A versatile wire mesh fencing available in different specifications to suit a wide range of livestock. For applications likely to get heavy use the high tensile version gives a much longer useful life.
Available in a range of different grades, C, L and HT (High Tensile.). These are usually stocked in 50 meter lengths in a number of different heights from 80 cm. to 200 cm, with the high tensile in 100 meter rolls. Stock fencing can also be used as a cost effective alternative to mesh and netting.
The Code Explained
e.g. L15 – 120 – 15
Often peeled and treated posts are used or alternatively machine round, both options can be round or half round; depending on application and customers preference.
The peeled are a cheaper choice, these are irregular in diameter and as the name suggests are literally peeled timbers. The diameters of these are ranged with an approximate size i.e. 75mm > 100mm meaning they are graded from, to and between these dimensions.
The machine round is a consistent diameter down its length with a smoother finish. But due to its production costs etc it is a more expensive option.
Straining Posts - Peeled & Treated
The most important factor in the strength of a strained fence is the straining posts. They take all the load of the strained wires, and the intermediate posts are only there to stiffen the fence, and to hold the wires at the correct height from the ground.
Used at the end of a run, corners or any change of direction; also on long straight runs two ways positioned at a maximum of 69m centres. Depending on the contractor these are either driven in with a tractor driver (for example) or dug in and back filled and rammed. (Back filled and rammed is the digging out of the soil, and then using that spoil to ram around the post to secure.) It would prove very difficult to drive these diameters in by hand.
2.1m long 125–150mm dia. tops and 2.1m long 150–175mm dia. tops are popular sizes. 2.4m long 150–175mm posts are also available. Posts are normally pointed for driving (subject to availability).
Struts – Peeled & Treated
These are used to support the straining posts, and to stop them being pulled in the direction of the straining wires.
One method is a diagonal timber dug into the ground often with a post driven in at the base of it to stop it being pushed into the ground. The top of the strut is either cut and notched into the straining post or some use the pointed end positioned into hole drilled into the strainer. The later method utilises the existing point well and does not require any mechanical fixing.
The other method is box strainers:
2.4m long 75–100mm dia tops with one end pointed. Will suit angled strutting or formation of horizontal for box strainers.
The load on the straining post comprises:
The total strain taken by the straining posts will therefore be greater for a fence of six line wires, each strained to 100kg, than for a fence of three line wires, each strained to 100kg.
The top wire has to bear the most strain from leaning animals, fallen trees and wind. In theory the top wire should be of a heavier gauge and/or greater tensile strength than the lower wires. Alternatively, ensure that the top wire can be easily re-tensioned. The 2 ply mild steel barbed wire, which is often used as a top wire, has a low tensile strength, and although of heavy gauge, can usually bear less strain than the lower wires.
The length of strain is only significant in dealing with any extra load on the wires, such as leaning animals. The longer the length, the more this extra load is spread, and the less likely that the wire will break or the straining posts move.
The type of ground
The following table gives an indication of the strength of various soils.
In weaker soils, anchorage should be increased by using larger and longer posts, and on straining posts, larger feet and thrust plates. In very weak soils, Box strainers are useful in difficult soils.
The amount of ground disturbance
It is not possible to backfill a hole in such a way that it is as firm as the undisturbed soil. The best method of erecting a post firmly is to drive it directly into the ground, using a mall or similar tool on smaller stakes, and a tractor mounted post driver for large posts. Posts driven into the ground are 150% firmer than posts rammed into holes dug oversize, measured by a sideways force.
They also withstand greater lifting forces. Tests by New Zealand Wire Industries showed that to lift posts out, the following forces were needed:
|Insertion Technique||Force Required For Extraction|
|Dug and rammed||90kg||Pilot hole and hand driven||900kg||Machine driven||1260kg|
There is thus obvious advantage in using machinery where possible, as posts are not only erected much more quickly, but are firmer than dug posts. The lack of ground disturbance compensates for the fact that a foot cannot be fitted to straining posts.
The height and diameter of the post
The depth the post is set in the ground is very important. The average rule is that straining posts are embedded to about half their total height, and intermediate posts to about one third.
Tests have shown that increasing the depth by one third will double the resistance to the post pulling out. For example, if a stake is normally knocked in 600mm, an addition of 200mm will double its holding ability. Extra long posts may be worth obtaining for fences in weak soils.
In strong soils, long posts are more likely to fail under extra strain by breaking, rather than pulling over. Tests have indicated a relationship between the diameter of the post, and the depth to which it is embedded. A post embedded to a depth of over ten times its diameter, in medium clay, will break rather than move. The same post must be embedded to a depth of up to fifteen times its diameter in soft soils before it fails by breaking.
In theory, fences which run up and down slopes should have straining and intermediate posts set at right angles to the slope, and not vertically. The posts then have the maximum depth in the ground, whilst holding the wires at the correct height. The parts of the strainer assembly stay in the same relationship to one another as on level ground.
Problems with this are:
Fences set across slopes should be set vertically, as shown. If set at right angles to the slope, there is increased gravitational pull on the fence. This, combined with the force of leaning animals, wind or snow acting on the upper side may push the fence over.
This is the most common type of strainer used on fences in Britain. However, a major difference in the design recommended here from the pattern normally seen is in the length and angle of the strainer. Usually the strut is attached near the top of the post, meeting the ground at an angle of about 45 degrees.
A much stronger structure results if a longer strut is used, attached to the post about one third down from the height of the top wire, meeting the ground at an angle of between 25 and 30 degrees.
The strut transfers some of the force from the wire downwards to the ground, making a pivot. This produces an upward force on the straining post. The recommended angle to minimise this pivoting action is 25 to 30 degrees.
The diagram shows the parts of a conventional strainer. Post and strut sizes for various fence designs are given in Chapter 1. However, the general rule for normal height (i.e. 1.1m) fences is that the post and strut are of equal length.
Strainers fail, that is either move or break when the full tension is applied to the wires, by one of the following ways:
|By pushing through the ground. This is prevented by the thrust plate, which takes the force transferred through the strut, and to a lesser extent, by the breast block.|
|By twisting to one side. This is prevented by ensuring that the strut is fitted in line with the wires, the joint of the strut and post is well made, and the strut cannot slip off the thrust plate. Twisting is also less likely if the wires pull off from the centre of the post, rather than from the edge and if the post has a foot.|
|By jacking out of the ground. This is prevented by the foot, and by having sufficient length of post in the ground, with the backfill carefully tamped.|
|By breaking. Use a sound post of sufficient diameter.|
Variations on the conventional strainer
The conventional strainer can be strengthened by fitting a retaining wire of spring steel or high tensile wire, as shown. This forms a triangle of great strength, similar in principle to the box strainers shown below. This method of building a strainer is recommended by the Forestry Commission (Forest Fencing, Forestry Commission, 1992).
Where a retaining wire is being used, the strut should be fitted to the post at the point that is three-quarters of the height from the ground to the highest strained line wire. The straining post should include a foot.
The Forestry Commission recommend the following method, when either stapling to attach, or using spiral fence connectors (Forest Fencing, Forestry Commission, 1992).
Another method of attaching the retaining wire is to make a simple loop of wire, joined with a Gripple, crimp or similar device.
The advice of the Ministry of Agriculture and Fisheries of New Zealand is to build straining posts on sloping ground as shown. In practice, this is not easy, because of having to dig a deep hole at an angle to the vertical. Normal vertical posts are suggested, with a box strut if necessary on the lower side, where the ground drops steeply away.
Another method sometimes seen is to build the fence from the top of the slope, and apart from the initial strainer, only erect struts on the upward side.
The section of the fence already strained then provides the anchorage for the next section. The disadvantage of this is that the strain of each section is not being fully taken by each straining post. If for example, section A on the above diagram fails due to a tree falling on it, post 2 may then be pulled towards post 3, because it has no supporting strut. Section B will then go slack.
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