Structures that adapt to loads through large shape changes: design and optimization

The construction industry is a major contributor to the consumption of mined material resources and to the global energy demand. It is thus important to consider energy and material efficiency criteria for the design of civil structures. Through sensing and actuation, structures can adapt their geometry and internal forces in order to maintain optimal performance during service life. Previous work has shown that well designed adaptive structures achieve substantial whole-life energy savings compared with passive structures. Whole-life energy consists of an embodied part in the material and an operational part for structural adaptation. This paper presents a new method to design structures that adapt to loads through large shape changes in order to redistribute stresses and thus to minimize material utilization. This method consists of two parts: (1) shape optimization is employed to obtain shapes that are optimal under each load case. This way extreme loads that typically have long return periods do not govern the design. (2) Stochastic search combined with the nonlinear force method (NFM) is employed to obtain an optimal actuation system that is able to control the structure into the target shapes obtained in (1). A 3D-truss roof structure is taken as a case study to illustrate the design process.

Design of structures that adapt to loads through large shape changes

Arka Prabhata Reksowardojo, Gennaro Senatore, Ian Smith

Adaptive structures can 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 with traditional passive designs. The whole-life energy comprises an embodied part in the material and an operational part for structural adaptation. 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. This paper presents a new design process for adaptive structures based on geometry and member sizing optimization in combination with actuator placement optimization. This method consists of two parts: (1) geometry and sizing optimization through sequential quadratic programming is carried out to obtain shapes that are optimal for each load case; (2) a formulation based on stochastic search and the nonlinear force method (NFM) is employed to obtain an optimal actuator layout and commands to control the structure into the target shapes obtained from (1). A case study of a planar statically indeterminate truss is presented. Numerical results show that 17% and 37% embodied energy savings are achieved with respect to an identical active structure designed to adapt through small shape changes and to a weight-optimized passive structure respectively. The combinatorial task of optimal actuator placement is carried out efficiently. The method formulated in this work produces actuator layouts which enable accurate geometric non-linear shape control under quasi-static loading through a low number of actuators compared to the number of members of the structure.

2020

Experimental Testing of a Small-Scale Truss Beam That Adapts to Loads Through Large Shape Changes

Arka Prabhata Reksowardojo, Gennaro Senatore, Ian Smith

Adaptive structures have the ability to modify their shape and internal forces through sensing and actuation in order to maintain optimal performance under changing actions. Previous studies have shown that substantial whole-life energy savings with respect to traditional passive designs can be achieved through well-conceived adaptive design strategies. The whole-life energy comprises an embodied part in the material and an operational part for structural adaptation. 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 embodied energy is reduced. A design process based on shape optimization has been formulated to obtain shapes that are optimal for each load case. A geometrically non-linear force method is employed to control the structure into required shapes. This paper presents the experimental testing of a small-scale prototype adaptive structure produced by this design process. The structure is a simply supported planar truss. Shape adaptation is achieved through controlled length changes of turnbuckles that strategically replace some of the structural elements. The stress is monitored by strain sensors fitted on some of the truss elements. The nodal coordinates are monitored by an optical tracking system. Numerical predictions and measurements have a minimum Pearson correlation of 0.86 which indicates good accordance. Although scaling effects have to be further investigated, experimental testing on a small-scale prototype has been useful to assess the feasibility of the design and control methods outlined in this work. Results show that stress homogenization through controlled large shape changes is feasible.

2019

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

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

Arka Prabhata Reksowardojo

EPFL2020

Design of structures that adapt to loads through large shape changes

Arka Prabhata Reksowardojo, Gennaro Senatore, Ian Smith

2020