My research interests revolve around deployable structures and adaptable materials and fit under three broad categories:

(1) Theory and Analysis of Deployable Systems:
  • The hierarchical nature of interconnected folded structures.
  • Structural dynamics and vibrations in deployable systems.
  • The transition between small and large displacements in thin-sheets.
  • Multiphysical functionality and behaviors of folding systems.

  • (2) Innovative Cellular Metamaterials and Components:
  • Engineered materials with unique characteristics. E.g.: high stiffness-weight ratios, high energy dissipation, ...
  • Materials with variable and adaptable properties. E.g.: damping, stiffness, thermal conductivity, transparency, ...
  • Manufacturing and self-assembly of origami inspired metamaterials.

  • (3) Hinged Folding Systems with Thickness:
  • The folding mechanics and kinematics of systems with thickened panels and hinges.
  • Actuation and localized behaviors in hinged systems.
  • Realization of practical applications in robotics, aerospace, and architecture.

  • My previous research has explored several different fields of structural engineering. A theme common through all of my research has been the development of analytical models and tools that simulate physical behaviors in structures. Short summaries of three of my previous projects are available below:
    Deployable Structures and Origami Engineering
    Multiresolution Topology Optimization
    Quasi-Isolation Systems for Bridges

    Deployable Structures and Origami Engineering

    Analysis Figure
    The three components of the bar and hinge model are shown in a way
    that can be used to analyze the foldable origami tube.

    Analytical models for thin-sheet structures
    We have improved a model for the analysis of origami structures that consist of flat panels interconnected by a pattern of prescribed fold lines. This bar and hinge model, first introduced by Schenk and Guest (2011) can capture the fundamental behaviors of thin folded sheet structures. The model consists of three components: (1) elastic bar elements that simulate the stretching and shear stiffness thin panels; (2) rotational hinges that simulate folding of the panels; and (3) rotational hinges that simulate folding along the more flexible prescribed fold lines. We have made the model scalable, and have incorporated material characteristics such as the elastic modulus, Poisson's ratio, and thickness of the thin sheet. Although this model is not as as accurate as a detailed finite element (FE) model it provides several useful advantages. The model is easy to implement, easy to use, it provides insightful results, and it is faster than FE analyses. The efficiency of the model makes it suitable for extensions to parametric studies, optimization or various specialized analyses.

    Analysis Figure
    Deployment and retraction of zipper-coupled
    tubes by actuating only on the right end.

    Deployable coupled tube structures
    We have explored a variety of origami tube systems and assemblages. The basic type of tube is constructed by placing two symmetric Miura-ori sheets together (see Tachi 2009). The tube is rigid and flat foldable meaning it can fully unfold from a flattened state with deformations occurring only at the fold lines. We used eigenvalue and structural cantilever analysis to investigate and compare different geometries of tubes and coupled tube systems. The "zipper"-coupled tube system (shown on the left) yields an unusually large eigenvalue band-gap that represents a unique difference in stiffness between deformation modes. The structure has only one flexible mode through which it can deploy, yet it is significantly stiffer for all other bending and twisting type modes. The deployment motion is permitted by the flexible bending the thin sheet along the prescribed fold lines, however all other modes require the significantly stiffer stretching and shear of the thin sheet. The zipper-couped tubes have the advantages of deployable origami, but also the stiffening effect that is common in cellular/corrugated structures and materials.

    Analysis Figure
    (Top) A cellular metamterial with the stiffness varying based
    on the structural configuration. (Bottom) Deployable bridge
    structure constructed from two different tubes.

    Extensions from metamaterials to deployable architecture
    Origami sheets and origami tubes can be coupled, combined, and arranged in a variety of methods to form new geometries and structures. We have shown different methods in which the zipper-coupled tubes can be assembled into cellular assemblages. By combining different types of coupled tubes together we can also enhance the structural characteristics of these systems. For example, the cubic cellular assemblage (shown top right) consists of zipper and aligned coupling, and has both space filling properties and the enhanced stiffness of the zipper tubes. This assemblage can have a variable asymmetrical stiffness depending on its configuration. Similarly, it is be possible to couple different geometries of tubes. To create a bridge type structure (shown bottom right), we use nearly square tubes to provide a smooth deck, and we use zippered zig-zag tubes to create a stiffer parapet. The deployable origami assemblages could lead to practical applications ranging in size from microscale metamaterials that harness the novel mechanical properties to large-scale deployable systems in engineering and architecture.

    Publications related to this research:

    1 Filipov, E.T., Paulino G.H., and Tachi T. “Origami Tubes with Reconfigurable Polygonal Cross-Sections”. Proceedings of the Royal Society - A , Vol. 472, No. 2185, 20150607. See paper, details, and media coverage.
    2 Filipov, E.T., Tachi T., and Paulino G.H. (2015) “Origami Tubes Assembled Into Stiff, yet Reconfigurable Structures and Metamaterials,” Proceedings of the National Academy of Sciences USA , Vol. 112, No. 40, pp. 12321-12326. See paper, details, and media coverage.
    3 Filipov, E.T., Tachi, T., and Paulino, G.H. (2015). “Toward Optimization of Stiffness and Flexibility of Rigid, Flat-Foldable Origami Structures,” In Origami 6, Proc. of the 6th International Meeting on Origami Science, Mathematics, and Education (eds K Miura, T Kawasaki, T Tachi, R Uehara, RJ Lang, P Wang-Iverson), pp. 409–419. Providence, RI: American Mathematical Society.
    Press coverage:

    Highlighted in PNAS commentary by Reis et al. 2015. Reported by: WXYZ TV – ABC News, Civil + Structural Engineer, Discovery News, Motherboard, City Lab, Fast Company, Gizmag,, Science Daily, Space Daily, Gizmodo, Sydney Morning Herald, also more on zipper tubes and more on polygonal tubes.

    Analysis Figure
    Optimization of the 1st eigenfrequency of a simply supported beam: (a) Geometry and
    boundary conditions, Regular approach with (b)2000 n-gons, and (c)34000 n-gons, and
    (d) Multiresolution Pn/n17 approach with 2000 n-gons and 34000 design variables.

    Multiresolution Topology Optimization
    We use versatile polygonal elements along with a multiresolution scheme for topology optimization. This approach allows for a computationally efficient and high resolution design for structural dynamics problems. The polygonal elements allow for fast and easy meshing of complex domains, or uneven refinements in discretization. The multiresolution scheme uses a coarse finite element mesh to perform the analysis, a fine design variable mesh for the optimization and a fine density variable mesh to represent the material distribution. We show applications of this multiresolution approach in the topology optimization of structural eigenfrequencies and forced vibration problems. With the multiresolution approach the finite element analysis on a coarse mesh is efficient, and a higher resolution can be obtained by distributing material in a finer mesh of design/density variables (part d in figure).

    Publications related to this research:

    1 Filipov, E.T.*, Chun J.*, Paulino G.H., and Song J. (2015) “Polygonal Multiresolution Topology Optimization (PolyMTOP) for Structural Dynamics,” Structural and Multidisciplinary Optimization. (In Press). doi: 10.1007/s00158-015-1309-x

    Bearing and Bridge Figure
    (Top) Lateral loading of an elastomeric bearing with
    side retainers (Bottom) Longitudinal deformation
    of the full bridge model used for seismic analyses.

    Quasi-Isolation Systems for Bridges
    We explore "quasi-isolation" which is a modern philosophy for seismic design of bridges. With this approach nonlinearity occurs in specific bearing components such that forces transferred into the substructure are reduced and isolation is achieved by sliding of bearings. This system can provide a low-complexity, low-cost approach to mitigate of earthquake effects in locations with risk at long recurrence periods, such as the eastern and central United States. The proposed system employs a set of fixed bearings at one intermediate substructure, and all other substructures are instrumented with elastomeric bearings that permit thermal expansion (top left). L-shaped steel side retainers are placed in the transverse direction of the elastomeric bearings. Along with the low-profile fixed bearings, these components prevent bridge movement during service loading, but break-off and permit sliding at high earthquake loads. We construct a finite element model (bottom right) that can capture a variety of nonlinear behaviors in the bridge and in the bearing elements. New models are formulated to capture the bi-directional stick-slip behavior in the sliding bearings and the bilinear (and eventual fracture) behavior of steel retainers and fixed bearings. These models are informed and calibrated based on a detailed experimental study. Longitudinal and transverse pushover analyses are performed to demonstrate local limit states and progression of damage in the bridge structure. A large scale parametric study with incremental dynamic analyses is carried out to investigate the quasi-isolated system on a variety of different bridges. Results indicate that bridges with quasi-isolation bearings can be resilient even for high seismic events in Illinois. However, the approach can be refined to improve performance and reduce damage to components such as the intermediate bridge piers.

    Publications related to this research:

    1 Steelman, J.S., Filipov, E.T., Fahnestock, L.A., Revell, J.R., LaFave, J.M., Hajjar, J.F., and Foutch, D.A. (2014) “Experimental Behavior of Steel Fixed Bearings and Implications for Seismic Bridge Response,” Journal of Bridge Engineering, Vol. 19, No. 8, SPECIAL ISSUE: Recent Advances in Seismic Design, Analysis, and Protection of Highway Bridges, A4014007. Link
    2 Filipov, E.T., Revell J.R., Fahnestock L.A., LaFave J.M., Hajjar, J.F., Foutch D.A., and Steelman J.S. (2013) “Seismic Performance of Highway Bridges with Fusing Bearing Components for Quasi-Isolation,” Earthquake Engineering and Structural Dynamics. Vol. 42, No. 9, pp. 1375-1394. Link
    3 Filipov, E.T., Fahnestock L.A., Steelman J.S., Hajjar, J.F., LaFave J.M., and Foutch D.A. (2013) “Evaluation of Quasi-Isolated Seismic Bridge Behavior Using Nonlinear Bearing Models,” Engineering Structures, Vol. 49, No. 14, pp. 168-181. Link
    4 Steelman, J.S., Fahnestock L.A., Filipov E.T., LaFave J.M., Hajjar, J.F., and Foutch D.A. (2013) “Shear and Friction Response of Non-Seismic Laminated Elastomeric Bridge Bearings Subject to Seismic Demands,” Journal of Bridge Engineering, Vol. 18, No. 7, pp. 612-623. Link