Wind pressures on permeably and impermeably-clad structures

The first European Standard for the design of commercial-production greenhouses passed through the formal voting stage in 2000 and its publication is imminent. For structural design considerations, it is based on the Eurocodes, and so for wind loading it is based on ENV 1991- 2-4: 1995, although it provides additional information where specific data are available. During the drafting exercise, one deficiency identified in available wind loading information was that of pressure coefficient data for permeably clad structures. Greenhouses, particularly flat shading structures and curved roof houses, are often clad in permeable shade or insectproof netting in southern Europe to reduce solar radiation gain and to increase ventilation. To facilitate reliable design of such structures, pressure data were obtained from large-scale tests conducted in the Jules Verne climatic wind tunnel at CSTB, Nantes, in November 1999. This paper reports the experiments conducted on an arch structure and on a flat-roof structure, where each was clad in turn in an impermeable plastic film, an ‘insect net’ of 33% open area, and a ‘shade net’ of 39% open area. The pressure coefficient data obtained with each cladding are compared for each of the two structural forms.