Load carrying capacity of greenhouse covering films under wind action: Optimising the supporting systems of greenhouse films

Abstract

The load carrying capacity of a typical greenhouse film used as covering for a Gothic-type greenhouse was investigated under negative wind pressure. The film was simulated as a membrane through appropriate shell elements and analysed with the finite element method (FEM). The design wind pressures were calculated based on the provisions of Eurocode 1 and EN13031. The FEM model boundary conditions corresponded to the real film supporting conditions of an industrial type greenhouse structure. The film was pre-tensioned at its initial state in the longitudinal direction, and then subjected to uniform negative wind pressure. The nonlinear elastic analysis with large deformations results showed that, as the wind pressure increases, the principal tensile stress in the film increases and gradually reaches the material stress at yield value. Plastic deformation is expected to take place and the film loses its capacity to return to its initial undeformed state. Consequently, the plastically deformed film loses its prestress state and its proper elastic behaviour. Depending on the final wind pressure intensity and the degradation condition of the film, premature failure may therefore occur. The analysis revealed that the film failure is due to insufficient supporting of the film on the steel structure. This behaviour was verified by examining a real greenhouse film failure case. Various supporting systems of the greenhouse covering films with additional intermediate supports were analysed and optimal design solutions are proposed. The proposed optimised supporting systems of greenhouse films covers a gap in the current European standard for greenhouse design.