Active Bending in Timber Structures: Case Studies in Design, Development and Construction

The realization of curvilinear forms in timber has generally been pursued through two distinct modes. In the first, straight members or flat panels are joined incrementally, at angles to approximate curvature through a strategy of subdivision and segmentation. In the second, thin lamellas or sheets are bent and glued to produce components that more fluidly reproduce an idealized curvature. Each of these poses a particular set of opportunities and challenges for structural analysis and constructive technique. In practice, most structures involve a combination of both techniques and Nevertheless, timber elements are not generally used to in-situ methods of heat forming or post-tensioning that are commonplace in concrete or steel, so this generaldivision between straight/flat and bent/curved components is an essential consideration in the structural and architectural definition of curved forms inwood.

This paper considers a technique of building that is situated between these well-established practices and takes advantage of lessons from both. Rather than accepting the shape of members as static forms, bending active structures are elastically shaped through bending and/or twisting to achieve a pre-determined structural geometry. Strength and stiffness of individual elements and of the entire structure are developed through a complex three-dimensional network of forces that must be carefully negotiated at every scale and especially at all points of connection.

Although some recent projects such as the Pompidou Metz by Shigeru Ban emulate forms of active bending, it is important to distinguish between works that draw geometric inspiration from active bending and ones that use the technique to structural advantage. This paper explores the development of two design and research projects executed at Aalto University that make use of active bending in differing degrees. It describes the design, development and laboratory testing of the bending systems and concludes with some suggestions about the possible applications active bending in larger constructions.

The first case study, Säie (strand or fiber in Finnish), was built as a 68 sqm pavilion and designed through extensive physical and computational modeling as well as full scale mock-ups and tests. The walls of the structure and its gridshell roof were developed through the use of 1:5 scale and full scale structural tests which produced data that could then inform the finite element modelling of the structure. Particular attention was given to the design of the nodal connections, which were executed through a combination of complex NC cut plywood elements which were assembled using simple analog connections on site.

The second case study, Aikalava (time-stage in Finnish), was built as a 243 sqm summer stage for the annual Areena event in the urban center of Pori. Building on the knowledge and techniques of the previous example, the structure makes use of active bending not as a gridshell but instead as a two-dimensional wall structure. At this larger scale, with much higher elastic energy contained in much larger structural members, the architectural and structural novelty of the system propose new ways of building that suggest an expanded field of potential for this mode of construction.