (e.g. wind storms, snow, earthquakes, unusual crowds but also moving loads such as trains). Instead of increasing the stiffness using material mass, strategically located active elements (actuators) provide controlled output energy in order to manipulate actively the internal flow of forces and to change the shape of the structure. In this way the stress can be homogenised and deflections can be kept within desired limits. However, the actuators only activate during rare occasions when external loading reaches a certain "activation threshold" (e.g. gale-force winds, larger crowds of people etc. figure 2). Therefore, the operational energy is only used when necessary.
Figure 1: Whole-life energy vs degree of adaptation
Figure 2: Load probability distribution (50 years)
Shape Control and Load Path Redirection
The Adaptive Structures Design Method (figure 3) has been developed to find the optimal load path (i.e. the flow of internal forces) and corresponding arrangement of material to achieve the minimum possible lifetime energy. To do this it needs to design a structure with an optimal 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 – step 1). If the MUT is high, the structure will have a low activation threshold, be very light (minimal weight passive structure) and therefore active system will use lots of energy for structural adaptation.
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 for strength only. If a change in the loads causes a deflection that violates a serviceability limit state (SLS), the load path must be redirected and displacements controlled by the active system (Step 3). For this reason, the method finds the minimum number and optimal position of active elements (i.e. actuators) (Step 4) whose work - in the form of length change - is needed for displacement correction and load path redirection. The process is repeated iteratively to find the configuration that corresponds the minimum whole-life energy.
Figure 3: Design methodology schematic flowchart
Extensive numerical simulations, which compare adaptive solutions against equivalent passive structures optimised using state-of-the-art methods, have shown that the total energy (embodied + operational for a 50-year design life) could be significantly reduced. Three case studies are presented here. The structure shown in figure 4 represents a section of a multi-storey building. The structure shown in figure 5 represents a section of a typical hangar building made of planar trussed portal frames. The structure shown in figure 6 & figure 7 is a simplified model of a tower building known informally as the “Gherkin”, a skyscraper in the City of London. This last building is assumed to carry external loads only using its own exoskeleton as structural system (i.e. no cores).
The external load is modelled using a stochastic distribution which is representative of typical building loading scenarios (design life of 50 years). The limit on deflection used in these examples are those commonly used for the design of civil engineering structures. For instance, main span/360 to limit displacements of structures spanning horizontally and height/500 to limit total building drift.
Results show that the energy savings stay in a range from 30% up to 70% for very slender structures. An optimal region in which adaptive structures outperform their optimised passive versions both in terms of energy and monetary cost savings was identified. This is broadly the region of stiffness-governed structures (e.g. long-span structures and high-rise buildings).
Figure 4: Cantilever (multi-storey building)
Figure 5: Arch bridge (catenary)
Figure 6: Exoskeleton tower structure, loads
Figure 7: Exoskeleton tower structure, passive vs adaptive
The Adaptive Truss Prototype
A large scale prototype (figure 8), designed using the methodology described in the previous section, was built at the University College London structures laboratory. The prototype is a 6 m × 0.8 m × 0.16 m (37.5:1 span-to-depth ratio, 75:1 if thought of as simply supported) cantilever spatial truss (figure 9). The truss consists of forty-five passive steel members and ten electric linear actuators strategically fitted within the tension diagonal members. The structure is designed to support its own weight (102 kg including actuators and cladding) and can take a live load of 100 kg at the tip of the cantilever (person walking along 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 steel members making up the truss have been sized to prevent collapse, but instead of adding more material, the more onerous requirements of deflection and movement are governed by a state-of-the-art control system. Due the fail-safe nature of the actuators, if the power is cut the actuators simply stop moving and load carrying capacity is not compromised.
Figure 8: The Adaptive Truss Prototype, UCL Structures Laboratory
Figure 9: Overall dimensions
The control system architecture was designed with the primary aim to achieve identification of the response to loading in terms of internal forces and displacements for the structure to be able to be control itself without user intervention nor predetermined knowledge of the external load (whose magnitude must be lower than that of the design load). Figure 10 shows the conceptual schematics for the architecture of the control system.
The deformation of each element, monitored using strain gauge-based sensors of type full-bridge (figure 11), together with the stroke position feedback from the actuators are fed into the main controller (strain feedback and position feedback in figure 10). The control algorithm processes the input feedback to first reconstruct the node spatial positions. In case the displacements exceed the imposed serviceability limits, the control algorithm computes the minimum length changes of the actuators that bring the structure back within desired serviceability limits. In addition, current sensors are installed at the mains supply to monitor the amount of power being used by the actuators and all the other electronic devices during load control.
Figure 10: Control System
Figure 11: 8-gauge full-bridge (rejects bending strain)
Figure 12: (a) signal cables, (b) actuators
Figure 13: Control unit
Displacement Control | Infinite Stiffness Structure
Extensive loads tests including asymmetric loading causing overall torsion showed that the displacements were practically reduced to zero 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 2 mm over 30 m. The difference in the vertical position between two consecutive nodes was within ±1 mm and between the supports and the free end nodes within ± 2 mm.
Figure 14 shows an example of the difference between the uncontrolled/deformed shape (a) and the controlled shape (b). Similar results in terms of displacements compensation were recorded when a person walked on the deck (video 1).
Figure 14:Displacements, (a) 100 kg no control, (b) 100 kg controlled
Displacement control person walking
Power Consumption | Whole-Life Energy Assessment
The main objective of this experiment was to gather experimental data to support the claim that adaptive structures allow savings on the total energy of the structure. The external load is modelled using a stochastic distribution which is representative of typical building loading scenarios (design life of 50 years). The limit on displacements is set to span/500 which is normally used as serviceability criteria to limit for the total drift of a high-rise building subjected to wind loading. Due to the pronounced slenderness the structure can be thought of as a scaled version for the super structure of a tall tower subjected to wind load.
Power consumption during structural adaptation under loading was recorded for all the electronic devices. The total energy of the adaptive truss prototype was benchmarked against that of two passive structures (figure 15) designed to take the same loads and to comply with the same serviceability limits used for the adaptive truss. The first structure is made of two steel I-beams, the second is an equivalent truss designed using state-of-the-art optimization methods. The adaptive truss achieves 70% savings compared to the I-beams and 40% compared to the passive optimised truss.
This experiment confirmed that the design method presented in this work is reliable and produces results that are overall conservative and more in general that adaptive structure can achieve substantive total energy savings compared to equivalent passive structures.
Figure 15: Energy comparison
Adaptive Structures, a new design philosophy
Adaptive structures present a new design philosophy in structural engineering. The design method proposed in this work produces structures that combine three performance objectives which are usually mutually exclusive: 1) the structure has a low overall environmental impact; 2) the displacements can be controlled within very tight limits; 3) the structure is extremely slender. Being able to combine these three objectives is unique in structural engineering. Applying the above new design philosophy, scenarios where adaptive structures could bring significant benefits include:
- Stringent, high-performance requirements for deflection, such as laboratory buildings, gantry crane runway beams, and bespoke facades.
- The structural design is governed by rare, but high loads, such as earthquakes and wind storms.
- Very high slenderness/shallow structural depths are needed. This could be driven by space limitations or by awe-inspiring aesthetics
- Long-span and high-rise building (skyscrapers, bridges, roofs) would benefit from the three main characteristics of adaptive structures (stiffer, lower weight, slenderer).
The adaptive truss prototype was exhibited at various key institutions amongst with the University College London, the International Association for Shell and Spatial Structures symposium (IASS) held in Amsterdam in 2015 and The Building Centre (London). The prototype was shortlisted for an IStructE Structural Award 2016.