Synthesis of minimum energy adaptive structures

This paper presents the formulation of a new methodology to design adaptive structures. This design method synthesises structural configurations that are optimum hybrids between a passive and an active structure. An optimisation scheme searches for an optimal material distribution and actuation layout to minimise the structure whole-life energy which consists of an embodied part in the material and an operational part for structural adaptation. Instead of using more material to cope with the effect of loads, here, strategically located active elements redirect the internal load path to homogenise the stresses and change the shape of the structure to keep deflections within required limits. To ensure the embodied energy saved this way is not used up to by actuation, the adaptive solution is designed to cope with ordinary loading events using only passive load-bearing capacity whilst relying on active control to deal with larger events that have a smaller probability of occurrence. The design methodology has been implemented for statically determinate and indeterminate reticular structures. However, the formulation is general and could be implemented to other structural types. Numerical simulations on a truss system case study confirm that substantial savings up to 50% of the whole-life energy can be achieved by the adaptive solution compared to a passive solution designed using state of the art optimisation methods.

Shape changes through geometric nonlinear control as a strategy for structural adaptation

Arka Prabhata Reksowardojo

Adaptive structures modify their geometry and internal forces through sensing and mechanical actuation in order to maintain optimal performance under changing actions. Previous work has shown that well-conceived adaptive design strategies achieve substantial whole-life energy savings compared to traditional passive designs. The whole-life energy comprises an embodied part in the material and an operational part for structural adaptation This thesis presents a new method to design adaptive structures capable of large and reversible shape changes achieved through actuation. To this end, linear actuators are strategically fitted within selected elements of reticular (e.g. trusses and frames) structures. Structural adaptation through controlled large shape changes allows a significant stress redistribution so that the design is not governed by extreme loads with long return periods. This way, material utilization is maximized and thus, embodied energy is reduced. A set of target shapes that counteract the effect of peak loads are first obtained through geometry and sizing optimization. Strategies have been developed to reduce the uncertainty due to the presence of multiple local minima so that the computational efficiency is improved and the convergence is guaranteed. A method is formulated to obtain a suitable actuator layout in order to control the structure into the target shapes. This is a challenging task due to the combinatorial nature of the actuator placement process which, in this case, includes geometric nonlinearity. A heuristic for near-neighbor generation based on the actuator control efficacy is employed to explore effectively the large search space. The heuristic has significantly improved convergence, which is important for structures with complex topologies that are made of many elements. A framework for real-time control combining shape optimization and nonlinear force method is proposed. The objective of the control framework is to minimize the operational energy for shape adaptation while satisfying stress and element buckling limits. A linear-sequential formulation is presented, allowing a computationally-efficient implementation of real-time shape control through a mechanics-based formulation. Through a nested univariate optimization scheme, an adaptive structure with minimum whole-life energy requirements is synthesized. Numerical case studies demonstrate that whole-life energy savings can be achieved compared to weight-optimized passive structures for the configurations studied in this thesis. Structures that adapt to loading through large shape changes achieve marginal whole-life energy savings with respect to structures that adapt to loading through small shape changes. Nevertheless, significant embodied energy (and thus material mass) savings are achieved with respect to weight-optimized passive structures as well as to structures that adapt through small shape changes. Experimental studies at various scales are carried out to verify numerical findings and investigate the feasibility of the design method. Experimental testing has demonstrated that significant stress homogenization through large-shape changes is achievable. The control framework allows for real-time shape adaptation to achieve stress homogenization under various loading conditions. An energy appraisal carried out on a near-full-scale prototype confirms that considerable savings of the total energy can be achieved by adaptive solutions.

EPFL2020

Exploring the application domain of adaptive structures

Gennaro Senatore

Using a previously developed design methodology it was shown that optimal material distribution in combination with strategic integration of the actuation system lead to significant whole-life energy savings when the design is governed by rare but strong loading events. The whole-life energy of the structure is made of an embodied part in the material and an operational part for structural adaptation. Instead of using more material to cope with the effect of loads, the actuation system redirects the internal load-path to homogenise the stresses and change the shape of the structure to keep deflections within limits. This paper presents a systematic exploration of the domain in which adaptive two-dimensional pin-jointed structures are beneficial in terms of whole-life energy and monetary costs savings. Two case studies are considered: a vertical cantilever truss representative of a multi-storey building supported by an exoskeleton structure and a simply supported truss beam which is part of a roof system. This exploration takes five directions studying the influence of: (1) the structural topology (2) the characteristics of the load probability distribution (3) the ratio of live load over dead load (4) the aspect ratio of the structure (e.g. height-to-depth) (5) the material energy intensity factor. Results from the main five strands are combined with those from the monetary cost analysis to identify an optimal region where adaptive structures are most effective in terms of both energy and monetary savings. It was found that the optimal region is broadly that of stiffness-governed structures. For the cantilever case, the optimal region covers most of the application domain and it is not very sensitive to either live-to-dead-load or height-to-depth ratios thus showing a wide range of applicability, including ordinary loading scenarios and relatively deep structures.

2018

Energy and Cost Assessment of Adaptive Structures: Case Studies

Gennaro Senatore

This paper demonstrates how adaptive design (details published elsewhere) can be employed to save, on average, 70% of whole-life energy on a range of spatial structures, the whole-life energy deriving from an embodied part in the material and an operational part for structural adaptation. Assuming some statistical distribution for the probability of occurrence of the loads, whole-life energy is minimized by combining optimal material distribution and strategic integration of the actuation system, which is only used when loading events exceed a certain threshold. Instead of using more material to cope with the effect of the loads, the active elements change the shape of the structure in order to homogenize the stresses and keep deflections within limits. Five case studies are investigated here: a tall building core, a trussed portal frame, a long-span arch bridge, a 3-pin roof arch, a double-curved shell, and an office tower supported by an exoskeleton structural system. The purpose of the case studies described in this paper is to study (1) adaptive structure performance in terms of mass and energy savings as well as monetary costs for both strength- and stiffness-governed design problems; and (2) design scalability to complex spatial configurations. The case studies confirmed that even for large complex structures, significant energy savings can be achieved, the more so as the structure becomes more stiffness-governed. In this case, the adaptive solution becomes competitive also in terms of monetary costs.

2018

Shape changes through geometric nonlinear control as a strategy for structural adaptation

Arka Prabhata Reksowardojo

EPFL2020