Transition from a tension wire structure to a tension skin structure
Introduction to the Almeria greenhouse
The Almeria tension wire structure is an existing and successful greenhouse structural design in wide use across the Almeria Region. It uses a system of wire ropes, which are placed in tension to provide a structural support that is almost horizontal but is sufficiently rigid to limit deflection of the layers of wire mesh and single plastic cover. The wire rope has a grid that are supported and held up by the columns and another grid that are pulled down by cables reach to anchors that are secured into the ground. The grid is almost flat initially but the cables are tensioned to create a shallow valley between the column rows. There is a grid of wire ropes crossing the bays and extending over the tops of the columns and this crossing grid shows the shape of the shallow valley and peak system that is created. The tie down cables form a row along the center line of the greenhouse bays and pull down to create a the valley that are typically 4 to 5 meters wide. The spacing between the cables in the cable row is the same as for the spacing of the columns in the column row and is typically 2 to 3 meters. This wire rope and cable system are placed under tension.
Another smaller gauge wire mesh is attached to the tension wires and they are laid out on a closer spacing of about 0.5 meters in a grid following the bay and crossing the bay. The transparent plastic cover is laid on top of this lighter wire mesh and then another light wire mesh is added to the top and is woven to the bottom mesh, through the plastic sheet and crossing under the lower grid of wires at the valleys so that it will hold down the plastic cover. Additional wire ties are used to connect the exterior wire mesh to the lower wire mesh by piercing the plastic covering. The covering sheets do not quite meet at the valley and water can fall through this opening into a gutter that is suspended from the wire rope at the valley. The tie-down cables actually pull on a metal bracket that supports the gutter and connects to the wire rope grid at the valley. Thus the gutter is positioned to catch the rainwater that may fall through the gap between the covering sheets.
Adapting the Almeria design to Sola Roof concepts
The covering system of inner, light wire mesh, plastic sheet and outer light wire mesh are not under tension but are only attached to the tensioned wire rope grid. Since the wire rope grid is not rigid and will deflect, it needs support frequently or it will billow outward in the wind or sag inward with the weight of rainwater, and thus there are frequent supports from close spacing of the columns and cables that are resisting these loads. In between these points of support the wire ropes and the wire mesh and plastic can be deflected by live loads of wind and rain and these deformations of the structure “envelope” are in two dimensions of curvature. This deformation will focus the force on critical points of the structure and create forces at these points that can far exceed the force that would exist if there was only a deformation in one dimension. A single curvature in a roof envelope will pass the load equally along a continuous line of attachment so the force is spread out and not concentrated at points. This is one of goals of the enhanced SOLAROOF design.
The Almeria tension wire structure typically has a low roof height because of the frequent columns and cables. The low elevation of the roof will limit the amount of metal used; however the trend for efficient climate control is towards greenhouses with high roofs. Because the Almeria tension wire roof is not rigid in between column supports there must be a close spacing of the columns – but our enhanced SOLAROOF design forms a rigid ridge along the column row by using a deep, lightweight, open web joist – and metal that would go into columns is eliminated and will offset the weight of metal that is used in the joist. Thus, with the improved structure needing fewer columns it is natural and more efficient to construct greenhouses that have high roofs.
The top and bottom chords of the SOLAROOF joists will span between the columns, and unlike the wire rope, the joists will have a very limited deflection because of rain or wind load and therefore there will be no curvature of the envelope in the direction of the column row (the length of the bay). Equipment and crop support can also be supported by the lower chord of the joists and this will not transfer any load to the envelope of the greenhouse. Please refer to the sketches at http://www.solaroof.org/gallery/AlmeriaProject
In the present Almeria greenhouse the wire rope roof grid is tensioned by a great number of cables that reach the ground along the centre line between the column rows. The improved system will tie the cables to the bottom chord (usually at a node with the web members as this is a strong point) of the joists or will eliminate the cables completely by using an inner cover material instead. The floor area will not have any anchors for the tension cables and therefore the floor plan is much more open and flexible. The cost of the earth anchors is eliminated. The uplift force of the wind is taken by the columns, acting through the bottom chord to the cables that extend from the chord nodes to a metal gutter at the valley of the roof envelope.
Replacing the wire tension system with a tension Double Skin
In a further step to enhance the Almeria greenhouse we go all the way to eliminating the metal gutter and the cable tension components; replacing these with an inner layer of plastic covering, which acts to tension the outer plastic covering and to resist the uplift force of the wind. This improvement eliminates all of the wire rope (heavy wire) and the layers of light wire, the metal gutter and the cable tension system, including the earth anchors. A great deal of construction labor is also eliminated. What is added to replace these components is another layer of plastic covering to create a two layer roof envelope. Plastic film is not suitable for use in this new roof envelope. The reason is that the tension that was in the metal wire system is transferred to the covering material and that requires woven and laminated materials that have a composition that can handle the tension without showing any elongation. Only elastic deformation is allowed and this is quite limited. Therefore films, which typically elongate 300 to 500 percent, are not acceptable. The covering material must resist the wind and rain load and also have a high transparency. Transmission of solar radiation is more important because of using two layers instead of one to create the roof envelope. The SOLAROOF concept has no use for metal wire and rope and instead, plastic or glass fibers and tapes are used to form a woven “substratum” and these are and then coated with transparent film layers to provide a strong covering material that will not stretch under tension.
This covering material is prefabricated into a two layer envelope that is made with special design to permit the easy attachment and tensioning of the envelope itself. The prefabricated envelope is attached continuously along the top and bottom chords of the joists that have special snap lock features incorporated into the extrusion profile. These modular panels are assembled to the frame to form the extensive roof area. The double layer creates a cavity space that can be filled with the liquid bubbles for shading and cooling. Also, a cooling liquid can be sprayed into the cavity space as a cooling system that can control the temperature and humidity within the greenhouse. The rain gutter is built into the design of the prefabricated panel system. The stressing of the double layer cover material creates a tension roof that resists wind and rain (snow load in cold climates) without any wire, cable or metal gutter system.
Spanish translation starts here:
I've started with the headings so as to make readers anxious for more };-> Lucas Gonzalez
Transición de una estructura con cables a tensión a una estructura con piel a tensión
Presentación del invernadero de Almería
La estructura de cables a tensión de Almería es un diseño estructural de invernaderos que existe y tiene éxito, con amplio uso en toda la Región de Almería. Usa un sistema de cables metálicos, que se colocan tensos para proporcionar apoyo estructural que es casi horizontal pero que es lo suficientemente rígido para limitar la deflexión de las capas de "mesh" de cables y una única cubierta de plástico. El cable de metal forma una malla que es sostenidas y mantenidas en alto por las columnas y por otra malla de la que tiran hacia abajo cables que llegan a anchas que se unen firmemente al suelo. La malla está casi plana inicialmente pero los cables se tensan para crear un valle de poca profundidad entre las filas de columnas. Hay una malla de cables de metal que cruza las bahías y que se extiende sobre lo alto de las columnas y esta malla entrecruzada muestra la forma del sistema de valles poco profundo y picos que se crea. Los cables que amarran hacia abajo forman una fila a lo largo de la línea central de las bahías del invernadero y tiran hacia abajo para crear los valles que suelen estar a 4 o 5 metros de distancia uno del otro. El espacio entre los cables de la fila de cables es el mismo que el espaciado de las columnas en la fila de columnas y suele ser de entre 2 y 3 metros. Este sistema de "wire rope and cable" se pone bajo tensión.
''Another smaller gauge wire mesh is attached to the tension wires and they are laid out on a closer spacing of about 0.5 meters in a grid following the bay and crossing the bay. The transparent plastic cover is laid on top of this lighter wire mesh and then another light wire mesh is added to the top and is woven to the bottom mesh, through the plastic sheet and crossing under the lower grid of wires at the valleys so that it will hold down the plastic cover. Additional wire ties are used to connect the exterior wire mesh to the lower wire mesh by piercing the plastic covering. The covering sheets do not quite meet at the valley and water can fall through this opening into a gutter that is suspended from the wire rope at the valley. The tie-down cables actually pull on a metal bracket that supports the gutter and connects to the wire rope grid at the valley. Thus the gutter is positioned to catch the rainwater that may fall through the gap between the covering sheets.
Adaptación del diseño de Almería a los conceptos de Sola Roof
Reemplazo del sistema de cables a tensión con una Doble Piel a tensión