Infrastructure

Design considerations

Shade cloth structures

Several forms of shade cloth structures are available including span structures (such as the case study on page 20), peaked sail structures, cantilever structures as pictured below and tent like structures with large central supports.

Design considerations for maximum effectiveness and useful life

Fabric material

For dairy cattle, use shade cloth which blocks at least 80% of sunlight, with a minimum 300 gsm (grams per square metre). Green or black coloured material is preferred. Shade cloth fabric should last at least 10 years. Higher quality and tighter weave fabrics last longer but cost more.

Shade cloth fabric can deteriorate relatively quickly with exposure to sunlight, dust, accumulated debris and water. Flexing will encourage deterioration and wear as well as failure or loosening of connections. Shade cloth is also prone to bird, insect and rodent attack and areas not able to be hosed down or easily inspected for maintenance are particularly prone.

Note: Cows are also not keen on going from bright areas to dark areas and prefer dappled shaded spaces to dark spaces, so they tend to get used to a shade cloth structure more quickly than a solidroofed structure, provided the cloth is not billowing or flapping noisily.

Fastening fabric to posts

Shade cloth must have sufficient tension applied to it to prevent the cloth from damage during windy conditions. The flexible and adjustable connections between the fabric and the support posts are therefore critical structural components.

The shade cloth fabric can be held to posts in different ways, ranging from load-carrying straps to chains and u-bolts and in the most advanced form adjustable cables and turnbuckles.

How long the connection lasts usually depends on how well it distributes the point load at the stanchion to the fabric. Reinforcing cables and seams help.

If excessive flexing of fabric is permitted the flapping can generate unacceptable noise during wind events which can disturb cows and irritate people. However, compared to metal frames and buildings, shade cloth structures suffer minimal temperature or water related deformation.

If a cable or cable connector fails due to overtensioning, it can cause instantaneous and catastrophic structural failure.

Turnbuckles for tensioning cable supported shade cloth structures are prone to loosening or failure through repetitive loading so they should be inspected regularly and tightened or replaced.

Chain connections should be avoided. In the event of overstress, breakage of a link can lead to the launch of a projectile whereas a cable will fray or unravel, allowing time for repair, replacement or escape from injury.

Shade cloth structures can become a hazard if damaged by storms. Blown cladding can provide serious injuries whilst broken cables and unsecured shade cloth can whip.

Different fastening methods.

Support posts and foundations

Ensure structural grade steel is used. For rigid framed and flexible shade cloth structures the wind loads, dead loads and live loads imposed on the structure need to be transferred through the structure into the ground. The foundations which secure each post to the ground are therefore critical structural components.

Deep, concreted footings are essential as the posts are subject to a ‘bending moment’ and need to transfer stresses into the ground, as well as to stop the structure lifting. Each post should be rigidly attached to a wide steel plate which is mounted on the foundation using bolted connections. Galvanised and threaded starter bars extending from the footing reinforcement are better for holding the plate than dynabolts.

Apart from central supports these posts usually lean against the applied horizontal load to maintain the tension rather than being vertically upright. They need to be free standing in a farm situation. Guy cables must be avoided.

Deep footings should be left to cure for an extended period before bearing any load.

Avoid collisions between animals and vehicles with posts, cables and cladding. If possible, position posts outside the animal traffic area so they are not in contact with manure and water or interfere with washing (If are located in the yard, place a raised concrete or PVC sleeve around the pipe to reduce corrosion potential).

Height

If the shade cloth is suspended too high, it will limit the area of shaded footprint.

A minimum height of 4.0 m is recommended to ensure:

cows do not interfere with it and are happy to walk under it (Low shade cloth structures can put cows off entering an area)

effective installation and use of sprinklers and fans

machinery can readily traffic the area without risk of tearing it or burning holes in it with their exhaust pipe

adequate airflow underneath the structure.

Orientation

The height of the structure, the angle of winter and summer sun and the required area of shaded footprint govern the orientation of the shade structure – northsouth or east-west. If the structure is aligned east west the passage of the sun will generally ensure that the northern side of the structure is more exposed to sunlight than the southern.

Drainage

Drainage from shade cloth must be considered, particularly if the paving is earth. Ideally, any drainage should be directed to formed drains rather than earth that is subject to animal traffic.

WEAKNESSES IN shade cloth structures are usually associated with: cyclic loading conditions lack of fabric strength inadequate fabric reinforcement at connections connections to stanchions which are prone to fatigue failure of footing connections, leading to loose posts excessive spans which can lead to billowing corrosion of metal degradation of fabric rain, hail or debris accumulating on top of the shade cloth.

Stresses on shade cloth structures

A shade cloth structure must be strong enough to withstand three types of load: ‘wind load’, ‘dead load’ and ‘live load’.

Wind load

Wind load is usually the largest load carried by a light-framed agricultural or industrial building taking the form of a tensioned shade cloth or a rigidly clad framed structure.

Wind loads impose unbalanced forces on a shade cloth, generating ripples or waves that may lead to premature failure of fabric or other components. This problem can be alleviated by ensuring that the shade cloth is adequately tensioned.

Horizontal wind loads can also contribute to structural failure. If the angle of inclination of a roof or shade cloth is 15° or so, it will generate lift (The roof behaves like the leading edge of an aircraft wing). Lift can be minimised by either flattening the inclination of the roof or shade cloth to between 10 and 14° or installing it with an inclination in excess of 20°. Roof angles of 15 to 18° should be avoided.

Sailcloth structures are particularly prone to ripping in response to high wind loads, inadequate fabric strength, lack of structural reinforcement and excessive spans which can lead to billowing.

Dead load

Dead load refers to the weight of the structure (downward force due to gravity) which must be withstood by the posts. Bracing in frames or cables supporting shade cloth are usually employed to ‘stay’ or brace posts.

Posts supporting shade cloth are frequently installed at an angle to help provide tension but this can also contribute to instability when the cable support is removed.

While the dead load of a metal clad framed structure is high by comparison with a shade cloth structure, the load imposed by shade cloth should not be underestimated, particularly if the loading is not well distributed. Cloth impregnated with dust or supporting leaves, twigs, hail, ponded water or wetted from beneath by sprinklers can also be quite heavy and the cables and connections which form the structural system are not light.

Live load

Live load refers to brief, temporary loads such as those associated with someone on a roof, hailstones or rainfall cascading down a roof, water pooling on the canopy or being shed through or from shade cloth.

If metal clad frameworks and shade cloth structures are not designed to carry human traffic it is not surprising that failure occurs when they do. Safe OH and S practice must restrict access to these structures.

While shade cloth is designed to pass some light and air as well as water it is not unusual for some of the gaps in the mesh to be clogged. If the mesh gets clogged and water pools, an excessive live load is possible which can lead to fabric failure at best or structural failure at worst.

If you intend to install a shade structure, consult a registered builder or structural engineer who understands how to design and build a structure which will withstand these loads. Alternatively , if you are buying a package shade cloth structure ensure that structural computations are supplied, the installers are experienced and local building regulations are met.