Construction Of Rigid Tensioned Frame Structure

Abstract

In a roof structure a flexible roofing membrane is supported and secured between at least two layers of tensioned flexible elements such as wires or cords. Each layer comprises a number of longitudinal tensioned wires and a number of transverse tensioned wires. The wires are attached to curved anchorage units, and the entire structure is supported on tensioned trellis trusses and purlins.

Parent Case Text

This is a continuation-in-part of my application Ser. No. 764,058 filed Oct. 1, 1968, and now abandoned.

Description

The present invention relates to rigid tensioned roof supporting frame structures in which the roof is formed of plastics material or other flexible roofing membrane. More particularly, the invention is concerned with a greenhouse or like horticultural or agricultural building (hereinafter referred to collectively as a "greenhouse") in which the roof covering is a plastics material or similar flexible membrane.

It is well known to those conversant with the art and theory of stressed cable design, e.g., suspension bridges, that the stability of a tensioned structure may be achieved by the massive deadweight, which in itself is sufficient to resist all downward and upward movement due to wind pressures. Alternatively, in some structures stability may be achieved by a number of downward stressed cables firmly anchored to the ground, as in many hanger type or large tent buildings, by the use of two mutually contra-tensioned cables stretched between two or more main supporting rigid units, themselves tied to standard heavy ground anchorages.

With particular reference to the erection of a stressed or tensioned structure for use in green-house development, none of the above systems are readily applicable since the most important aspects of a green-house structure, i.e., the angle of incidence of sunlight and the diffusion of light throughout the hours of daylight are not rigidly controlled and further the application of forced fan or natural ventilation equal to one-sixth of the ground area of the structure, which is desirable in green-house constructions, is difficult to achieve. Additionally it is well known, in the art of green-house growing of vegetables and flowers, that the light transmission factor to the interior of the structure is directly related to the vertical depth of the glazing bars in relation to their spacing apart, the orientation of the structure; the angle of roof pitch; the angle of incidence of sunrays on the roof; and the slope of the site. The loss of light at the most critical period of growth and plant propogation, mainly winter and early spring, in latitude 50.degree.-59.degree. (Britain) and 51.degree.-55.degree. (Ireland) amounts to 60-70 per cent of the total light available for growth.

An object of a preferred construction of green-house according to the present invention is to provide a green-house structure which eliminates all the aforementioned inherent structural, economic and horticultural disadvantages of orthodox green-houses, in providing an artificial climate and environment for maximum commercial growth of flowers and vegetables.

According to the invention, there is provided a curved roof structure comprising at least one layer of flexible roofing membrane supported and secured between at least two layers of tensioned flexible elements, each layer comprising a plurality of longitudinal tensioned flexible elements and a plurality of transverse tensioned flexible elements so arranged so as to bear against one another. In one preferred construction of green-house, the tensioned flexible elements comprise two layers of wire, cables of steel or the like or plastics cord, the roofing membrane advantageously being translucent polyethylene or polyvinyl chloride (P.V.C.).

In a simple construction, one of the layers of flexible elements may comprise a plurality of longitudinal tensioned flexible elements and may be supported by and connected between a pair of curved wire anchorage units, one of which is disposed at each end of the structure and both of which are substantially at right angles to the ground, while the other layer of flexible elements may comprise a plurality of transverse tensioned flexible elements and may be supported by and connected between spaced apart parallel purlins connected to and between said wire anchorage units. If desired, the wire anchorage units and certain of the purlins (for example the purlin disposed at the apex of the structure) may be supported by vertically disposed trellis trusses securely located in ground foundation or support.

A complete green-house structure according to the invention may be formed of:

a. A number of self-supporting tensioned trellis trusses consisting of one or two independently tensioned members, for example, bars or steel cables or other strong thin element, which are supported and tensioned by alternating vertical rigid compression and vertical tensioned members, carried by dual purpose tension/compression foundations of a permanent nature below cultivation level, and movable above same.

b. A roof supporting structure consisting of two or more rigid or reinforced/tensioned elements of wood, steel or plastics, hereinafter called purlins, which are firmly fixed to external members of the trellis trusses and disposed in parallel position in relation to each other, at right angles to each of the trellis trusses connecting the trellis trusses together longitudinally to form a roof supporting framework.

c. Two or more layers of tensioned thin resilient members such as wire, thin bars, plastics cord, supported by the aforementioned trellis trusses and purlins, framework, in such a manner as to maintain in position between the layers a sheet or a number of sheets of flexible roofing membrane, the sheets of flexible roofing membrane being held firmly in position by the individual bands of tensioned resilient members in compression against each other and forming a plurality of planar surface units which may be described as curved hyperbolic-paraboloid planes, to form a complete covering roof membrane for the entire structure.

The flexible roofing membrane -- usually clear or translucent plastics material -- is applied between layers of the aforesaid tensioned resilient members, in any sizes or shapes outlined by any selected pattern of structural members found most convenient for use with the minimum size of sheet membrane available commercially. In the case where the sheet membranes are larger in width than the maximum width of a single supporting unit or of unlimited length the membrane may be fixed to the main longitudinal tensioned purlins only by any suitable method which eliminates the use of nailed battens, screws or clips. The fixing of the membrane to the main longitudinal tensioned purlins should be such as to allow for removal and/or renewal of the plastics roofing membrane without interference with or damage to the supporting structure. If the membrane sheet thus fixed in position is to be adjoined transversely on structures of greater length than the membrane length available, this may be readily achieved by roll lapping adjacent sides of the two sheets together and fixing same in position by extra internal and external tensioned wires, rolled in lap, and held in position firmly by two main tensioned layers of resilient members. When the plastics roofing membrane is of a narrower width than a single supporting unit, it may be affixed diagonally or parallel between any two or greater number of supporting members which are fixed in position at a distance from each other, most appropriate to the width of plastics membrane used to form the roof and side covering of the structure. Further, any odd sizes of plastics membrane that lends itself to jointing by the simple process of pressure heat welding may be used to cover the structure and welded in situ to form a weatherproof joint, in any position on the surface of the structure, regardless of its location in relation to the tensioned resilient members, purlins or other elements of the structure.

The rigid tensioned frame structure as described above may be sheeted over its complete external surface with a single layer of flexible membrane. In the construction of greenhouses the provision of heat insulation is desirable. This is particularly important when plastic sheeting, which has a 10-15 percent greater heat loss than glass is used, as a roofing material. Consequently, the invention provides means for cladding the structure with a two or more layers of flexible membrane.

It is to be understood that in comparison with orthodox metal or timber and glass structures, which at present generally form the artificial environment for growing of plants and vegetables, the plastic membrane covered tensioned structure as described in this invention lends itself to all standard forms of both forced and natural systems of ventilating and heating. Ridge and side-wall vents, having inward and/or outward openings, may be incorporated in the plastic covered structure with maximum efficiency. The plastic covered greenhouse, according to the present invention, forms a sealed structure free from air leaks, such as occur between laps in glazing in metal and wood orthodox greenhouses and due to this particular attribute, the sealed structure lends itself most efficiently to forced ventilation by motor driven fans, which are designed either to ventilate by forcing air into the greenhouse, the escape of the air being controlled by vents or louvers, or by the extraction of air from the interior of the structure, thereby causing air to flow from the external atmosphere into the greenhouse through the controller vents or through vents with plastic internal ducting positioned throughout the interior length of the structure. The heating of the structure can be achieved by any of the well known heating systems used for horticultural purposes with improved efficiency.

The invention will hereinafter be described more particularly with reference to the accompanying drawings which illustrate, by way of example only, preferred embodiments thereof, and wherein:

FIG. 1 is a perspective view of the roof and gable section of a single span greenhouse according to the invention,

FIG. 2 is a perspective view of a single span greenhouse having a construction different to that of FIG. 1,

FIG. 3 is a diagrammatic plan of a single span greenhouse having a gable construction corresponding to FIG. 2 and a roof support structure corresponding to FIG. 1,

FIG. 4 is a diagrammatic plan of a single span greenhouse with a further alternative gable construction and showing the position of ridge opening ventilators,

FIG. 5 is a transverse cross-sectional view of the greenhouse illustrated diagrammatically in FIG. 4,

FIG. 6 is a longitudinal elevation of the greenhouse illustrated in FIG. 4,

FIG. 7 is a diagrammatic cross-section of a single span greenhouse,

FIG. 7a is a diagrammatic side view of a greenhouse similar to that illustrated in FIG. 1,

FIG. 7b is a diagrammatic side view of a greenhouse similar to that illustrated in FIG. 2,

FIG. 7c is a diagrammatic side view of the greenhouse illustrated in FIG. 3,

FIG. 8 is a diagrammatic end view of a greenhouse similar to that illustrated in FIG. 1 showing an alternative construction of curved anchorage unit,

FIG. 9 is a diagrammatic cross sectional view of a greenhouse illustrating the use of fans and vents,

FIG. 10 is a diagrammatic cross sectional view of a greenhouse suitable for medium span structures,

FIG. 11 is a diagrammatic cross section of a greenhouse similar to that illustrated in FIG. 8 for a wider span greenhouse,

FIG. 12 is a diagrammtic view taken along section lines XII--XII of FIG. 11,

FIGS. 13 and 14 are further diagrammatic cross-sections of further green houses,

FIG. 15 is a cross-sectional elevation illustrating details of FIG. 1,

FIG. 16 shows, in perspective, a detail of FIG. 15,

FIG. 17 is a diagrammatic representation of a method of securing a second layer of roofing membrane to a roof support member.

FIG. 18 is an exploded view of portion of the roofing membrane in position,

FIG. 18a is a view in the direction of the line XVIIIa-XVIIIa of FIG. 18,

FIG. 18b is a view similar to FIG. 18a showing two layers of roofing membrane,

FIG. 18c is a view similar to FIG. 18b showing the layers of roofing membrane after air has been introduced between them,

FIG. 19 is a longitudinal elevation showing two alternative systems of pulleys for tensioning of the top layers of resilient members for supporting of the roof membrane,

FIG. 20 is a perspective view of a second tensioning system for use in tensioning the top layer of resilient members for supporting of the roof membrane,

FIG. 21 is a cross-section of a greenhouse having a removable layer of insulation material suspended from roof,

FIG. 22 is a longitudinal section of the greenhouse in FIG. 21 with the insulation material in the partially removed position, and

FIG. 23 is a longitudinal section of the greenhouse in FIG. 21 with the insulation material in the position of use.

Referring to the accompanying drawings, and initially to FIG. 1 thereof, there is illustrated a greenhouse of relatively simple construction in accordance with the present invention and which comprises concrete foundation members or elements (not shown) which are located below cultivation level in such a manner as to allow the structure to be readily removed to another site and/or to allow the soil to be cultivated with mechanical machinery. The concrete foundation elements support a series of tensioned trellis trusses which are located in spaced apart rows, each row containing three trusses 2, 2a, the central truss 2a in each row being higher than the outer trusses 2.

Extending from the upper extremity of each of the outer trusses 2 in each row of trusses and connected at the upper extremity of the central truss 2a are stabilizing elements 3 while extending directly between the upper extremities of the outer trusses 2 in each row is a crop support member 4 the height and strength of which is determined by the weight and quantity of the crop to be grown in the greenhouse. The crop support member 4 is parallel to the ground and perpendicular to each of the trusses 2, 2a, to which it is connected.

Additionally supporting each of the outer trusses 2 in each row is a strut 5 operatively connected between the upper extremities of each of the outer trusses 2 and between a tensile foundation member (not shown). The strut 5 may be tensioned by a turnbuckle 5a.

The rows of trellis trusses 2,2a, at each gable end of the structure, are additionaly stabilized by struts 6, 6b operatively connected between the upper extremities of the trellis trusses 2 and ground anchorage units (not shown), the struts 6, 6b being tensioned by turnbuckles 6a. The gable end rows of trusses 2 support a main wire anchorage unit 7, which is additionally stabilized by struts 6d, operatively connected between the said main wire anchorage unit 7 and ground anchorage units (not shown) and tensioned by turnbuckles 6a.

Connected to the upper extremities of the trusses 2a in each row of trusses 2 is a purlin 11 which defines the apex of the structure, while connected between the upper extremities of the outer trusses 2 are a pair of purlins 12 each of which is parallel to the other and to the purlin 11, the latter and the purlins 12 forming the main framework of the structure. Furthermore purlins 13 are connected between opposed wire anchorage units 7 and are located adjacent ground level.

Curved stabilizing units 14 extend diagonally from an outer truss 2 in one row to an outer truss 2 on the remote side of a spaced apart row of trusses 2.

At each side of the structure is a longitudinal side board 15 of marine plywood, the upper edge of which is secured to the adjacent purlin 13 and the lower edge of which is embedded in the soil.

A curved base plate 16 is fixed to the ground below cultivation level and is utilized in association with the main wire anchorage unit 7 to form a frame for the gable end of the structure.

Longitudinal wires 21 are connected between the base plates 16 at each gable end of the building and extend through pulleys 22 on the main wire anchorage units 7. Each wire 21 passes through a turnbuckle (not shown) located adjacent one of the base plates 16. Application of tension to each wire 21 through the intermediary of the associated turnbuckle ensures that the framework of the entire structure is rigid and capable of resisting wind loads.

Located over the longitudinal wires 21 is a flexible roofing membrane of plastic sheeting 23 and securing the plastic sheeting 23 in position are transverse wires 24 connected between the purlins 13, the wires 24 being also tensioned by turnbuckles 1. Transverse wires 24 also support the flexible roofing membrane 23 against wires 21 at the gable ends of the structure.

Between the strut 6b, and one of the struts 6d parallel thereto, there is connected a transverse strut 6c which defines a door to one gable end of the greenhouse. A further door may similarly be provided at the remote end of the structure.

FIGS. 2 and 3 show greenhouses similar, but not identical, to FIG. 1. In the constructions illustrated in FIGS. 2 and 3, however, a pair of curved wire anchorage units 31 are positioned as shown so as to form equal angles at each corner of a greenhouse construction.

Supporting the wire anchorage units 31 are purlins 11, 12 borne by trellis trusses 2, 2a identical to the correspondingly numbered trellis trusses shown in FIG. 1. Additional purlins 12a bridge contiguous wire anchorage units 31 at each end of the structure, and render unnecessary struts corresponding to struts 6, 6b shown in the gable end of the structure illustrated in FIG. 1. The gable plan of the single span greenhouse illustrated in FIG. 2 is shown in FIG. 3 and the stabilising action of the longitudinal wires 21 is shown whereby the tension of the longitudinal wires 21 between the curved wire anchorage units 31 at one gable end is balanced by the tension exerted by the same longitudinal wires 21 between the two curved wire anchorage units 31. There is therefore a bending moment exerted on each wire anchorage unit 31 which is counteracted by the trellis trusses 2 and 2a. The struts 6 and 6b are therefore not required.

Intermediate curved wire anchorage units 32, supported by trellis trusses 33 and purlin 12a, define the framework for a door (not shown) to the construction of FIG. 2.

As in the greenhouse shown in FIG. 1, the construction illustrated in FIG. 3 has diagonally extending curved stabilizing units 14. In FIG. 2, however, associated with each intermediate row of trusses 2, 2a, is a curved stabilizing unit 14a located in the plane of the trusses to which the unit 14a is connected. Other details of FIG. 2 are substantially identical to the corresponding details of FIG. 1.

The principal elements of the invention as described above may be employed to form a plurality of single span and multispan structures. FIGS. 4, 5 and 6 show the construction of a single span greenhouse similar to that described in FIG. 1 but which has two main wire anchorage units 35 each disposed at an acute angle to the ground and inclined so that the trusses 2 and 2a are also inclined and in the same plane. The main wire anchorage unit 35 is similar to the main wire anchorage unit 7 previously described with reference to FIG. 1. The trusses 36, and 36a are substantially similar to the struts 6 and 6b respectively.

As shown in FIGS. 5 and 6, the central truss 36a projects above the upper curved extremity of the wire anchorage units 35. Located below and on each side of the central purlin 37 is an intermediate purlin 38 between each of which, and the central purlin 37, are standard outwardly opening roof ventilators 39 whic may be operated by any of the orthodox manual or automatic control means. The curved stabilizing units 14 shown in FIG. 4 and 5 are not continued across the apex of the structure as in FIG. 1. It will be appreciated that the central purlin 37 and similarly the purlin 11 (reference FIG. 1) are not required when the structure is ventilated by extractor fans. The use of extractor fans is described hereinafter.

The constructions shown diagrammatically with reference to FIGS. 7 to 14 inclusive illustrate various span structures incorporating the principles described with reference to FIGS. 1 to 6. The construction shown in FIGS. 7 to 14 inclusive are generally self-explanatory in the light of the foregoing description and demonstrate the manner in which single or multiple span greenhouses may be constructed. Similar reference numerals are used to illustrate parts similar to those previously described with reference to FIGS. 1 to 6.

For example, FIG. 7 shows a diagrammatic cross-section of a simple structure involving the use of a pair of curved main anchorage units 7 at each extremity of the structure without the employment of internal trellis trusses. The construction of FIG. 7 is normally employed only in minimum span structures where the use of trellis trusses is not warranted.

FIGS. 7a, 7b and 7c show in diagrammatic form the three main wire anchorage systems described with reference to FIGS. 1, 3 and 4. FIG. 7a illustrates a vertical main anchorage unit 7 with or without trellis trusses stayed in the vertical position by struts 6, 6b and 6d which are connected to the curved base plate 16 and is thus similar to the greenhouse illustrated in FIG. 1. FIG. 7b illustrates the main anchorage unit 35, with or without trellis trusses, disposed at an acute angle to the ground and supported by means of the struts 6, 6b and 6d. FIG. 7c is a diagrammatic side view of the greenhouse illustrated in FIG. 3. It will be appreciated that the tension in the longitudinal wires 21 will cause a bending moment around the junction of the curved wire anchorage unit 31 and the ground, which is indicated by the arrow of FIG. 7c. It will be appreciated that the pair of curved wire anchorage units 31, forming equal angles at the corners of each gable end of the structure, the trellis trusses 2 and 2a, the purlins 12a and the longitudinal wires 21 form together a three dimensional rigid structure. Such a structure does not require the use of tension struts which need only be applied to ensure more effective anchorage of the structure on to the ground.

FIG. 8 is a diagrammatic representation of a span structure, larger than those shown in FIGS. 7, 7a, 7b and 7c, illustrating the use of trellis trusses and capable of supporting longitudinal purlins. It will be appreciated that for single span structures of up to 32-35 ft. span, a standard mild steel tubing is of sufficient inherent structural strength for use in construction of the curved anchorage unit 7 and trellis trusses 2, 2a as illustrated in the previous Figures. However, for wider spans of between 40-100 ft. wide the use of single mild steel tubing would not be technically feasible or desirable. Accordingly, in FIG. 8 there is illustrated the substitution of a curved lattice steel anchorage unit 7a. The use of internal trellis trusses 2 and 2a with curved lattice anchorage units 7a is optional and would obviously be only used for the wider spans of greenhouse or other structure.

FIG. 9 shows diagrammatically a structure which has outer trusses 2 but no central truss. There is provided a forced extraction electric fan 40a of conventional construction, housed in a ducting 15. On the opposite side of the structure there is provided a series of inward opening vents 40. In use air from the inside of the structure may be exhausted to atmosphere by operation of the fans 40a, such air being replaced by fresh air through the vents 40.

The construction shown in FIG. 10 is that of a medium span structure showing the location of longitudinal purlins supported by trellis trusses and having natural outwardly opening ridge ventilators of the kind described with reference to FIG. 4 together with similar ventilators 40 on both sides thereof at ground level. FIG. 11 shows the construction of a wide single span greenhouse having a curved lattice steel truss anchorage unit 7a. FIG. 12 is a longitudinal section of the greenhouse illustrated in FIG. 11 while FIGS. 13 and 14 are illustrations of multi-span buildings.

FIG. 15 illustrates one preferred method whereby the flexible roofing membrane 23 may be secured to a longitudinal purlin 11. In a simple construction, the purlin 11 may be manufactured from a rectangular section of pressure preserved and waterproofed timber, two longitudinal portions of which are cut away to provide abutment members 41 for containing the flexible membrane 23 as described hereinafter. Immediately below the co-planar faces 42 of the purlin on opposite sides of the projecting portion 11b of the purlin 11 are provided spaced apart and axially parallel apertures 43 extending transversely through the purlin 11 for accommodating double headed bolts 44 each of which has a wing nut 45 on each end thereof. Between each wing nut 45 and the contiguous side of the purlin 11 is provided a flat steel washer or pressure plate 46 so that, upon tightening of the wing nuts 45, the steel washers 46 cause the abutment members 41 to press into and against the adjacent edges of the purlin 11. The abutment members 41 may be the same length as the purlin 11, or may be cut into shorter sections, fixed independently by any required number of wing nuts 45, for convenience in handling.

As shown in FIG. 15, the flexible membrane 23, on each side of the purlin 11, is fully twisted around the appropriate abutment member 41 and firmly secured against the abutment member 41 by the steel washer 46, the area of contact between the steel washer 46 and the abutment member 41 being substantially airtight and watertight.

The purlin 11 is secured to a vertically disposed truss 2a by a bracket 51 shown separately in perspective in FIG. 16. The bracket 51 is secured to the purlin 11 and to the trellis truss 2a by means of a vertically disposed bolt 52. The bracket 51 has side wings 53 to which curved stabilizing units 14a are secured by bolts 55.

It is necessary, at the apex of the structure, that the exterior surfaces of the purlin 11 (including the abutment members 41) and the stabilizing units 14a should be maintained in the same plane in order to allow the wires 21 and 24 to act firmly against each other and to avoid possible projecting portions of the purlin 11 abutment members 41 and stabilizing units 14a may protrude through the flexible membrane 23. A channel shaped top rail 56 is bolted to the projecting portion 11b of the purlin 11, which rail may be used as a support for a roof service gantry or like machine.

A second layer of roofing membrane 23a may be incorporated in the manner shown diagrammatically with reference to FIG. 17. The layer 23a may serve to reduce the likelihood of failure of the primary roofing membrane 23, particularly at the connection point between a support member and the roofing membrane. The layer 23a is used to form a longitudinal wear resistant strip against the abutment members 41 where a single sheet of roofing membrane only is fixed to the structure. Alternatively, the layer 23a may span the entire roof area to form a second fixed insulating layer which is referred to hereinafter.

It is envisaged that alternative means may be employed to tension the wires supporting the flexible roofing membrane which defines the roof and gable portions of the structure. The simplest and most convenient method is to use a plurality of wires as shown in FIGS. 1 and 2 and to tension each wire individually by providing a turnbuckle on each wire.

In order to provide continuous support to the upper and lower surfaces of the roofing membrane, the invention provides in addition to the two main tensioned wires 21 and 24 described above, a plurality of secondary wires. When the main longitudinal tensioned wires 21 have been fixed in position a lower transverse stabilising wire 24a may be fixed at right angles to the main longitudinal tensioned wire 21 between the longitudinal purlins 12 and 13. (reference FIGS. 18 and 18a).

The function of this layer of transverse stabilising wires is to support the roofing membrane between the parallel lines of the main longitudinal tensioned wires 21 against the contra-pressure of the main transverse tensioned wires 24. This lower transverse stabilizing wire is preferably tensioned sufficiently to prevent sag in the plastic membrane between the main longitudinal tensioned wires 21. After the roofing membrane has been placed on the top of this layer of wires an upper stabilising wire 21a similar to the lower stabilising wire 24a may be laid longitudinally so as to press the roofing membrane down to the main longitudinal tensioned wires 21. To obtain this pressure it is preferable to apply tension to the upper stabilising wires 21a. It will be appreciated that the stabilising wires 21a are directly beneath the transverse tensioning wires 24. The location of the main tensioning wires 21 and 24 in relation to the stabilising wires 21a and 24a and to the sheet plastic membrane 23 fixed between them is shown clearly in FIG. 18. Beneath the plastic membrane 23 is located the first layer of wires, i.e., the main longitudinal wires 21 and the transverse stabilising wires 24a. On top of the plastic membrane is located the second layer of wires in other words the secondary stabilising longitudinal wires 21a and the main transverse wires 24. FIG. 18a illustrates this in cross-section. It should be appreciated that it is not possible to show these four sets of wires in all the figures.

The advantages of this method of securing the roofing membrane in position, namely supporting and securing it between two layers of wires each layer of which comprises a number of longitudinal tensioned wires and a number of transverse tensioned wires so arranged as to bear one against the other will be readily appreciated. It has been shown that in structures of the type illustrated in this Specification that the wind causes a negative pressure or lift on 75 percent to 90 percent of the roof surface. (See : "Wind tunnel studies of pressure distribution on elementary building forms" by Ning Chien, Yin Feng, Hung-ju Wong and Tien-To Siao, published by Iowa Institute of Hydraulic Research, State University of Iowa, Iowa City, U.S.A., 1951). For example a building similar to that illustrated in FIG. 1 of this Specification and having a curvature of 50 feet would be subjected to a maximum lifting force of 600 to 700 lbs lift per foot run of building in a 70 miles per hour gale. Accordingly, a building 300 feet long would be subjected to a maximum lift of over 200,000 lbs. It is for this reason that roof structures of the kind illustrated in the present Specification have heretofore necessitated the use of a roofing membrane which is inherently strong.

While it is readily appreciated that if it were possible to support and secure the roofing membrane within the confines of relatively small frames or supports that it would not be necessary to use a roofing membrane of inherent strength. Referring to FIGS. 18 and 18a it will be appreciated that the sheet plastic membrane 23 is supported and secured along all four sides of a series of rectangles or squares formed by the wires 21, 21a, 24 and 24a. For example when the maximum lift force on a building is 100,000 lbs the maximum lift on a sheet roofing membrane 1 foot square, so secured, would not exceed 40 lbs.

If the force of a wind causes the roof to lift the upper layer of wires will continue to bear down on the lower layer of wires and accordingly the roofing membrane will continue to be secured in position. Any movement of the roofing membrane will entail movement of the whole roof structure including both layers of wires.

Various ingenious solutions to this problem have been proposed for example the placing of a roofing membrane between two nets or mazes of wires. These constructions of roof do not retain and secure the roofing membrane in a series of small squares or rectangles as in the present invention but merely serve to limit excessive movement of the roofing membrane. It is useful to consider the effect of wind loads on such a roof structure. The roofing membrane first lifts and bears against the upper net or maze of wires, the roofing membrane bearing the load across its entire surface, being restrained but not supported by the net or maze of wires. As the force of the wind reduces, the roofing membrane collapses. This intermittent loading quickly causes the roofing membrane, unless of itself inherently strong, to collapse.

The transverse tensioning of wires 24 and similarly wires 24a, which for simplicity are not shown, may be achieved by the use of an individual pulley attached to each wire 24 or by a selected number of pulleys 66 fixed to a longitudinal rigid unit 64 as shown in FIGS. 19 and 20. The wires 24 can be individually attached to the longitudinal unit 64 as shown in FIG. 20 or a single wire may be threaded through a series of small holes 65 in the longitudinal unit 64 as shown in FIG. 19. The pulleys 66 are connected by a continuous wire or cable 62 to a series of pulleys 67 which are suitably attached to the rigid purlin 12 forming a tensioning system as shown in the left hand side of FIGS. 19 and 20. An alternative tensioning system may be formed by the use of a double pulley 68 attached to purlins 12 in place of the single pulley 67, as illustrated in the right hand side of FIGS. 19 and 20.

Tensioning of the longitudinal tensioned wires 21 and 21a may be achieved by the connection of pulleys and tensioning units to the main curved wire anchorage units 7 and the gable base plates 13 (reference FIGS. 1, 3, 4, 5 and 6).

Where ground anchorage for wire tensioning is difficult to attain, a single tensioned wire 21 and hence forming both roof and gable end planes. For this method of construction, it is necessary to have the external trellis trusses 31 erected to form an oblique angle outward from the centre of the structure as described already with reference to FIG. 3.

To improve the heat insulation properties of the construction an alternative construction envisages double glazing the structure. This is achieved by a double layer of roofing membrane 23 and 23a which is fixed to the structure in the same manner as described above. This double layer of roofing membrane forms, when in position, a plurality of sealed envelopes between the apertures formed by the contra tensioned wires 21 and 24, and secondary stabilising wires 21a and 24a a shown in FIG. 18b. Air under pressure is introduced between the layers of roofing membrane. The air pressure forces the two layers apart in each mesh opening forming an insulated roof as illustrated in FIG. 18c. Similarly it is also possible to reverse the process, evacuate the air space and hence ensure that the double layer of membrane operates as a single layer.

It will be appreciated that the disadvantage of using the methods of heat insulation as hereinbefore described is that the light penetration is reduced, during daylight hours, by the extra layer of roofing membrane or other material. Accordingly an alternative embodiment envisages the suspension from the roof of one or more easily removable layers of insulation material.

Referring to FIGS. 21, 22 and 23 there is provided a plurality of longitudinal tensioned steel wires 69, fixed to the bottom portions of the curved stabilizing units 14a said wires 69 stretching the complete length of the structure beneath the layer of contra-tensioned flexible elements namely the longitudinal wires 21 and transverse wires 24 again for simplicity the wires 21a and 24a are not shown. Two layers 70 of insulation material are slidably suspended by means of eyelets 74, of known construction, from the wires 69 one end of each layer 70 being connected to one of the curved stabilising units and other end of each layer 70 being connected to a buffer element 71 slidably mounted on the wires 69. Each buffer element 71 has a front face 75 constructed from a resilient material such as foam rubber. To each buffer element 71 there is connected two control wires 72 and 73 each of which is operatively connected to an electric motor (not shown).

In operation, when it is desired to draw the layers 70 of insulation material beneath the roof structure, the control wires 73 are drawn across the structure by means of the electric motors (not shown) until the facing buffer elements 71 are in contact, the resilient materials of their front faces 75 forming a substantially airtight point. The control wires 72 are used to draw the insulation material in the opposite direction. Referring to FIG. 23 it will be noted that in the closed position the layer of insulation material forms a series of concave sections hence eliminating any longitudinal stress on the insulation material and allowing drops of condensation which may occur as the surrounding atmosphere cools, to fall from the interior of the roofing membrane and run off to the lower edge of the structure where the water may be drawn off by suitable means.

It is envisaged that suitable control means such as a photoelectric call may be utilised to control the electric motors (not shown) used to draw the control wires 72 and 73 across the structure. The insulation material is drawn across the structure when the incident light is below a desired minimum, while the insulation material is removed when the incident light is sufficient for horicultural purposes.

Where "short day" culture is required, as in flower bloom timing the insulation layer may comprise a layer of opaque material which may be drawn across the structure during some of the daylight hours to control the growth of the plants.

Having now described the rigid tensioned structure in detail in connection with the erection of plastic covered greenhouses, it will be appreciated that the same basic structure can be adapted by a choice of covering membranes to suit many other types of building requirments such as agricultural buildings, light factory structures, warehouses and other buildings which may have to be erected in remote areas under difficult operating conditions. In any of the above mentioned adaptations of the basic structure, much stronger stanchions, bars and contra-tensioned cables may be used to carry such roofing material as sheet steel or aluminum, bituminous felt, wire mesh and cement, flexible timber panels or shingles fixed to a wire grid etc.

Claims

I claim:

1. A curved roof structure comprising in combination at least one layer of flexible roofing membrane supported and secured between at least two layers of tensioned flexible elements, each layer comprising a plurality of longitudinal tensioned flexible elements, a plurality of transverse tensioned flexible elements so arranged as to bear against one another, curved anchorage units for the tensioned flexible elements of one of said layers, said units being disposed in respective planes spaced apart and substantially perpendicular to the ground, a plurality of stabilizing purlins disposed between the anchorage units substantially parallel to the ground, a pair of said purlins disposed adjacent the level of the ground, said flexible elements being steel wires, the steel wires of said one layer being connected between the ground purlins, the wires of one layer being substantially perpendicular to the wires of the other layer.

2. A roof structure of claim 1, wherein the roofing membrane consists of at least one plastic material sheet.

3. The roof structure of claim 1, further comprising curved stabilizing units disposed intermedially of the anchorage units.

4. The roof structure of claim 1, further comprising tensioned trellis trusses supporting the anchorage units and extending substantially perpendicularly from the ground.

5. The roof structure of claim 1, further comprising tensioned trellis trusses supporting at least one of the stabilizing purlins and extending substantially perpendicularly from the ground.

6. The roof structure of claim 1, wherein one of said stabilizing purlins is an apex purlin.

7. The roof structure of claim 6, wherein the flexible roofing membrane is detachably secured to the apex purlin.

8. The roof structure of claim 1, further comprising means for tensioning each of said wires.

9. The roof structure of claim 8, wherein said means includes a turnbuckle connected to each wire for individually tensioning each wire.

10. The roof structure of claim 8, wherein said tensioning means comprises a pulley system, each layer consisting of a length of steel wire associated with the pulley system and arranged in grid formation.

11. The roof structure of claim 1, further comprising a slidable sheeting movably mounted under the layers of contra-tensioned elongated flexible elements.