Redpath Bulletins & Articles

BULLETINS & ARTICLES

HORTICULTURAL PRODUCE & PRACTICE

Artificial Shelter Specifications for construction

These tips & specifications are general information only & they do not include all of the required information or complete engineering data for all applications.

The provision of adequate shelter from wind is necessary for successful production of most horticultural crops.  This is especially so for citrus and subtropical fruit.

Wind adversely affects many aspects of crop production by:

• Slowing growth and delaying establishment.
• Accentuating drought stress.
• Deterring pollinating insects.
• Disrupting satisfactory application of sprays, fertiliser and irrigation.
• Removing shoots, foliage and fruit.
• Causing blemishes by rubbing of fruit.
• Reducing temperature, which affects the internal quality of fruit.

Shelter is normally best provided by planting trees, bamboo, pampas grass or other live shelter.  However, live shelter has several disadvantages:

• Slow growing.
• Prone to attack by pests or diseases.
• Host to pests and diseases of crops.
• Roots rob the crop of moisture and nutrients.
• Sometimes prone to wind-throw.
• Prone to branch breakage.
• Roots block drains.
• Needs constant trimming.
• Some shelter plants are too dense in growth habit, leading to wind turbulence.

To avoid these problems the alternative of artificial shelter can be considered in some circumstances.  The advantages of artificial shelter are:

• Instant shelter – no wait for establishment before planting crop.
• Not prone to pests or diseases.
• No roots to compete with the crop.
• No trimming required.
• Degree of porosity can be selected.
• No roots to block drains.
• On some crops it can be run over the top, and avoid wasting space.

The biggest advantage is that of being instant shelter.  On new land the crop can be planted immediately, without having to wait for the shelter belts to grow high enough to plant the crop.

The high cost of artificial shelter is a disadvantage.  It may not be as aesthetically pleasing as trees, although trees may be planted in conjunction with it.

The support structure for artificial shelter must be constructed with materials that are substantial enough to withstand the stresses to which it will be subjected, and must be erected properly.

SHELTER FORMULA

Spacing
The general formula for spacing for horticultural crops is that on level ground a shelter belt will provide shelter for approximately 8 times its height.  However, the height of the crops must first be deducted from the height of the shelter to find the distance of effective shelter in any given situation.  Examples of the calculations are shown in Table 1.

On land sloping away from the wind, the times factor can be increased, but for land sloping into the wind it must be reduced.
 

Table 1: Examples of spacing calculations

Shelter belt = 20m high

Crop of avocados = 10m high

20m – 10m = 10m

10m x 8 = 104m effectively sheltered

Shelter belt = 15m

Crop of kiwifruit = 2m

15m – 2m = 13m

13m x 8m – 104m effectively sheltered

Height

The minimum height for shelter is approximately twice the height of the crop it is protecting.

Length

The minimum length is about 11 times the shelter height.

Porosity

Any shelter should have a porosity of about 50%, he ideal being 40-45%.  If too dense, it will cause eddying behind the shelter.  If too porous, the shelter will not reduce the wind velocity enough to protect the crops.  A gap underneath or between shelters causes wind funnelling, which may damage crops.

STRUCTURAL VARIABLES

Wind

Wind velocity has the greatest effect of all the variables on the strength of structure required.

Specific Tonne loadings per post for shelter belt construction can be provided by Redpath.

Factors that determine the design wind speed are:

• Location – e.g., Tauranga or Wellington.
• Site exposure – e.g. on top of a ridge.
• Proximity of other shelter.
• Life of structure, i.e., designing for a 1-in-5 year or 1-in-25 year wind.

Timber

Roundwood timber is much stronger than sawn timber, therefore poles are usually used. There are several factors to consider when selecting poles:

Diameter:  This has the greatest effect on ple strength e.g., an increase in diameter of 10% means an increase in strength of about 33%.  The critical dimension is the ground line diameter (GLD).  Poles are normally purchased by the small end diameter (SED), so check the GLD when purchasing.  The taper in pine poles is about 25mm in 3m.

Type of timber:  Corsican pine, Douglas fir, larch and radiata pine from Matakana Island northwards are of similar strength.  Radiata pine from elsewhere in the country is about 20% weaker.

Knots:  can be a problem if too big or there are too many at one point.  A guide is that an individual knot should not be greater than one-fifth of the pole diameter at that point.

Shaving:  Trimming or smoothing at the pole at the nodal swellings (i.e., around the knot) considerably reduces pole strength and causes greater variability in strength between poles.  Shaven poles are not recommended for artificial shelter.

Steaming:  Some treatments involve preliminary steaming as part of the process.  This reduces pole strength slightly.

Wire

The strength of the wire determines the maximum pole spacing, which for 3.15mm high tensile (HT) wire is 9-10m.  This however, is very dependent on tension applied to the wire at time of installation, i.e., the pretension.  It should not be greater than 1kN; about 0.5kN is ideal.

The fence must be allowed to billow, otherwise the wire tensions get too great when the wind blows and failures occur.  In most situations with artificial shelter the wires are operating at near maximum strength.  As knots reduce the strength of wire, they should be avoided if possible.  If they are necessary, only figure 8 or reef knots should be used.

CONSTRUCTION

Strainer assemblies

The horizontal stay and the tie-back are the most common assemblies. A diagonal stay may be used but there are problems with uplift of end post, which must be footed (support or foundations for the diagonal post in the ground and connection of the diagonal to the vertical post).

The horizontal stay has a big advantage over the tie-back in that there are no wires to block access ways.  Points to note in their conjunction are:

Horizontal stay:  Length of horizontal – usually one of the upright poles is used.  The longer the horizontal, the stronger the assembly.

Diagonal wire:  Carefull choice of the angle of the diagonal wires relation to the horizontal is required, the wire should have a strength similar to the total strength of the line wires.

Tie-back:  Deadman – the strength of the assembly depends on this.  The most satisfactory is the Redpath screw anchor.

Wire:  the wire should have a strength similar to the total line wire strength, e.g. for 10 x 3.15mm line wires, 10 x 3.15mm tie-back wires are required.

Stayed or guyed poles 

Generally guyed poles are not acceptable in an orchard situation, because guys create access problems.  Guys can substantially reduce both the size of pole required and the depth the pole is in the ground.  However, the cost is not reduced because of the wire, deadman, and extra labour for erection required.

If the tops of the poles are guyed with the wires at a 45º angle to the ground, pole sizes can be reduced a little to that of a cantilevered pole, i.e., no guy wires.  This generally means at least two 3.15mm HT wires for guys.  If the guys are tied three-quarters of the way up the pole, pole sizes can be reduced further, but stronger guy wires and deadmen are necessary.

Breast blocks

Breast blocks are not necessary, but are recommended if the shelter is a permanent one.  A half-round post pn either side just below ground level is adequate.

With continual buffeting the poles eventually become loose in the hole.  The breast block overcomes this but is only successful if the base of the pole is secure.

Windbreak material

Redpath Windshield & Shadeshiled life is very dependant on correct construction ethods.  They all have a porosity around 50%.

Redpath Shadeshield:

Woven polyethylene fabric which is supplied in 50m rolls, either 1.83m or 3.66m wide.

These fabrics are held up by wires.  For pole spacings of 6.0m or greater, it is recommended that 3.15mm (10 gauge) HT wire is used for the top and bottom of each 1.83m width of fabric.  For intermediate wires, i.e., between the 3.15mm wires, 2.5, HT wires are used.  For exposed sites, 2 intermediate wires each side of a 1.83m width are recommended.  For protected sites one each side is adequate.  The fabric is not connected to the intermediate wires.

The fabric is connected to the wires with clip rings or by sewing.  The method of connection is important.  The fabric is folded around the wire to make a hem with the wire contained within.  The clip then secures the loose end of the fabric back to itself.  The clip does not go around the wire (Fig.1).  The only clips around the wires are those holding the two 1.83m widths meet.  If sewing, an ultraviolet-resistant yarn is necessary.  An edge hemming service is available from Redpath.

The wires are secured to the poles by staples.  When all wires and fabric are up, a 50mm x 12mm batten is placed over them and secured to the poles with nails at 300mm centres.  This batten is essential as it stops chafing of the fabric between wire and pole, and secures fabric and wires to the pole, enabling the structure to take winds that blow the fabric away from the pole.

If the pole is nobbly, two battens are used, in which case 100m galvanised flat-head nails are required.  If only one batten is used, 75mm galvanised flat-head nails are adequate (Fig 2).

3.15mm HT wires at 0.5m intervals or 2.5mm HT wires at 0.4m intervals are required.  These must not be over-strained.  Material and wires are secured to the poles with battens, as for Redpath Windshield or Shadesheild.

The recommendations in Table 2 are based on a 5-year-return windspeed of 27m/sec, It was assumed that peeled Corsican pine poles at 6m spacings were used.  Four wind conditions were assumed, depending on location:

1. A boundary shelter on an exposed ridge or where there was wind funnelling.

2. A boundary shelter in open country, i.e. there are no hedges in the immediate vicinity.

Table 2: Ground line diameters for artificial shelter poles.

Shelter height (m)	Ground line diameter (mm) for peeled Corsican pine Wind condition according to location
.	1	2	3	4
4.5	210	185	170	155
5.4	240	210	195	180
6.3	270	240	225	200
7.2	300	265	250	225

Fig.2: Connection to poles.

The wires are secured to the poles by staples.  After all wires and fabric are up, a 50mm x 12mm batten is placed over them and secured to the poles with nails at 300mm centres.  This batten is essential as it stops chafing of the fabric between wire and pole, and secures fabric and wires to the pole, enabling the structure to take winds that blow the fabric away from the pole.
If the pole is nobbly, two battens are used, in which case 100mm galvanised flat-head nails are required.  If only one batten is used, 75mm galvanised flat head nails are adequate (Fig 2).
 

3. A boundary shelter in an existing orcharding area, i.e., the artificial shelter is getting some protection from existing hedges.
4. An internal shelter of an orchard where the fence is getting good protection by existing shelters.

The dimensions given are the ground line diameters, which are the critical dimensions for artificial shelter poles.  Approximate small-end diameter can be obtained from table 2 by allowing for 25mm in 3m taper.

Modifictions
Modifications to the given assumptions are obtained by multiplying the diameter by the following factors:

• If the shelter is to be permanent, the 25-year return wind speed is 32m/sec.  Factor, F25=1.12.
• A 10% change in spacing requires a 3% change in pole diameter, e.g., at 9m spacing, Fg=1.15.
• If different timber is used, pole sizes may vary.  For Douglas fir larch and radiata pine from Matakana Island and north, the sizes are about the same. 
• Shaving, or removal of the nodal swellings (swellings around the knot) reduces pole strength, therefore, requiring a larger diameter pole. 
• Steaming during treatment also reduces pole strength.  Suggestion factor, FSt=1.06.
• Where there is more than one factor they are multiplied together.

Table 3:Recommended minimum embedment depths for poles on ash soils 

Height of shelter	Pole embedment depth
(m)	(m)
4.5	1.2
5.4	1.5
6.3	1.8
7.2	2.0

Wind speeds from a 50-year-return period are given in Fig.3.  The factors given in Table 4 are for 5-year-return wind speeds for temporary shelter and 25-year-return wind speeds for permanent shelter.

Table 4:Modifications for area outside.

.	50 year return wind speeds (m/s)
.	35	40	45	50
Temporary shelter
(Fot)	1.04	1.14	1.23	1.32
Permanent shelter
(Fop)	1.14	1.25	1.35	1.45

These tips & specifications are general information only & they do not include all of the required information or complete engineering data for all applications.