IV. should be strong enough to support

IV. Based on the
Span

a. Single span structure: When only one
span is used as described in shape, and then they are called as single span
structures.

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–       
Many
single span structures can be erected. It permits convenient expansion /
contraction of business.

–       
It
permits the production of a different plant material by maintaining different
micro-climate in different units.

–       
Flexibility
can be seen with this type and gutter is not required.

 

b. Multi spans
structure

It is an economy in construction, for
cooling and heating operations and cultural operations. There is no flexibility
and different crops cannot be grown in one structure. The gutter is required.

 

Constructional
material for the greenhouses

            A greenhouse
maintains the favorable climate suitable for growing crops.

¿
This
objective needs to be achieved at an affordable cost.

¿
The
greenhouse structure should be light yet it should be strong enough to withstand
certain forces/loads.

¿
The
greenhouse is considered to be semi-permanent structure and should be designed
for service life of twenty-twenty five years.

¿
The
greenhouse must resist a load of its own weight, Wind load, Snow load and Crop
load.

 

Minimum values
of GH designs loads

      Load                                                                                 Minimum value (kg/m2)

Dead:  a. Pipe frame                                                                          10

b. Bass frame, lapped glass,                                                   25

c. Hanging baskets                                                                 20

Live:   a. Snow  
                                                                                75

            b. Wind                                                                       To
be estimated based on area

 

ü 
Maximum
recommended a live load of 68 kg/m2 of covered ground area.

ü 
A
greenhouse should be designed to resist a wind velocity of 100 km/hr.

ü 
Wind
load is affected by the direction of the wind, shape of greenhouse, wind breaks
and the opening of the greenhouse.

ü 
Snow
load depends upon the roof slope, snow fall, single span or multi-span and
whether heated or unheated. 

 

Method
of construction

a.       
Foundation

v
Foundation
should be strong enough to support the super structure against various forces.

v
Pit
or pile foundation is common because columns are widely spaced and the depth
should be at least 60 cm reaching too hard strata with a width of 60 cm.

v
The
foundation may be Reinforced Cement Concrete (RCC), CC or telescopic, whereas
the telescopic foundations are easier and cheaper than RCC or CC.

 

b.     Floors

Greenhouse crop may be grown in soil or
soilless. The floor of the greenhouse should have drainage. For soilless
cultivation, sometimes porous concrete floors having the depth of 10 cm are
prepared.

 

c.      Frame

v
The
common materials of frame construction are steel, wood, bamboo, aluminum and
RCC.

v
The
frame includes columns, bottom cord, truss members, purlins, knee bracing, wind
bracing, fitting members, etc.

v
Wood
should be treated for protection from high moisture and termites and painted
for better light conditions in the greenhouse.

v
Coal
tar is a good preservative for underground portion of wood.

v
Steel
components to be used in greenhouse frames should be hot dipped galvanized to
avoid rusting.

v
Sometimes
to reduce the cost, mild steel is used for the frame. Mild steel should be
painted with two coats of red oxide and two coats of oil paint to avoid
rusting.

v
Painting
should be carried out at least once in two years.

v
Steel
or aluminum frames transmit more light as compared to wooden ones.

v
The
frame members should be smooth to avoid damage to the covering material.

 

d.    
Glazing material fixing

c      
Commonly
used glazing materials are glass, poly film, poly carbonate sheets, acrylic
sheets and FRP sheets.

c      
Except
for poly film, all other materials are rigid and comparatively heavy in weight.

c      
However,
the material used for fixing/fitting the glazing material remains more or less
same.

 

They
are fitted with any of the following materials.

i.
Wooden battens:
Generally 2.5-3.5 cm battens of suitable length are used. However, if the
length is less, then work of fitting increases. For poly-film, length of batten
may be 3 m or more to have convenient working. Battens may be salwood, teak,
mango, season or silver oak. Covering materials are fitted in the battens with
nut and bolts. 

 

ii.
Plastic grippers:
The gripper made in ‘U’ shape from plastic is used. They are available in coils
of 100, 200, 300 m. This type
of gripper has a protruding surface for
ensuring grip. Nevertheless, for additional firmness, spring clamps are
also used. Over the period, due to properties of plastic gripper may be
degraded.

 

iii.
Aluminum profiles: Aluminum
profiles are shaped in ‘C’. The film is fixed in ‘C’ profile with hammering the
PVC pipe. Sometimes the film is also fixed with ‘Z’ shaped continuous GI wire.

 

Glazing
Materials for greenhouses

The covering of a greenhouse or any
structure with the transparent material is known as glazing or cladding. The
glazing material includes glass, poly film, poly carbonate, FRP, PVA, acrylic,
polyester, PVF. Except for poly film and PVC, all other materials are rigid
panels/ sheets.

 

Radiation
properties of glazing material

            When a beam of radiation is
incident on a material, a part of it may be reflected back, apart may be
absorbed by the material and remaining portion will be transmitted.

D     
Depending
upon the material composition and surface properties, the radiation absorbing
ability, reflectivity and transmissivity values are determined.

D     
For
a given material, the values of three coefficients must add to 1.0 with
reference to a particular radiation beam.

D     
A
transparent material transmits a large portion of light through it, whereas, an
opaque material either reflects or absorbs the incident radiation.

D     
A
translucent material may transmit a large portion of radiation but the
transmitted light is diffused in nature.

D     
In
case of greenhouses, the transmissivity of radiation property is important. The
values of transmissivity for various glazing material are given below (Table
2.1).