REVIEW PAPER: Review of Structural and Functional ......Tunnel greenhouses and multispan green-houses with arch-shape roof are the most representative types built with plastic "lms

J. agric. Engng Res. (2000) 75, 111}126doi:10.1006/jaer.1999.0512, available online at http://www.idealibrary.com on

REVIEW PAPER

Review of Structural and Functional Characteristics of Greenhouses in EuropeanUnion Countries, Part II: Typical Designs

B. von Elsner1; D. Briassoulis2; D. Waaijenberg3; A. Mistriotis2; Chr. von Zabeltitz1; J. Gratraud4;G. Russo5; R. Suay-Cortes6

1Institute of Horticultural and Agricultural Engineering, University of Hannover, Herrenhaeuser Str. 2, D-30419 Hannover, Germany;e-mail corresponding author: [email protected]

2Department of Agricultural Engineering, Agricultural University of Athens, 75 Iera Odos Street, 11855 Athens, Greece;e-mail: [email protected]

3IMAG-DLO, P.O. Box 43, 6700 AA Wageningen, The Netherlands; e-mail: [email protected], 361, Av. JF. Breton, P.O. Box 5095, 34033 Montpellier, France; e-mail: [email protected]

5Institute of Agricultural Buildings, University of Bari, Via Amendola 165/A, 70126 Bari, Italy; e-mail: [email protected] Engineering Dept, ENSAM University, Placa Viala, 34060 Montpellier, France; e-mail: [email protected]

(Received 3 November 1997, accepted in revised form 6 November 1999)

The most characteristic greenhouse designs used in six European Union countries contributing to this work arepresented in detail. Their advantages and disadvantages are critically described in relationship to the datapresented in the "rst part of this work. In this way the origin of the various greenhouse designs is clari"ed andthe in#uence of climate, availability of construction materials, tradition, and local regulations is analysed. Theoutcome of this work may be utilized to improve the various greenhouse designs dominating the Europeanmarket.

( 2000 Silsoe Research Institute

1. Introduction

Cultivation in greenhouses has steadily expandedthroughout Europe during the recent decade. This kindof production has been proved to be a highly competitiveand pro"table sector of agriculture in most of the Euro-pean countries. In the "rst part of this publication (VonElsner et al., 2000), the local conditions in#uencing thegreenhouse design were investigated. The causes givingrise to the large variety of greenhouse types existing inEurope, built with di!erent covering materials and de-signed to "t the local climate, were presented.

In the present work, the most representative green-house types in six European countries (participating inthe relevant research project) are presented in detail. Thissystematic presentation allows the evaluation of the in-#uence of various factors, such as the climate, the localbuilding regulations, the cultivated crops and the avail-ability of building materials, on the greenhouse design.

Climate is a major factor in#uencing both the struc-tural and the functional characteristics of greenhouses.The design of a greenhouse aims at exploiting the ex-

0021-8634/00/020111#16 $35.00/0 111

ternal climatic conditions for improving the indoormicroclimate. For this reason, the overall greenhousedesign is strongly in#uenced by the climate and thelatitude of the location. Moreover, various load require-ments for greenhouses depend on the climatic conditions.This is re#ected in the national and international stan-dards for greenhouse construction.

It is obvious that there is no straightforward solutionfor designing a greenhouse. Improvements are continu-ous and originate from both experience and scienti"cresearch. All major types of greenhouses have importantfunctional and structural advantages and disadvantages.The choice of a greenhouse type by the grower dependson local technical, legal and economic conditions. Inmany cases, local tradition and considerations of com-patibility with existing greenhouses play an importantrole in the decision making. In particular, local economicconsiderations as well as the open European and interna-tional market strongly in#uence the popularity of certaingreenhouse types.

The mechanical behaviour and the radiometric andphysical properties are important factors taken into

( 2000 Silsoe Research Institute

Fig. 1. Wide-span glasshouse with roof and sidewall ventilation

Fig. 2. Venlo-type glass-covered greenhouse with trellis girders

B. VON ELSNER E¹ A¸ .112

account in the selection of the covering materials. In fact,the large variety of covering materials make the selectionof the cover a very complex technical problem while theoptimal design of the ventilating openings remainsa mostly unresolved problem. Glass has been the tradi-tional greenhouse covering material in Northern Europe.Nevertheless, the use of #exible covering materials whichallows designs with improved structural and functionalcharacteristics such as higher light transmittance, etc.,has increased. Tunnel greenhouses and multispan green-houses with arch-shape roof are the most representativetypes built with plastic "lms.

Most of the available greenhouse designs in Europefollow the corresponding national standards and codes ofpractice. A draft European standard for commercial pro-duction greenhouses prEN13031-1 (prEN 13031-1, 1997),currently under development, represents a "rst attemptfor standardizing the greenhouse design methodology atan European level.

In Section 3, detailed descriptions of the structural andfunctional characteristics of the most representativegreenhouse types currently used in six European Unioncountries are given. This thorough overview providesa clear picture of the present state-of-the-art greenhousetechnology in the most important horticultural countriesin Europe.

2. Greenhouse structures

2.1. ¹he most important greenhouse types

2.1.1. Glass and rigid plastic greenhousesGlass has been the traditional greenhouse covering

material in Northern Europe where greenhouses wereextensively used before the appearance of plastic cover-ing materials. It has important advantages, such as

(1) very low degradation due to environmental causesand agrochemicals;

(2) low thermal radiation transmittance (&greenhouse'e!ect); and

(3) high visible radiation transmittance (light).

Glass or rigid plastic covered greenhouses are charac-terized by long lifetime and are usually well-equippedsystems.

Greenhouses covered with glass or rigid plastic sheetsare categorized into wide-span greenhouses, and Venlo-type greenhouses, named after the Dutch town Venlo,where they "rst appeared. This distinction is based on theroof design. The Venlo-type greenhouse has only oneglass pane placed in glazing bars between gutter andridge. On the contrary, the wide-span greenhouse has

more than one glass pane on the roof between gutter andridge.

Both types are constructed with a metallic (steel oraluminium) frame. The most typical covering material isglass (Briassoulis et al., 1997a, 1997b).

Figure 1 shows a wide-span greenhouse. These green-house types can be built as single span or multispanstructures, where neighbouring spans are connectedalong the gutter. The span width varies between 6 and15 m. They are equipped with continuous ventilators atthe ridge and, if needed, at the sidewalls.

Figure 2 shows a Venlo-type construction with a stan-dard roof width of 3)2 or 4 m. The roof is supported bycolumns, placed under each gutter, or one under everysecond or third gutter. In the second case, the gutter(s)between the columns is (are) supported by a trellis girder.More information about typical designs of Venlo-typegreenhouses is given in Section 3.5.1.

Both types have important advantages and disadvan-tages.

(1) Venlo-type greenhouses are standardized to a largedegree. As a result, their construction and mainten-ance is easier and cheaper. A large variety of equip-ment is available for Venlo-type greenhouses, whilethe design-related physical processes in#uencing theindoor climate has been extensively studied.

113GREENHOUSE DESIGNS IN EUROPEAN UNION COUNTRIES

(2) Wide-span greenhouses o!er larger indoor volumeand better ventilation capacity.

Attempts have been made to replace glass with rigidplastic sheets, such as polymethyl methacrylate(PMMA), polycarbonate (PC), and polyvinyl chloride(PVC), that exhibit higher insulating performance. How-ever, these materials are still too expensive. For thisreason and due to the reduction in light transmittance(when used as a double-layer sheet), they are rarely usedso far (Briassoulis et al., 1997a, and 1997b).

2.1.2. ¸ow plastic tunnelsLow tunnels and walk-in tunnels could be called

miniature greenhouses. Their typical dimensions areshown in Fig. 3. Many types of tunnels, consisting ofa semi-cylindrical supporting framework covered withplastic "lm, have been developed. The supporting frame-work can be made of curved wood, bamboo or steel. Thearches are secured into the ground at a spacing of 2}3 m.The plastic "lm is stretched over the arches and buriedinto the ground along the sides. In addition to the sup-porting arch, a string or wire retains the "lm in place. The"lm can be lifted between the supporting arch and theretaining wire on both sides for ventilation.

Low tunnels are normally built only for one croppingperiod and taken down for tillage. They are used forprotecting plants against low temperature, wind, rainand hail, birds and insects. They are normally unheated.

The main advantages of low tunnels are their low costand the simple construction. On the other hand, theyprovide small heating capability, allow poorly controlledventilation and make plant husbandry di$cult.

Fig. 3. Low tunnel and walk-in tunnel:(a) low tunnel for lowgrowing plants; (b) low tunnel for high growing plants; (c) walk-intunnel; (d) construction of a low tunnel; (e) construction of a walk-in tunnel; (1), plastic xlm; (2), support hoops; (3), retaining cord

or wire; (4), soil anchor

2.1.3. Plastic ,lm greenhousesIn many countries, especially in those with a warm

climate, plastic "lm covered greenhouses are extensivelyused. The "rst plastic "lm covered greenhouses weredeveloped as plastic covers installed over simple woodenframes, which also were used to support the culture. Eventhough plastic "lm greenhouses passed through severalstages of development and important improvementshave been introduced, they still remain cheaper thanglasshouses. Moreover, if their design follows certainspeci"cations, they provide important advantages withrespect to their functionality.

However, several of the existing plastic "lm green-house designs have also disadvantages. Among them

(1) in most cases, too much work is required for install-ing and replacing the "lm;

(2) the "lm stability deteriorates due to ageing caused bysolar radiation and friction against the structure;

(3) poorly "tted "lms #utter against the structure,weakening the resistance of the cover to storms;

(4) inadequate condensation behaviour of the "lm com-bined with horizontal #at segments of the roof cancause falling of drops condensed on the "lm; and

(5) the ventilation is inadequate in multispan green-houses.

A large variety of plastic "lm greenhouses have beendeveloped worldwide. They range from simple structuresof wood with nailed-on plastic "lm and round-archedtunnels to complex constructions. A few characteristictypes are described below (Von Zabeltitz, 1990).

2.1.3.1. ¹unnel greenhouses. The simplest form ofplastic "lm greenhouse is the single-span tunnel green-house. Figure 4 shows the dimensions of two typicaltunnels made of galvanized steel tubes. Here, the width ofthe tunnels is 9 m as an example, the height 3)25 m and

Fig. 4. Round-arched tunnel greenhouse with dimensions as anexample

Fig. 5. Round-arched tunnel greenhouse with continuous sidewallventilation

Fig. 7. Plastic xlm construction with continuous roof ventilation


the length 40}60 m. The space between the arches next tothe gables is 2 m (because of higher wind loads), while theintermediate arches have distances of 2)5 m from eachother. Steel tubes and wires are used to stabilize thestructure in the longitudinal direction. However, usingwires in plastic "lm greenhouses is a disadvantage, sincethe "lm area in contact with the wires can be damaged byfriction (Briassoulis et al., 1997a, 1997b). A space of 2 m isrecommended between two tunnels. The most importantadvantage of the single-span tunnels is the relativelysimple construction system and its good wind resistance.On the other hand, tunnel greenhouses have some func-tional disadvantages (Von Zabeltitz, 1990; Feuilloley etal., 1995). The ventilation is insu$cient when the &sheetparting'mechanism is used (see Fig. 15). However, a roll-up ventilation system in the sidewalls can improve theventilation e$ciency (Fig. 5) (Feuilloley et al., 1995).

2.1.3.2. Plastic pitched roof greenhouses. Di!erenttypes of pitched roof greenhouses with plastic "lm coverare built around the world (Fig. 6). The columns aremade of wood, steel or concrete. In many cases, they aresimple structures. The roof frame is made of wood. The

Fig. 6. Pitched roof construction covered with plastic xlm

plastic "lm often is nailed onto wooden laths, whichmakes the replacement of the "lm di$cult and reducesthe "lm durability (Briassoulis et al., 1997a, 1997b).

2.1.3.3. Arch-shaped roof greenhouses. The frame ofarch-shaped roof greenhouses is made of steel tubes. Theshape of the roof is either a round arch (Fig. 7) ora Gothic arch shape roof (Fig. 8). Gothic arch structureshave advantages over round arches because condensa-tion water can #ow o! better on the inner side of the "lm.Therefore, fewer water drops fall onto the plants. Gothicarches also reduce the snow load because snow can slideeasier down the roof to the ground or the gutter. How-ever, the mechanical resistance of the Gothic arch islower than that of the normal arch due to curvaturediscontinuity.

Roof and/or side openings provide ventilation. Plastic"lm structures have been developed, where one-half ofthe roof can open (Fig. 7 ). Greenhouses with ventilatorsalong the ridge can also be equipped with rolling ventila-tors at the sidewalls (Fig. 9).

2.1.3.4. In-ated, double plastic ,lm, greenhouses. Re-cently, a new type of plastic "lm greenhouses with higher

Fig. 8. Steel tube Gothic arch construction

Fig. 9. Plastic covered greenhouse with ridge and sidewall venti-lation


insulation performance has been developed for use undercold climatic conditions. The cover of these greenhousesconsists of two layers of plastic "lm tightly "xed togetheralong the frame elements of the walls and the roof. Thespace between the "lms is in#ated at a pressure of40}60 Pa. In this way, the insulating performance and theresistance of the structure to wind or snow loads isenhanced.

2.2. Materials for greenhouse construction

Greenhouses are preferably constructed from cheapand easily available materials. Wood, bamboo and steelare usually suitable for the construction of plastic "lmgreenhouses. The frame of glass and rigid plastic green-houses is usually made of aluminium or steel. Foun-dations are usually constructed of concrete. All these ma-

Fig. 10. *Swinging mechanism+ for the operation o

terials have advantages and disadvantages. For example,wood can swell and shrink due to humidity. Encouragedby humidity, fungi and insects can infest the wood, caus-ing rotting and decay. Therefore, wood has to be pro-tected for longer usage. In addition, it has to be protectedfrom termites.

The choice of the construction material depends on thelocal prices for timber and steel. For the time being, woodconstruction is cheaper in many countries. Wood used ingreenhouse frames is (or should be) protected by impreg-nation with protective substances, namely synthetic ornatural resins or chemicals, such as zinc chloride, coppersulphate or borax. Toxic substances should be avoidedwhenever possible. Steel has to be protected from cor-rosion. Nowadays, steel is usually galvanized with zinc,but zinc drained into the irrigation water may causeimportant environmental pollution problems. Alumi-nium does not need anticorrosive protection.

2.3. <entilators in greenhouses

2.3.1. <enlo-type greenhousesThe Venlo-type greenhouse was designed so that it can

easily be built with standard glass panes (Fig. 2). Theventilators are not continuous. The windows usuallyconsist of two or three half-glass panes or, more rarely, ofone whole glass pane. They are placed at both sides of theroof near the ridge (Fig. 10) and can open at a maximumangle of 443. For operating the windows a &swing mecha-nism' (Fig. 10) or a &truss rail mechanism' (Fig. 11) is used(Waaijenberg, 1995). The latter has the advantage ofbeing situated above the trellis (truss) girder so that itdoes not increase the total shadow due to extra frameelements.

f ventilation windows in a Venlo-type greenhouse

Fig. 11. *Truss rail mechanism+ for operation of ventilation windows in a Venlo-type greenhouse on top of the trellis girders


2.3.2. =ide-span greenhousesWide-span glasshouses used to be the most common

greenhouse type in Northern Europe before the introduc-tion of the Venlo type.

Their ventilators are continuous windows extendingalong the longitudinal axis of the greenhouse. The roofopenings usually are placed near the ridge (Fig. 1), ormore rarely near the gutter. Also, continuous side venti-lators exist along the sidewalls of these greenhouses.Generally, wide-span plastic greenhouses are not stand-ardized as a product (e.g. shape and dimensions are not

Fig. 12. Typical operation mechanism for continuous ventilators us

standardized). Therefore, each designer makes his ownvariations for the size and the location of the ventilators.The maximum opening ratio varies depending on theclimate in which the greenhouse operates and the cultiva-tion for which it is used.

The operating mechanism for the windows depends onthe covering material in use. A typical operating mecha-nism for the roof ventilators consists of a turning shaftwith gears translating toothed rods on a rack and pinionprinciple (Fig. 12). Similar operating mechanisms areused for the side ventilators.

ing a rack and pinion device to convert rotary into linear movement


2.3.3. Multispan plastic greenhouses witharch-shaped roof

These are semi-rigid plastic or plastic "lm coveredgreenhouses with metallic frames. This type of green-house is widely used in Southern European countries.For this reason, it is usually equipped with large ventila-tors.

The usual roof ventilators consist of continuous longi-tudinal segments of the roof moving outwards. In somedesigns, half of the roof can pivot around the ridge(Fig. 7). In other cases, the roof ventilators are placed justabove the gutter. These design variations are the result ofstructural considerations, while the functionality withrespect to ventilation is more or less the same. However,maintaining large opening ratios is desirable even thoughnot necessary, for obtaining high ventilation e$ciency.Similar large opening ratios can be obtained in "lm-covered greenhouses by rolling or folding the roof cover(Waaijenberg, 1995).

Fig. 13. Rolling-down side ventil

The side ventilators usually are gaps in the sidewallscreated by rolling up or down the plastic "lm (Fig. 13).Special care is taken in designing the rolling mechanismso that the "lm remains properly stretched during opera-tion in order to avoid tearing by #apping. Moreover theoperation mechanism should close the ventilators tightlywhen it is needed.

2.3.4. ¹unnel greenhousesThese are plastic "lm covered structures with metallic

frames. The ventilators are usually continuous side open-ings operated by a rolling mechanism (Fig. 5). However,more rarely, windows opening outwards are used. Ina few recent designs, in#ated plastic tubes control theventilation through the side openings.

In addition, opening the greenhouse doors at thegables usually enhances ventilation. In a few designs,a &swing half-moon' ventilator is added at the upper partof the gable (Fig. 14).

ator in an arch-type greenhouse

Fig. 14. *Swinging half-moon+ ventilator in the front gable ofa tunnel greenhouse


In the case of simple tunnels where the "lm is notattached to the frame, but stretched over it, ventilatorsexist only at the gables. However, it is possible to haveside ventilators also in this type of greenhouses by usingthe &sheet parting technique' (Fig. 15).

2.3.5. Plastic pitched roof greenhousesThese are usually simple wooden structures covered

with plastic "lm. They are poorly equipped, so theygenerally lack mechanically operated ventilators. Never-theless, the main design characteristics of the ventilatorsare similar to those of the commercial greenhouse typesdescribed above.

Hand-made greenhouses are usually wide-span struc-tures with one or two spans. The side ventilators aremore common since they are easier to construct andoperate. Plastic "lms, which are manually rolled orfolded like curtains, close these side ventilators.

Roof ventilators are generally avoided since theyweaken the roof cover. However, the sheet parting tech-nique can be used for creating temporary ventilators.

When the greenhouses are operated in hot countries,parts of the cover are permanently removed during thesummer for increasing the ventilation.

3. Analysis of the structures with respect to the designrequirements and the adjustment to local in6uences

3.1. Greenhouse cultivation in France

Of the total greenhouse area of approximately10 000 ha, two-thirds is used for vegetables and one-thirdfor ornamentals (#owers, potplants) (Briassoulis et al.,1997a, 1997b). The area covered by plastic greenhouses ismostly concentrated in the south of France.

The standardization of greenhouses in France hasbeen stimulated by insurance companies, which are in-terested in avoiding extensive damage. All insuredgreenhouses must be designed in accordance with the co-rresponding standards. Compliance of the industriallyproduced greenhouses with the standards is certi"ed byspecialized agencies supervised by the government, and isenforced through the loaning services of banks which"nance only insured greenhouse structures.

3.1.1. Most common greenhouse types in France3.1.1.1. Glass greenhouses. More than 95% of the glass-houses are covered with single glass panes (drawn sheetand tempered glass). Glasshouses for vegetables aremostly of the Venlo type, while glasshouses for #owerproduction are usually wide-span structures with pitchedroof, 9)6 or 12)6 m wide.

3.1.1.2. Plastic multispan greenhouses. Multispan plasticcovered greenhouses in France usually have arch-shapedroofs. Before 1990, the span width of plastic multispangreenhouses was 6)4 m. Recently, the span width in-creased to 8 or 9)6 m. The height from gutter to groundvaries between 3 and 4 m. Roofs are mostly covered byplastic "lm (more than 95%), and more rarely (5%) bysemi-rigid plastic sheets. The walls are usually covered bysemi-rigid plastics (PVC mainly, and Glass ReinforcedPolyester). During recent years, the use of plastic multi-span arch-shape roof greenhouses (Figs 7 and 8 ) ex-panded and they gradually replace the older tunnelgreenhouses.

3.1.1.3. ¹unnel greenhouses covered by plastic ,lm. Mostof the tunnel greenhouses are covered by polyethylene(PE) "lm 120}200 km thickness with a life expectancy of2}3 yr. In France, the tunnel greenhouses can be categor-ized into two characteristic types.

Small tunnels of 4}5 m width. The arches are made ofsteel pipes. They are "xed into the ground with metallicanchors (usually of the screw type). The "lm is placedlengthways and "xed on the arches with strings. Rollingup the "lm along the greenhouse sides ventilates thetunnel. It is possible to connect frames of such tunnelsand form multispan structures.

Fig. 15. *Sheet parting technique+ ventilator


¸arge tunnels of 7}10 m width. The most commonwidth of large tunnels is 8 m. The arches are made of steeltubes. The "lm is placed transversely onto the frame and"xed to the ground by burying its edges into the soil. Thetunnel is usually ventilated by a &sheet parting' mecha-nism (Fig. 15).

Recently, a new tunnel type with columns appeared inthe market. The important advantage of this structure isthat it allows several arches to join together and forma multi-tunnel (or multispan) greenhouse (Fig. 16).Foundations are necessary for this greenhouse type.Ventilation is obtained either by rolling up the "lmat the sidewalls or through ventilation windows in theroof.

3.2. Greenhouse cultivation in Germany

Greenhouse cultivation expands all over Germany.More than 80% of the greenhouses in Germany areglass-covered greenhouses. During the last decadeVenlo-type glasshouses became more and more popularamongst the German growers as a result of the relativelyhigh prizes of the wide-span glasshouses. Plastic "lmgreenhouses are mainly used for unheated production ofvegetables, summer #owers, shrubs and tree nurseries.

3.2.1. ¹he German standard greenhouseThe Glasshouse Manufacturer Association, the

Growers Association, research institutes and planningauthorities stimulated the development of a standardisedgreenhouse design.

The product standard DIN 11536 &Greenhouse in steelconstruction, galvanized, 12 m nominal width' (DIN11536, 1974) was "rst created in 1971 and speci"es thefollowing characteristics (Von Zabeltitz, 1986):

(a) length (3)065 m) of a unit element, which is the distancebetween two consecutive lines of columns, the width ofa 6, 9, 12 or 15 m greenhouse (only the 12 m type isstandardized) being a multiple of this unit length;

(b) vertical sidewalls 2)3 or 2)8 m high;(c) pitch slope with a ratio of 1 : 2 (26)53) to guarantee

proper slippage of snow, an e$cient indoor climatecontrol with e$cient ventilation, high light transmit-tance and enough space for the installation of shadingsystems or other equipment;

(d) dimensions of the glass panes 0)60]1)74 m, this sizebeing easier to handle during repair works than the2 m pane;

(e) the gutters at both sides of the greenhouse formingstructural elements and allowing for the expansion ofthe greenhouse to a multispan block;

Fig. 16. Multitunnel xlm-covered greenhouse in France


(f ) the ventilation opening at the ridge with a width of1)74 m and opening up to 153 over the horizontalplane; and

(g) the design being certi"ed, without the need for anadditional recalculation by a structural engineer.

This standard had been revised and extended in 1994to a new code: DIN 11535-2 &Greenhouses; steel andaluminium construction 12)8 m in width with a longitu-dinal grid dimension of 3)065 m' (DIN 11535-2, 1994).The main advances in technology were incorporated. Itincludes two types of greenhouses with 24 and 26)53 pitchangle of the roof and takes into account the possibility ofusing aluminium as the frame material. The solar radi-ation transmittance of the greenhouse cover shouldexceed 93% for a period of 10 yrs.

Nevertheless, many German growers favour the Venlo-type glasshouses, because they are lighter and cheaper.

3.3. Greenhouse cultivation in Greece

The "rst greenhouses in Greece appeared in1955}1956. However, the rapid expansion of the coveredagricultural area began after 1961, following the intro-duction of the plastic "lms as a covering material. Today,greenhouse cultivation is one of the most signi"cant anddynamically developing sectors of the Greek agriculture.According to the statistics of the Ministry of Agricultureand the Agricultural Bank of Greece, between 1967 and1994 the area of greenhouses increased from 269 to4200 ha (Von Elsner et al., 2000).

Greenhouse types in Greece can be generally classi"edas

(a) &commercially built' greenhouses (i.e. greenhouses de-signed and constructed in accordance with speci"ca-tions set by the industries following national or inter-national standards); and

(b) &grower-built' greenhouses (i.e. greenhouses made bythe farmers themselves; usually the design and con-struction of these structures is empirical).

The most common structural materials used forgreenhouses are wood, steel and aluminium. All thesematerials are used separately or in combinations. Steelis the dominant structural material in the category ofthe commercially built greenhouses, whereas wood ispreferred as a construction material for the grower-built greenhouses. The covering materials for the&&commercially built'' greenhouses are glass or plasticwhereas the covering material for the grower-builtgreenhouses is almost exclusively plastic "lm. In general,PE "lm is the predominant covering material for allcategories (Briassoulis et al., 1997a, 1997b; Tsirogiannis,1996).

The area of plastic "lm greenhouses increases continu-ously, while the area of glasshouses remains almoststable. The majority of greenhouses (60% in 1994) haveno heating equipment, whereas only a small percentageof them (13% in 1994) have a heating system su$cientlysatisfying their needs.

Most of the greenhouses made in Greece are used forthe cultivation of vegetables (92%). The majority of thegreenhouses (57% in 1994) are grower-built structures(mainly wooden structures with plastic "lm cover). Thesesimple structures have a lot of disadvantages and defectssuch as lack of functionality, insu$cient ventilation, lowresistance against loads, etc.


3.3.1. Most important greenhouse types in GreeceIn the past, when grower-built structures represented

the large majority of greenhouses in Greece, the maindesigns were the &Ierapetra' type and the &Macedonian'type with pitched roofs and the &Filiatra' type with arch-shaped roofs. These types or their variations have alsobeen used as a model for the design of the Greek &com-mercially built' greenhouses. Today, the most commongrower-built and commercially built greenhouse types inGreece*which are constructed either in a single ora multispan form } are the tunnel type, the arch-shapedroof type with vertical sidewalls and the pitched rooftype. Table 1 shows the typical suggested dimensions ofthe basic structural units of the di!erent greenhousetypes in Greece (Gra"adelis, 1980; Agricultural Bank ofGreece, 1986; Castilla, 1994). Comparing the designspromoted by the greenhouse industries with the grower-built ones, the heights of the ridge and the gutter arelower, the slope of the roof varies only between 15 and203 (Fig. 6) at the typically grower-built greenhouses(Tsirogiannis, 1996; Gra"adelis, 1980), whereas the com-mercially built types are equipped with larger roof andside ventilators.

3.4. Greenhouse cultivation in Italy

Greenhouses are widespread all over Italy witha greater concentration (about 60% of the total) in south-ern regions, where the so-called Mediterranean green-house has been established (Castilla, 1994; Stanghellini,1994; Tesi, 1991). It consists of low-cost structures,with plastic "lm covering and no heating equipment.&Cold greenhouses' cover approximately 70% of thetotal protected area, whereas the remaining 30% ofthe protected area is equipped with indoor micro-climatic control systems (Tesi, 1991; Scarascia-Mugnozza, 1995).

TablCharacteristics of the basic structural unit of the most common gre

Speci5cations: (Agricultura

Dimension Tunnel

Commercial Grower-built

Derived from traditional type * *

Min. height of the ridge, m 3 3Min. height at the gutter, m * *

Slope of the roof, deg.Minimum width per span, m 7 7Minimum side opening ratio, % 7 10Minimum roof opening ratio, % 18 10

3.4.1. Greenhouse structures in ItalyIn Italy, 90% of the protected area is covered with

plastic material. Only 10% of the total greenhouse areacorrespond to glass-covered structures. Approximately87% of the greenhouse area is used for vegetables and13% for #owers or potplants.

Two major tendencies are currently observed in theItalian greenhouse market.

(1) The use of low technological input greenhousessteadily expands. They are simple structures used forvegetables and low-temperature #ower crops. Theyare designed to require low investment and low oper-ating costs and to exhibit fairly good energy e$cien-cy. They usually consist of a pitched roof structure(Fig. 6) built of lightweight steel, timber or concrete,covered with plastic "lms, usually without heatingsystem or rarely with simple supplementary heatingequipment (Tesi, 1991; Scarascia-Mugnozza, 1995).In Sicily, greenhouses are usually built of woodand/or concrete, covered by single- or double-layerplastic "lm. Ventilation is obtained by rolling orfolding the "lm upward along the sidewalls (e.g.Fig. 13).

(2) Improved modern greenhouses with high technolo-gical input used for #ower growing, ornamentalplants and nurseries are also developed. Their frameis usually made of steel and they are equipped withcontrol systems for a more e$cient regulation of theindoor microclimatic parameters, aiming at high-quality products, low-energy consumption and re-duced labour cost.The most common greenhouse types of this categoryare either multispan arch-shaped roof structurescovered with plastic "lm (Fig. 7), or pitched roofframes covered with glass or rigid plastic (Fig. 1). Thiskind of greenhouses are ventilated with openingssituated in the sidewalls in the case of only a few

e 1enhouse types in Greece according to the 9Technical Greenhousel Bank of Greece, 1986)

Arch-shaped roof Pitched roof

Commercial Grower-built Commercial Grower-built

&Filiatra' &Ierapetra' &Macedonia'3)5 3)1 3)5 3)12)6 2)2 2)6 2)2

20}305 5 5 57 10 7 10

18 10 18 10


spans (less than three) and by means of additionalopenings in the roof in the case of more spans.

3.5. Greenhouse cultivation in ¹he Netherlands

The Netherlands is the European country with thelongest tradition in the use of greenhouses for extendedcommercial production of vegetables and #owers. TheDutch greenhouse is a highly sophisticated unit wellequipped which allow optimal conditions for the growthand the production of plants. The construction of sucha greenhouse requires a relatively high initial investment,which aims at maximum production.

The covered greenhouse area in The Netherlands isa little more than 10 000 ha (in 1997) of which 93% isheated (LEI-DLO and CBS, 1998). An area of 4250 ha isused for the cultivation of vegetables and fruit and5800 ha for ornamentals (#owers, potting plants, shrubs,etc). The most important concentration of greenhouses islocated in the west part of the country in the so-calledZuid-Hollands glasdistrict (Westland): here 4800 ha issituated. The two provinces Noord- en Zuid-Holland(North- and South-Holland) together count 6900 ha(68% of the total area).

Glass is the most commonly used covering material forgreenhouses in The Netherlands due to its high lighttransmittance, its long lifetime and the local greenhousebuilding tradition and the low costs of standard glasspanes. About 97}98% of the total greenhouse area iscovered with single glass (Von Elsner et al., 2000).

In the Netherlands where labour costs are high, thedurability and the easy maintenance of the greenhouse isimportant. The greenhouse frame is usually made ofgalvanized steel or aluminium and it is covered withglass.

3.5.1. Greenhouse structures in ¹he NetherlandsThe glasshouses can be divided into two categories: the

wide-span houses and the Venlo-type houses. About85}90% of the newly built greenhouses in the Nether-lands are of the Venlo type (Waaijenberg, 1992).

A wide-span greenhouse is a conventional construc-tion with steel or aluminium purlins attached to steeltrusses. These purlins together with the steel or alumi-nium gutter support the glazing bars, on which the glassis placed. The span width of the wide-span house isstandardized on a multiple of 0)8 m, and thus may be 6)4,8, 9)6 or 12)8 m. Its disadvantage is the higher investmentcost compared to a Venlo-type greenhouse.

The Venlo-type greenhouse, is the most popular green-house type in the Netherlands. It consists of a structurecovered by multiple small-pitched roofs of 3)2 or 4 mspan. They are built in continuous big blocks covering

a total ground area of 1}2 ha per grower (average area ofnewly built greenhouses).

In order to achieve a su$ciently high crop productionlevel, even during periods of the year when little light isavailable, greenhouses were developed with a better lighttransmittance. A modern Dutch greenhouse has manynew improved features such as smaller glazing bars andgutter pro"les (steel and aluminium), increased trussspacing and integration of installation and structuralparts (Waaijenberg, 1992).

Plastic "lm covered greenhouses correspond only to1}2% of the total greenhouse area. The most commontypes of plastic-covered greenhouses built in the Nether-lands are the tunnel and the "lm-covered multispangreenhouse with arch-shaped roof. To improve the qual-ity of plastic "lm greenhouses, an initiative has beentaken to develop a special construction guideline for thiscategory (Waaijenberg, 1997). It establishes rules for thecalculation of loads in plastic greenhouses, tunnels, shad-ing halls, etc., and provides the strength of materials(steel, aluminium, concrete and plastic). It also o!erspractical advises for the design of the foundation, thestructure and the cladding of this greenhouse type.

The Venlo-type greenhouse is developed due to theavailability of standard glass panes with traditional sizesof 0)73 by 1)65 m for the roof. The standard span width is3)2 m, consisting of two glass panes of dimensions 1 m by1)65 m or 1)125 m by 1)65 m joined at the ridge (Fig. 2).Extra trusses do not support the ridge, since the glasspanes and the glazing bars are self-supporting. Duringthe recent years, there is a change in the glass panedimensions. New glass panes of 0)80 m by 2)08 m or 1 mby 2)08 m, allow the span width to increase to 4 m. Trellisgirders (trusses) are used to support the roof and guttersand the service loads inside caused by equipment, trans-port systems and crops (Fig. 2). Their lengths are, respec-tively, 6)4 m (2 times 3)2 m) and 8 m (2 times 4 m). Thecentre-to-centre distance of the columns in the directionparallel to the gutter is 4 m or 4)5 m (Waaijenberg, 1992).

The foundation of a greenhouse column mostly con-sists of a pre-fabricated concrete pole cast in a concreteblock at a certain depth below the surface. The dimen-sions of these concrete blocks depend on a number ofparameters, such as the greenhouse height, the number ofconnected spans, the number of stability bracings, thesoil type, etc.

A few years ago, attempts have been made for develop-ing greenhouses with insulating covering materials, suchas double glass, synthetic double-web sheets or coatedglass with low emissivity. Similar structures with highthermal insulation were also developed by applyinga second glass layer to an existing single cladding.Although these designs clearly improved the insulatingperformance, they also reduced the light transmittance


while the investment costs were often too high to beeconomically attractive for the grower. Now single glasswith a thickness of 4 mm is the most popular coveringmaterial for greenhouses. Only for the gables and side-walls a system with two parallel glass panes of 4 mmsupported by glazing bars with double grooves is used.

In Dutch horticulture about 70% of the total glass-house area is equipped with a type of "xed or movablescreen for energy saving, shading or blackout. Thescreens can be used for several functions and contributeto a more e$cient control of the indoor greenhouseclimate. In many cases when the screens have to combinedi!erent requirements, they are constructed as a doublescreen.

3.6. Greenhouse cultivation in Spain

Greenhouses in Spain are spread along the Mediterra-nean coast. With 28 000 ha of covered area, Spain is theleading country in the use of plastic "lm covered green-houses in the Mediterranean basin and the EuropeanUnion (Von Elsner et al., 2000). The densest concentra-tion of plastic "lm greenhouses is located in the southernregion of Almeria in the south of Spain.

3.6.1. Parral-type of greenhouse structure in SpainThe so-called &Parral-type' is the plastic "lm green-

house most frequently met in the Almeria region. Theselow-cost structures are used to grow vegetables fromSeptember throughout June.

The basic structure of the Parral-type is made ofwooden posts placed vertically. Their upper ends areconnected together by continuous tension wires. Out-ward facing buttresses give stability to the whole con-struction (Fig. 17 ) (PeH rez Parra, 1992).

Fig. 17. Typical structure of the *Parra

The tension wires support two galvanized steel wirenets, which hold the "lm tight between them (&sandwich'structure). In this way, the stability of the cover is ensuredin a region where wind speed is quite high.

Experienced technicians are required to erect this typeof structures. The nets and the "lm must be stretchedcorrectly and carefully, otherwise the "lms may #utter inthe wind. Wind is the most important factor of theAlmeria climate. Parral greenhouses have #at or slightlypitched roof (less than 103). In many cases, the ridge axishas an east}west orientation to resist the prevailingwinds. The deposition of dust is intense and frequent inthe area and results in reducing the solar radiation trans-mittance of the cover. Moreover, in winter, the angle ofincidence of solar radiation on the horizontal roof islarge. Therefore, the re#ectance of the cover is high. TheParral-type greenhouse is poorly designed with respect tooptimal control of the indoor climate. During the coldestmonths, average outdoor minimum temperatures arebetween 7 and 93C. Under these conditions, plant growthis retarded due to the lack of a heating system. On theother hand, crops can also su!er from excessive heat.Under such circumstances, the poorly designed naturalventilation openings of the Parral-type greenhouse arenot su$cient to lower the indoor temperature to thedesirable level. Therefore, more e$cient natural ventila-tion systems are required to improve growing conditions.Summer months are not suitable for greenhouse cultiva-tion in the area.

4. Conclusions

The present review describes the current state-of-the-art with respect to various greenhouse designs prevailingin six European Union (EU) countries where 90% of the

l-type+ greenhouse in Almeria (Spain)


total EU greenhouse-covered area is located. Eventhough, important protected cultivation activity alsoexist in other EU countries (e.g. UK and Denmark), itwas not possible to include any speci"c designs of thosecountries here due to lack of the corresponding informa-tion. However, the scope of this work is not only todescribe systematically the most important types ofgreenhouse structures used in several European countriesbut also to explain the causes, which have led to thedevelopment of such a large variety of greenhousedesigns as well as the current trends. In this way, it con-tributes to a further clari"cation of the design require-ments for greenhouses as well as promotes technologytransfer between the EU countries. Summarizing thispresentation, the main factors in#uencing the greenhousedesign requirements are stressed.

4.1. In-uence of climate

Climate is the main factor in#uencing the greenhousedesign. The optimization of a greenhouse structure withrespect to local climatic conditions still remains a chal-lenge for the designer not only from the technological butalso from the "nancial point of view. The large diversityin climatic conditions between the regions of the Euro-pean Union makes it impossible for a manufacturer too!er greenhouses suitable for all kinds of climate. Forthis reason, local markets have been developed wherelocally designed structures became popular (e.g. Almeriain Spain, The Netherlands, Crete in Greece). Each ofthese designs aims at solving speci"c problems related tothe local climate. For example, high solar radiationtransmittance is a major requirement in particular forgreenhouses in the North. For this reason designs withhighly translucent cover and small structural elementswere developed.

Related to climate are also the "nancial parameterssuch as the energy prices. Gas, electricity and petrolprices in#uence the greenhouse design. For example, ifthe cladding area increases by increasing the pitch angle,the solar radiation transmittance improves but the insu-lation performance of the cover decreases.

Climate variations impose important barriers for theinternational or even the inter-European trade of green-houses. Greenhouse builders must have the knowledgefor developing suitable structures for their exportcustomers.

4.2. In-uence of available materials

The selection of the building and covering materials isbased on both technical and economic requirements. The

low price of standard glass panes in the North Europeancountries combined with its high solar radiation trans-mittance, its high infrared absorption and its durabilityto environmental factors have made glass the most fa-vourable covering material in the North of Europe. Des-pite the recent developments in the production of rigidplastic sheets with functional and structural properties ofhigher quality, glass remains popular.

In the early-industrialized Northern European coun-tries the use of steel for building the greenhouse framewas a natural development. Moreover, steel structuresallow smaller volume of the structural elements*castinga smaller shadow*than wooden frames.

In the Southern European countries, plastic "lms arethe most common covering materials for greenhouses.The plastic "lm is a relatively cheap product, which caneasily "t to any greenhouse frame while it can o!er goodfunctional properties such as high solar radiation trans-mittance and good insulation performance. The southernEuropean growers have no trouble to use it as a cover onany kind of simple structure.

Although steel is nowadays quite common as a framematerial for the southern European greenhouses, wood isalso extensively used since possible losses of light are notconsidered as "rst priority. Wood o!ers higher #exibilityin the greenhouse structure and the grower can intervenewhen it is necessary to adapt the frame to speci"c localconditions (grower-built greenhouses in Greece and Italy).

4.3. In-uence of tradition

Tradition is also a factor in#uencing the developmentsin greenhouse technology. Successful designs of the pastusually remain in use for a long time even after newimproved products appear in the market. The lack ofcompatibility between old and new products is usuallythe main reason that growers prefer older techniques.Moreover, the experience and the skill acquired throughusing speci"c techniques make it more di$cult to advo-cate drastic changes. In most cases, greenhouse growersstarted their production by using simple cheap construc-tions. When they were successful, they invested theirpro"ts in building better-equipped, more sophisticatedgreenhouses. However, a level of compatibility betweenthe modern and the old greenhouses was preserved. Inthis way, the in#uence of local tradition generated speci-"c trends in the European greenhouse technology.

4.4. Standardization versus adaptability

Designing economic and e$cient greenhouses requiresa successful exploitation of experience, materials and


equipment, which are available locally. The availabilityof the necessary structural materials and accessories ina standardised form can strongly support the deve-lopment of e$cient greenhouse designs. In return, thestandardization of important functional and structuralfeatures of the greenhouse can lead to the production ofcheaper and more e$cient greenhouse components.

However, a degree of adaptability is required whenvery diverse climatic and socio-economic conditions aretaken into account. In several cases, however, the incom-patibility of the structural components undermines theintended target for a cheaper greenhouse by increasingthe building costs. Moreover, the poor co-operation be-tween industries that are producing the main structuralcomponents for greenhouses, makes it di$cult to realizethe optimization of the design in an e$cient way. In thatrespect standardization is required where this is accept-able by the market.

4.5. Greenhouse design and standardization

In all European countries, horticulture under cover isa pro"table branch of agriculture involving considerableindustrial activities. Despite the large number of innova-tions applied in the present day greenhouses, no designcan be considered the perfect unique solution. Green-house construction must meet local needs and climaticconditions. For this reason, the standardization of green-houses at a European level involves di$cult technical,economic, social and legal problems. However, a certaindegree of European harmonization is required in order tofacilitate the trade of greenhouses and greenhouse com-ponents among the EU countries.

At this moment, the Eurocodes for design of conven-tional building structures in Europe represent a commonmethodology for designing compatible greenhouse struc-tures in Europe. In addition, a recent e!ort for the prep-aration of a draft European standard for greenhousedesign (prEN 13031-1, 1997) attempts to unify the loadcalculation methods and the designing methodology inall EU countries. The general methodology provided inthis standard follows the methodology of the correspond-ing Eurocodes in a complementary way. Consequently, itcan be adapted to regional conditions by the appropriatesafety or combination coe$cients in accordance withexisting national standards.

The e!ort for the development of a European standardfor greenhouse design, is the starting point for the devel-opment of a uni"ed approach with respect to construc-tion regulations in greenhouses all over Europe. This,along with the implementation of the various relevantEurocodes and European Standards for materials, is ex-pected to contribute to the integration of the correspond-

ing European market. The review contributes to thisuni"cation process by presenting the most successfulgreenhouse types used in EU countries and explainingthe reasons of their success.

5. Acknowledgements

This work is partially funded by the EU Human Capitaland Mobility Programme (Contract ERBCHRXCT930384). One of the authors, A. Mistriotis is fully sup-ported by the EU-TMR Individual Fellowship Pro-gramme (Contract ERBFMBICT960972). We wouldalso like to thank Prof G.P.A. Bot, Prof G. Scarascia-Mugnozza, Dr G. Papadakis, Ing. P. Feuilloley, and Ing.A. Molina-Llorca for supporting this work by providingimportant data.

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Documents

REVIEW PAPER: Review of Structural and Functional ......Tunnel greenhouses and multispan green-houses with arch-shape roof are the most representative types built with plastic "lms