Erecting gridshell structures with inflatable membrane technology – simulations and techniques

By Alessandro Liuti

This thesis aims to develop and test a novel technology, based on inflatable membranes, for the erection of post-formed gridshell structures.

Gridshell structures (gridshells) are lightweight form-resistant timber structures which discretise the features of double-curved shells into a lightweight grid made by continuous timber laths running through the joints. This allows to have reduced structural self-weight and to carry loads through membrane action.

Design is generally performed through form-finding– commonly implemented through either physical or digital tools such as dynamic relaxation. This stage has been widely covered by literature, and it intrinsically matches principles of structural efficiency.

Construction generally takes place by loosely connecting layers of linear laths in a flat grid of quads with hinge joints; such a grid is subsequently post-formed into a double-curved shape and hence stiffened by means of bracing, shear blocks and connection tightening. During the bending process gridshells can undergo a stress state which is higher than the ones it will react during its steady state. Erection is thus a key phase for gridshells. Furthermore, another criticality related to construction is that expensive and complicated formworks are often required during the post-forming phase.

Presently, only three techniques have successfully been used for the erection of such structures: the ‘lift-up’ (which consists of assembling the flat grid on the ground and then lifting it up by means of cables and cranes); the ‘push-up’ (low-budget version of the lift-up, implemented by using jacking towers and forklifts); the ‘ease-down’ (which consists of assembling the grid flat configuration on a raised level to later bend it down by means of modular scaffolds and mechanical formworks). These erection techniques feature several criticalities related to both stress state; for instance, applying concentrated and asymmetric loads during erection can induce much stress states that can break the laths; empirical erection can be imprecise and difficult to monitor; bending can require long periods of time; a large amount of workers can be demanded  to operate in a hazard situation under an unfinished dome.

Bending air to form lightweight timber shell-structures

A fourth technique, based on “inflatable membrane technology”, has been theorised as a possible alternative but never applied. Preliminary studies prove that, on the one hand, this technology can benefit construction with good speed and replicability – lowering the chances of imprecisions and OHS accidents. On the other hand, it allows distributing lifting forces more homogenously – lowering the induced stresses in each lath and hence lowering the risk of breakings.

The early inspiration draws from Dante Bini’s inflatable-based construction systems: Binishell (for the construction of reinforced concrete shells) and Binistar (for the erection of steel spatial structures). Operational Research is resolved through a series of different, interdependent, stages: Preliminary numerical modelling; Physical small-scale prototyping; Refined numerical modelling; Physical large-scale prototyping.

The first numerical models are drafted within the environment of Rhino / Grasshopper / Kangaroo / Karamba – and then refined in Abaqus FEM; here there are developed and compared form-finding algorithms for both bending the laths and inflating membranes. These simulations are hence validated through physical prototyping; these preliminary stages, condensed within Case study1, show preliminary results such as: (1) inflatable membrane technology can reduce the stress and imprecisions during to erection if compared to other erection techniques; (2) the technology can provide an precise shape control on the final outcome.

After developing and testing a general case study framework with case study 1, it gains relevance to validate the method by shifting to a more complex and relevant case study. Case study 2 (a large-scale prototype) is the last stage of research; this case study allows: (1) further developing technological details such as ground connections and membrane cutting patterns; (2) validating the precision of the digital tools; (3) validating the efficiency and the flexibility of the system.