A. A STUDY OF GREENHOUSE FOUNDATIONS - NEW SYSTEMS

Ing. J.C. Spek, arch. I.M.A.G. Wageningen Netherlands

The Dutch standard for greenhouses NEN 3859 specifies that all verti-cal and horizontal forces acting on the greenhouse must be transferred to the ground via the foundations. For example, with greenhouses having 6.4 m lattice girders (2 bays)* this is done by drilling a hole into the ground and pouring into it an in-situ cast concrete base block 400 mm below ground level. This then combines with the undisturbed soil: shear stresses and skin friction are also found to be present in tests and thus can be taken into account. A prefabricated reinforced concrete pile is placed into the concrete base and extends about 300 mm above ground. The pile is 120 x 120x 1000 mm long. The stanchion of the house is then attached to the head of the pile to resist displacement and uplift. The attachment must act as a hinge joint. The reinforcement consists of four bars.

On the basis of information from soil mechanics and practical tests, the size of the base blocks has been calculated with the aid of a com-puter program for three types of soil and four different widths of the greenhouses, having variable numbers of connected spans, different eave heights and three types of piles (gable pile, pile for the bracing bars, central pile). 0.5 kgf cm2 is chosen as the permissible earth pressure for sustained loads.

In 1982 a large number of concrete piles of the foundation type, es-pecially those from the 1976-79 period, were found to be attacked by sulphates. This applies to something of the order of 80 000 - 200 000 piles. At a size of compartment of 3x6.4 each pile supports 19.2 m2 of greenhouse. This means that it involves about 1 600 000 - 4 000 000 m2 of greenhouses out of a total area of 9 000 ha = 90 000 000 m2.

This sulphate attack, which leads to disintegration of the concrete, starts in the core of the pile and causes swelling (figure la). This swelling first becomes visible in the form of small cracks on the sur-face of the concrete pile above ground, but is also audible when the pile is knocked gently. A pile thus attacked loses all connection over a considerable stretch (figure lb) and is, therefore, no longer able to function properly under load, particularly in the presence of gusts of gales.

In August 1984 study has started on:

- the causes of such attacks,-- a traditional composition of concrete that would resist such attacks over a period of at least 15 years;

- the prior application of coatings; - possible remedial treatment.

A report on these problems can be expected in October, after which investigations are likely to follow.

*This type of greenhouse is increasingly becoming the "all-purpose" one of the Dutch glasshouse industry Acta Horticulturae 170, 1985 Greenhouse Construction, Materials

11

It is well known that:

- when soil samples are taken from the greenhouse (i.e. instantaneous sampling) the soil must be regarded as presenting an aggressive environment for traditional concrete;

- as a result of strong evaporation in the greenhouse a continuous pro-cess is initiated on the concrete surface in the stabilizing block outside the damp soil, which is far more hazardous than would be suspected from the instantaneous sample;

- that this process is promoted by the high temperature and ventilation in the greenhouse;

- that no corrosion of the reinforcement has been observed with piles examined so far; in some cases there was little covering (10 - 15 mm);

- that sulphate attacks are encountered with Portland and blast-furnace slag cement.

At present, there are some builders thinking about repairing propo-sals (figure 4). Others are thinking mainly of replacement solutions.

Of these, one person is thinking of a solution involving monolithic construction (everything in one piece), while another is talking about a combination of a prefabricated pile with a prefabricated base. Neither system has so far been tested theoretically with respect to its strength and durability, nor verified by tests on prototypes. Impediments to this are: the durability of the concrete composition since even blast-furnace cement may be attacked by sulphate; the soil mechanics aspect because the pile is partly placed into completely disturbed soil; and above all the extremely uncertain market, which is in some cases influenced by pronouncements and articles in the trade press that are not always technically justified. Moreover, some growers are leaning toward mecha-nical replacement rather than the very moderate durability requirement of 15 years, or towards the strength. The strength is now increasingly becoming accepted as a criterion. In the Netherlands, greenhouses last for 20-25 years.

In addition, I.M.A.G. has developed two entirely different systems which conform well to the greenhouse standards with respect to strength and stability, as well as meeting the requirement for a life of at least 15 years.

Verification on the basis of prototypes has been commissioned for one of the systems. A common feature of both systems is that the pre-sumed continuous flow between the damp soil and the air in the green-house is no longer hazardous with regard to attacks on the piles. The soil mechanics design is based on completely disturbed soil in the top layer of 0.4 m. Apart from these, completely different systems are feasible, but no plan has yet been put forward.

A. With the replacement system "Prefabricated concrete pad with an inert pile fixed therein" (figure 2) (no attack over a period of 15 years) no use can be made of any soil property other than earth pressure. The concrete pad is placed 400 mm below ground level, so as to be able to render the pile as small as possible, as well as to make the construction no more difficult and heavy than necessary.

Having removed the existing pile with its base block, a horizon-tal surface (sand fill) should be provided, on which the concrete pad is placed at the relevant depth. The pile is fitted into the pad and accurately set up. This is followed by backfilling with the soil

12

that had been dug up and with top soil. The computation proceeds as follows:

I Choice of type of greenhouse; II Determination of loads; III Permissible stresses; IV Preservation-treated round timber piles (120 mm diam., Pinus

Sylvestris); V Square-section Azobe heartwood pile (110x 110 mm) ; VI Practical steel pile (100x 100x 3 mm); VII Concrete pad (300 x 600x 140 mm; two 8 mm diam.); VIII Normal stress O of the soil (0.41 kgf/cm2 centric, sustained

during a storm, 0.65 kgf/cm2 for a short time in the presence of gusts of gales);

IX Resistance of pile to uplift.

B. The system "Steel + ground anchor" (figure 3) is based on a specified monolithic combination of a base plate + a hot-galvanized steel pile treated with epoxy tar. This is a complete new system for greenhouse foundations.

1. In order to prevent uplift of the greenhouse, use is made of a hot-galvanized ground anchor, the tie bar of which is stuck through the pile and is fixed by means of the threaded end and a nut on a washer. This pile is given a practical cross-section with reference to the attachment of the steel stanchion of the greenhouse. In the calculation the contribution of the two gable piles is eguated to that of one centre pile.

2. The strengthened steel base plate 300x600 mm is placed 400 mm below ground level on to the sand fill after the ground anchor has been screwed in.

3. The computation proceeds as follows: I Type of greenhouse and loads; II Permissible stresses; III Normal stress O of the soil (0.53 kgf/cm2 centric, sustained,

0.85 kgf/cm2 for a short period during gusts of gales); IV Ground anchor (1 = 1 500 mm; d = 150 mm) ; V Steel pile (100x 100x 3 mm); VI Steel base plate (600 x 300 x 4 mm, ribbed) (or a crash barrier

of the German standard type). 4. The justification of the durability has been mentioned already:

the whole system consists of a hot-galvanized steel part, treated on top with an epoxy tar coating. In the activities of the Rijkswaterstaat (Ministry of Public Works) care is taken to en-sure that a life of 25 years is obtained in soil. The require-ment for greenhouse assemblies and foundations is: at least 15 years (Netherlands Code of Practice on Greenhouse Construction).

This appears to be essential in the justification of the com-putation procedure (a) in order that both the input data of the loads and (b) the introduction of soil mechanics effects, which sometimes enter into the calculation.but which in reality are absent, or are possible present after some years, will be viewed critically. Herewith some figures for information:

13

(A)-Type of greenhouse: Venlo twin-bay 3.20 m; lattice girders 6.40 m; eave height 3.0 m; spacing between stanchions 3.0 m; roof angle a = 26°; h for gales: 3.40 m thus the thrust is 50 kgf/m2 (Netherlands standard 3859). The vertical base loads are taken to be as follows:

Substructure 18.5 kgf/m2 Heating 5.0 kgf/m2 Crop 15.0 kgf/m2 Snow 25.0 kgf/m2

In the absence of gales for each stanchion; 3 cases with exclusive-ly vertical loading.

A. Empty greenhouse, no heating (uplift hazard) 360 kgf B. Greenhouse with crop, with heating 740 kgf C. Empty greenhouse, no heating, with snow 835 kgf

Gusts of gales (for each gable or roof area) see figure 5:

At right angles to the ridge, against the side facade (3 m) (horizontal) 540 kgf

In line with the ridge, against the end gable (6.4 m) (horizontal) 1 200 kgf

In both directions(uplift) on the roof (19.2 m2 ) (vertically upwards) 576 kgf

(B)-The 400 mm top layer of loose top soil does not play any part. During construction it should, moreover, be necessary to give some reasonable indication of how the footing makes contact with the undisturbed soil. With such a philosophy, verification tests are absolutely essential. Moreover, the minimization of settlement remains a crucial point, because at greater lengths of the green-houses uneven settlement is unacceptable in view of the drainage of rain water. It should be easily possible to compensate for small settlements.

Now a few final remarks:

1. Acrylic concrete may also be considered as a material for piles inert to sulphate attacks, the rectangular cross-section of piles made of traditional concrete being modified to save expen-sive material.

2. A monolithic solution has the advantage that a sealing in the form of a hard watertight and highly moisture-resistant coating can be applied all over direct at the factory, but there are placing difficulties.

3. The view that replaced foundations do not meet the durability requirements of 15 years is fundamentally wrong. A greenhouse has a life of 20-25 years.

4. The trend towards decreeing a diameter for the base block on the basis of tradition without verification of the soil a in the presence of a gale and of the safe resistance to uplift is irresponsible.

14

5. The clean steel foundation is particularly suitable for wide-span houses* and greenhouses of light-weight construction, having one or two layers of film. The ribbed rectangular plate from the replacement circuit can be turned into a circular one in new construction, which should accelerate its placement. The large concrete blocks for wide-span greenhouses are not to be encoun-tered at all in this system, which is a big advantage.

C. New forms of greenhouses

Up to the present, plastic panels are being used mainly as a roof cladding, following a technique derived from glazing with glass. In the past, it has at times been investigated whether it is possible to work with large polyester roof units. However, the pro-duction method as well as the expected price bearing in mind the limited demand rendered this idea unrealistic.

With other plastics it is easily possible to deform flat sheets. The idea is to start off from the traditional frame structure of a Dutch greenhouse with 6.40 m lattice girders and to place pyramids with a square base of 3.2x 3.2 m in a row at the eaves, which are spaced 3.20 m apart. A light gain is obtained and the air capacity of the greenhouse restricted thereby. The problem of natural venti-lation can be solved by traditional movement of part of the pyramid. A standard frame in a standard unit is also possible; the ventila-tion does not then respond to any definite wind direction.

D. Screen with integrated systems

The present-day screen installations are additions to a tradi-tional construction. Another approach is to adapt the main suppor-ting structure in the details of the section members in such a way that the installation is fitted therein. The same system should be considered for simple greenhouses with plastic film cladding, in which an internal lining can be suspended, that is either permanent or acts as a screen. The section members can be made in a combina-tion of steel and aluminium. With plastic cladding, the dead weight of the greenhouse is very low and the heavy concrete structure (to counteract uplift) can be replaced by the steel foundation + ground anchor discussed above. A further step is then to integrate the foundation system into the superstructure.

Wide-span greenhouses are those with the trusses and purlins span-ning freely across 6.4, 8.0, 9.6 or 12.8 m.

15

Summary

Innovations in greenhouse design and construction

Conventional greenhouse design is based on the limitations of the com-mon construction materials as glass, steel, aluminium and reinforced concrete. But new materials, notably plastic films and rigid plastics, require a re-evaluation of the traditional greenhouse design with the aim of maximizing illumination being central. In the same time, concern for energy saving has produced developments in screen installations. Some innovative solutions to the problem of reconciling heat conservation with maximum illumination in the green-house have been described. Another aspect of greenhouse construction, namely the foundation, is also being researched in an attempt to prevent the occurrence of severe désintégration affecting existing concrete foundations of greenhouses erected between 1976 and 1980. Two new systems developed at IMAG are described. The first is based on the traditional approach using very reliable materials such as hardwood and hot-dip galvanized steel. The second, more revolutionary solution to the problem, involves a new de-sign in one reliable material which enables the foundation and the superstructure to be integrated into one unit. It eliminates the need for a heavy concrete foundation and is therefore a break-through for wide (6.4 - 12.8 m span) greenhouses. Until now greenhouse floors have been made of 100 mm thick solid re-inforced concrete with polyethylene tubing for floor heating. Weight reduction by more than 50%, easier removability and a quicker response to temperature requirements are the present problems. A suggestion is made .

Fig. la - Presumed process in the existing state.

Zones of supply of agressive substances

0 300

Temperature and ventilation

Evaporation - zone

Ground level

First stage of attack on core : swelling

Concrete block in undisturbed ground

16

300H

4 0 0 -

Tight f i t t ing in the concrete base block 700

aj *->

Ï u c o o

-O (C

H— a> Q.

Q.

Concrete base block

Complete désintégration

Completely sound

Fig. lb - Concrete quality after attack.

Coating

Space to al low for ground cul t ivat ion

4 0 0 -

o 3 0 0

Fig. lc - Repairing of stabilizing units.

17

201

Pile resistant to

J 5 1!

sulfate attacks 7

Prefab concrete 1 r h

pad 1! 1 -i / 1 1 , .

As

Preservation treated timber P Azobe heartwood

Steel Acrylic concrete

200 Stabilised sand

Fig. 2a - IMAG System: pile and pad.

.40 Pile resistant to attacks

t \ 208 406

t i r — m —« — — «»•» W— V»» **** — — ~

C J

(.4. 44

IT) CM 00

IT) CN 00

600

Appr 60 kg

«- -t I I

Fig. 2b - Prefab concrete pad.

18

Fig. 2c - IMAG System: steel and groundanchor.

19

Fig. 4 - Windload in direction latice girders

7 bays - 6 centre piles - 2 gable piles

6 . 4 m

La t t i ce g i rder

W i n d 1 1 1 •

T w — < T w —H i w 14 — >

• Hinge connec t ions

( - Fixed connec t ions

W i n d

Ì >

Fig. 5 - Windload in direction gutter.

A Fig. 6 - Diagram of the replacement-repair concept

B C

7 Steel

Ground anchor

p r

6

Inert pile

Concrete pad

I M A G systems w i th just i f ied durabil i ty aspect

ss = 2 0 0 mm stabil ised sand

4

2 parts

.Î "I

Monolith

Solutions in prefabricated concrete

v^y

Casing Coatings / Bindings

Concrete base in undisturbed ground

is not replaced

A

Retardation repairing system

Fig. 7 - Principle futuristic Venlo deck in plastic elements.