Fence mechanics

General principles

• 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.
• Wire should be attached to a running fit at the intermediate posts, so that any extra load, for example from an animal leaning on the fence, is transferred along the length of that strain.
  
• Always build the fence so that the wire can be re-tensioned. This means either fitting tensioning devices, or fastening off at the straining post in such a way that the wire can be released, re-tensioned and re-attached. Excessive use of staples is not advisable.
• At any hollows in the fence line, the wire will require a ‘tie-down’, in order to keep it stock proof at ground level. The strength of the tie-down must increase for steeper hollows and greater wire tension.
• Although mature living trees can make very strong straining posts, they should never be used for this purpose, because the staples and wire become embedded in the wood of the tree as the girth expands. This damages the tree, and reduces its commercial value if it is felled for timber. Damage to chainsaws from embedded metal is so frequent that many operators will only fell trees above fence height if they think the tree may have been used as a fence post. It is not worth using stumps of dead trees as posts, as they will rot.

Load

The load on the straining post comprises:

• The number of horizontal wires.
• The tension put on the wires by the Monkey strainers or other tensioning device.
• Any extra load, such as leaning animals, fallen trees, wind, drifted snow.

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.

Posts

The holding ability of a fence post depends on:

• The type of ground.
• The amount of ground disturbance caused as the post is put in.
• The length and diameter of the post.
• The presence and size of the foot.

The type of ground

The following table gives an indication of the strength of various soils.

Table 4k Soil types and strengths

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, the type of underground structure shown here may be necessary. 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 mell 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:

Dug and rammed…………………………………………90kg force
Pilot hole and hand driven……………………………900kg force
Machine driven …………………………………………..1260kg force

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.

An average size for a fencing stake is 1.7m x 75mm diameter, embedded 600mm. Although a 1.85mm x 75mm diameter post, embedded 750mm (ie ten times the diameter) would be stronger in medium strength soils, there is no advantage in having one longer than this, except in weak soils. There is also little advantage in having a wider post, unless it is correspondingly longer.

Slopes

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:

• It looks ‘wrong’ to most people, and may therefore not be acceptable.
• It is awkward to dig post holes at an angle to the vertical, and difficult to drive in posts using a post driver.
• The straining post must be vertical at a corner or on a three-way assembly, where a fence line is being taken off across the slope.

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.

Strainer assemblies

Conventional

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 (ie 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.

Table 5b Sizes of Posts and Struts

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).

• Make a U-bend in the end of the wire and attach it to the straining post.
• Take the wire around the thrust-plate, and loosely staple it so that the wire is just above the base of the strut. Take the wire back around the base of the strainer post.
• Fasten the strainers as shown, and secure the end of the wire back on itself with a fence connector. Alternatively, staple the end to the straining post.

Another method of attaching the retaining wire is to make a simple loop of wire, joined with a Gripple, Wirelok, crimp or similar device.

• Pull out a suitable length of wire, and pass it round the straining post and thrust plate, securely it loosely on both with guide staples.
• Using the ‘mid-strain’ method, attach the Monkey strainers, and then cut the wire leaving sufficient to join.
• Strain the wire, then cut to an appropriate length for the particular joining device. The Gripple should be threaded onto the ends of the wire first, and then tensioned with the Gripple tensioning tool.

Sloping ground

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.

Box strainers

Like many fencing techniques, the use of box strainers was developed in New Zealand. As in a conventional strainer, the box strainer relies on the principle of the triangulation of forces, but in the case of the box strainer, the force is taken by the strained wire.

• The box strainer is as strong as a conventional strainer in firm soil. In weak or very shallow soils the box strainer may perform better than the conventional strainer.
• The posts in a box strainer do not need feet, and where possible, are best driven into the ground. The fitting of the horizontal stay and the tensioned wire takes longer than fitting a strut.
• The components of a box strainer are only slightly more expensive than those of a conventional strainer, because although two posts are needed, they are of smaller diameter.
• The box strainer is not suitable for areas where vandalism is a problem, as the whole assembly will fail if the wire is cut.

The box strainer is a more complex structure than a conventional strainer, with more to go wrong, and its use is probably best restricted to situations where a conventional strainer is difficult to build. Single and double box strainers are widely used for high tensile fencing. Box strainers can also be used for high tensile deer fencing, although the Forestry Commission prefers the conventional strainer with retaining wire, as shown above.

Where ground conditions are difficult, or very high strains have to be held, a double box strainer can be constructed. This has over twice the strength of a single box strainer. Although the optimum depth is 1m (3′), it is possible to construct a double box strainer in only 300mm (1′) depth of soil, for example over bedrock.

Construction of box strainer

1. Knock the two posts into the ground, the distance apart of the horizontal stay.
2. Cut recesses of about 15mm (1/2″) near the top of each post. Place the stay in position.
3. Drill a hole through the post into the stay, and then hammer in a 200mm (8″) length of 12mm (1/2″) mild steel bar. Repeat at the other end of the stay.
4. Follow the procedure for straining and fixing a retaining wire, as described above.

Alternative design

A ‘windlass’ can be used to tension the wire where mild steel wire is used for the loop. This can be tightened further as necessary if the wire slackens. Preferably use a piece of hardwood, about 50 x 50 x 450mm (2 x 2 x 18″), inserted between the wires. Twist until tight, and then lock behind the stay. Fasten it with wire to discourage vandalism and to prevent stock interfering with it.

Box struts

The same principle is employed in the box strut, which is a very useful device for fence corners, slopes, and for repairing and strengthening posts and strainers. The following diagrams show some of these uses. An incidental advantage, for farm use, is that the horizontal stay provides an easy way of getting over fences, without damaging yourself or the fence. However, this may be a disadvantage if it provides unwanted access.