Brace Assemblies for Wire Fences

FUNCTION AND DESIGN OF BRACES

Corner, end and inline brace assemblies are the foundation of a good fence. The failure of one such assembly may result in the failure of a whole section of fence. Particular attention must be given to details such as location in suitable soils, depth of post placement, connection of the rail to the posts and tying off of the wires.

What is a Brace ?

A brace is a made-on-site anchor assembly consisting of one or more posts driven into the ground connected with one or more rails and tensioned together with wire. It forms the tie off point for the fence wire. It is named for the number of driven posts it requires, i.e. a two-post brace has two posts driven into the ground (and is completed with a rail and tensioned brace wire).

What is a brace really doing ?

As a brace assembly is an anchor point it must withstand all the load from the fence wires. Under normal conditions, the tensioned fence wires will exert a pull on the brace of 1,000 to 1,500 lb. With cold weather contraction the pull may be as much as 2,500 lb. In addition, shock loads to the wires, such as from livestock pushing or fallen trees, also have to be withstood.

In barbed wire fences, some of the load is absorbed by line posts when the barbs catch at staples. In smooth wire fences, with line wires not stapled “home”, the total fence load is transmitted to the brace assembly.

A brace must withstand the fence loads by transferring them to the earth. This means a brace is a combination of structure and soil – both are important for an effective brace. A brace can fail in one or more ways, by:

• structure failure (broken post or rail)

• soil failure (soil shear in front of the post allowing the post to move)

• end post pull out (indicating poor brace design for the soil conditions

Forces in a End Brace Assembly

What are the forces in a brace ?

Above, illustrates the forces on a typical two-post, horizontal rail end brace. It can be seen that the fence wire tension load tends to bend the end post slightly, which, through the rigid rail, also bends the brace post. Movement of the top of the brace post adds tension to the brace wire, transferring part of the load back to the bottom of the end post.

The effect on the end post of this load transfer is in relation to the angle of the brace wire. This transferred load can be broken down into:

· a horizontal force F1 trying to move the end post through the soil

· a vertical force F2 trying to pull the end post out of the soil

The “steeper” the angle of the brace wire, the greater this vertical pull-out force. This is why short span braces (with “steeply” angled brace wire) may fail at low fence loads by end post pullout. Longer span braces (with “lower” angled brace wire) apply more horizontal load to the soil and post, failing at higher loads. Longer span braces will use the full load bearing capacities of both the soil and the post. This added strength can also be achieved by lowering the horizontal rail and therefore the brace wire angle.

Typical Two-Post Brace Assembly


What size should a brace be?

Short and long as used in the previous discussion are relative terms. As a rule, a brace should be 2 1/2 times as long (between each driven post) as the height of the fence (if the rail is located at fence height). Therefore, for example, a fence brace should be ten feet long for a four foot fence height.

To avoid having to use a ten foot rail, use a shorter rail, set lower between the brace posts. A rule-of-thumb is to use a rail 2 times the fence height, set at 3/4’ s of the fence height. The example brace is then eight feet long and the rail at three feet from the ground on four-foot high brace posts. This uses economical common sized posts to achieve full strength. This is considered the standard brace portions and is shown above.

Where to locate a brace ?

A brace is required wherever an anchoring point for the fence wires is needed. Some of these points are dictated by the terrain (dips, rises, etc.) or things like gates. Otherwise, braces are installed at the appropriate wire tie-off distances (660 ft. for barbed or woven wire – 1320 ft. for smooth wire).

What types of braces are used ?

A wide variety of brace designs have been used. Table 1 illustrates designs that have been proven effective. To be effective in resisting fence loads, these designs must be properly constructed and installed in suitable soils.

All the designs have a tie off post (shown larger) that is braced in front, in rear or in both directions to resist movement. Other designs not shown may have the required strength but generally are either too costly, too difficult to construct or both.

Tests conducted by various researchers indicate that two-post brace assemblies under favorable soil conditions are capable of withstanding loads from 3000–6000 lb. of pull; whereas, three-posts assemblies are capable of almost double that capacity. In good soil conditions, a two-post brace assembly will be adequate for fences of 6 line wires or less.

In soils of low cohesive properties, such as sandy, very wet plastic clays or peats, or when using more than 6 line wires, three-post brace assemblies are used.

Where to tie off on a brace ?

Normally, the fence wires are tied off on the end post when two-post assemblies are used. To achieve optimum strength using three-post assemblies, line wires are tied off at the centre post and the rear panel is “slack” wired. This is to prevent any misalignment of the three driven posts from causing brace failure. Pulling from the center post of a two-post brace can actually re–align the posts whereas pulling from the rear post will cause buckling and failure.

What are the factors in brace rigidity ?

Some factors effecting the rigidity of a brace (in similar soil conditions):

• the deeper the post is set into the ground, the greater the load capacity

• the larger the diameter of the post the greater the load capacity

• a post driven into undisturbed soil has a greater load capacity than one placed in an over-sized hole and filled by ramming

• dry and clay type soils generally have more resistance than wet and sandy soils

A properly built three-post brace assembly has a greater load capacity than a two- post assembly, but is generally only required in poor soil conditions.

When are double brace wires used ?

Some braces, such as inline braces located in straight fence runs, anchor wires that pull in two directions. In these cases a diagonal tension wire is installed on the brace to resist each load. It is important that these two diagonals are allowed to function separately and they must not be wrapped together where they cross each other.

This type of brace will also be located at the hinge post side of a gate opening where the end brace assembly is anchoring the fence wire loads in one direction and supporting the weight of the gate in the other direction. The end brace assembly on the gate latch side will only be anchoring the fence wire loads and therefore require only one diagonal tension wire.

What are good construction methods and materials?

The importance of proper construction of brace assemblies cannot be over stressed. Posts must be properly sized and must be driven into the ground to the full depth required (i.e. 31/2 to 4 feet for most end posts). The driven posts must be in line with the fence as a small off-centre pull will tend to collapse the whole assembly. End or corner posts should lean away from the line of pull (up to 2 in from vertical).

As a rule-of-thumb for most fences, brace posts and rails are one diameter size larger than fence line posts. Fences that may have very heavy loading require brace posts two sizes larger (i.e. for confined areas such as pens or fences for large animals such as bison). The driven posts should be treated to resist rot but the brace rails do not normally require protection.

Standard high tensile steel wire is preferred for the brace wires as it has adequate strength. Two complete wraps are required. For fences with four strands or less, two wraps of a low tensile steel wire (such as most barbed wire) will suffice, but this brace wire will likely be a weak point. Fences that are expected to be ‘tested’ by livestock or windfalls should be constructed with superior braces. All brace wires should have Class 1 galvanization; Class 3 is preferred, especially in wet and coast locations.

Braces constructed with the ‘drill & pin’ method use steel rebar pins. Galvanized pins are unlikely required except in the very wettest environments.

Wooden brace ‘twist sticks’ should be a minimum 1.5 inch x 1.5 inch cross sectional size. They should be attached to the brace rail (a nail will do) to prevent accidental unwinding. If attached, the stick need not extend up past the brace rail and therefore will present no danger to livestock or wildlife that may attempt to jump the brace.

How to build a horizontal rail brace

While there are many possible brace designs, the horizontal rail brace is the most often used design. It provides the required strength in an easy to construct, economical layout.

This brace may be constructed as either a two-post brace (for normal soil and fence conditions), or a three-post brace (for poor soil or high fence load conditions). The two-post, horizontal rail is the standard brace for fences of 6 wires or less in suitable soils.

Construction steps for a two-post, horizontal rail, end brace

his brace may be constructed as either a two-post brace (for normal soil and fence conditions), or a three-post brace (for poor soil or high fence load conditions). The two-post, horizontal rail is the standard brace for fences of 6 wires or less in suitable soils.

Two driven posts are connected by a rail and tensioned together by a brace wire. The rail-to-post connections are drilled (to prevent splitting) and pinned or nailed. Two diagonal wraps of 12 1/2 ga high tensile smooth wire complete the brace giving it the required strength while maintaining flexibility under shock loads (such as falling trees).

Construction sequence is as follows:

• drive the end post with a 2 inch lean away from the fence load

• drive the brace post spaced twice the fence height from the end post

• mark the drill holes on both posts at approximately 3/4 fence height from the ground (i.e. 3 ft on a 4 ft fence)

• drill a 3/8 inch diameter hole through each post in line with the fence

• cut the rail length to fit snug at this 3/4 point

• drill a 3/8 inch diameter hole 3 inches deep in the center at each end

• place the rail in position lining up the drilled holes

• drive a 3/8 inch rebar pin through the posts and into each end of the rail

• leave 1 inch of pin protruding from the brace post; drive the pin flush in the end post

• drive a staple into the base of the end post about 4 inches from the ground

• hook one end of the brace wire onto the protruding brace post pin

• wrap two full turns around the staple and the pin

• pull all slack out of the brace wire

• hook the remaining end of brace wire around the pin

• staple off both ends of the brace wire

• twist the brace wire using a twist stick to remove the wire slack

• secure the twist stick to the rail (nail or wire on)

• attach the line wires

TYPES OF BRACES

Refer to Table 1, for an outline of the types of braces being discussed. They include single-post, two-post and three-post styles with horizontal and diagonal rails, for end and inline requirements.

SINGLE-POST BRACES

These braces are the simplest and least expensive but are generally used for only the lowest fence loading conditions. They should be constructed in firm soil. There are three basic types all using a post diameter size one larger than the line posts.

Basic Single-Post Brace

A wooden post driven 3-1/2 to 4 feet in firm soil can be used as an end post. One to three wires can be tied off at this post. These wires should only be low tensioned wires such as in an electric fence. Because it lacks any bracing the single post relies only on post-to-soil friction. Effectiveness can be increased by burying a 2 foot long block of wood (pressure treated) just below ground level at right angles to the pull of the fence wires to act as a thrust block. Refer to Figure 1. This design is suitable for an end brace but not generally for a corner brace.


Figure 1 Single-Post Brace


Single-Post, Deadman Anchor Brace

Figure 2 Single-Post, Deadman Anchor Brace

Adding a deadman anchor to the single post increases the load rating (Figure 2). Buried steel or wood anchors or steel screw-type anchors are used. Firm soil is required for best results. Steel “foot and cable” anchors are available that are driven into the ground then pulled to set the foot. Screw-type anchors (manual or machine assisted installations) are difficult to install in rocky soil. In light soils, the post may tend to be driven downward by the interaction of the fence and anchor loads.

This design can be used as either an end or corner brace. For corners, use two anchors to resist the fence loads of both fence lines. This design can support loads equal to a double span brace with suitable soils and deadman anchors.

Single-Post, Kiwi Brace

The third single driven post brace design is one termed the “Kiwi” brace from its use in New Zealand. It’s both simple and effective with load capacity equal to the single span braces. It is a modification of a diagonally supported post without the tendency for the diagonal to “jack-out” the end post. It is most important that the lower end of the diagonal rail be allowed to move freely to reduce this “jacking-out” tendency. A slider block is used for this as shown in Figure 3.

Figure 3 Single-Post Kiwi Brace

Two versions are used; one with a steel rod for a lower member and one with two wraps of htsw fence wire. Both are effective; the rod is likely to be more expensive but will resist side-to-side movement better. In either case a slider is used that will allow free movement of the diagonal in response to fence loads. Note that side to side movement of this rail is not desirable and a short stake may be required on either side of the lower end of the rail.

The lower tension member (rod or wire) should not be over tightened, only enough to remove any slack. The two wraps of htsw do not need to be twisted together. Once the fence wires are tied off to the brace and tensioned, the brace will “set”.

This design is suitable as an end and inside brace. When constructed as a corner brace, two diagonal braces and tension wires are used to resist the fence load in the two directions.

TWO-POST BRACES

These are the most commonly used braces and are built using either a horizontal or diagonal rail. Where soil conditions are suitable (firm, not sandy) they can be used in virtually all livestock fences. Generally the driven posts are one diameter size larger than the line posts and the rail the same diameter size as the line posts. They are simple to construct and effective if the following points are considered.

Two-Post Brace Recommendation

The two types of two post braces (horizontal rail and diagonal rail) can be considered to have very similar load capacities (in similar soils). However, the construction of the diagonal brace requires good connection of the rail ends to each post. These take more time and poor joints will allow movement and encourage failures. As well, the horizontal rail design can be loaded in both directions (as an inline brace) simply by locating the brace wire(s) in the correct direction. The diagonal rail design can only be loaded in one direction (reverse loading will cause the brace wire to slacken). It is for these reasons that the easier-to-construct horizontal rail design is usually recommended and the most often built brace assembly.

Two-Post, Horizontal Rail, End Brace

This is the standard design for fences of 6 wires or less in suitable soils (Figure 4, below). The construction sequence is given below.

This brace can be used as an inline brace where long runs of straight fence have no corners for tie off. In this case, fence wires will be pulling from two directions (not one as in an end brace) so two brace wires are required. The brace wires should be free to move independently, not twisted together. Note that double span braces are often used as inline braces because of their added strength (Figure 7, and in Figure 9).



Figure-4-Two-Post-Horizontal-Rail-End-Brace.


Two-Post Brace Construction

Two driven posts are connected by a rail and tensioned together by a brace wire. The rail-to-post connections are drilled (to prevent splitting) and pinned or nailed. Two diagonal wraps of 12 1/2 ga high tensile smooth wire complete the brace giving it the required strength while maintaining flexibility under shock loads (i.e. falling trees).

The two-post, horizontal rail is the standard brace for fences of 6 wires or less in suitable soils. Construction steps are:

• drive the end post with a 2 inch lean away from the fence load

• drive the brace post spaced twice the fence height from the end post

• mark the drill holes on both posts at approximately 3/4 fence height from the ground (i.e. 3 ft on a 4 ft fence)

• drill a 3/8 inch diameter hole through each post in line with the fence

• cut the rail length to fit snug at this 3/4 point

• drill a 3/8 inch diameter hole 3 inches deep in the center at each end

• place the rail in position lining up the drilled holes

• drive a 3/8 inch rebar pin through the posts and into each end of the rail

• leave pin protruding 1 inch from the brace post; drive pin flush in the end post

• drive a staple into the base of the end post about 4 inches from the ground

• bend the end of the brace wire into a “U” shape and hook onto the protruding brace post pin

• wrap two full turns around the staple and the pin

• pull all slack out of the brace wire

• hook the remaining end of brace wire around the pin

• staple off both hooked ends of the brace wire

• twist the brace wire using a twist stick to remove the wire slack

• secure the twist stick to the rail (nail or wire on)

• attach the line wires

Two-Post, Diagonal Rail, End Brace

This design is very similar to the previous except the rigid rail is placed diagonally instead of horizontally between the two driven posts (Figure 5, below). When loaded by the fence wires, this brace reacts as a horizontal rail brace except the diagonal rail applies its load to the bottom of the brace post rather than the top. The brace wire is under tension as the brace post is moved in the direction of the load. Good connection of the rail to the post is important to prevent the diagonal rail from slipping under load.

This brace tends to have less end post movement in the direction of the load (compared to the horizontal rail brace) but has a greater uplifting force on the end post. Deeper placement of this end post is recommended for all diagonal braces.


Figure 5 Two-Post, Diagonal Rail, End Brace


THREE-POST BRACES

When either fence loads or soil conditions require increased brace strength, a two-post brace is expanded by using three driven posts. Because of the difficulty in driving these three posts in direct line with the pull of the fence wires, three-post braces can fail prematurely if the incorrect tie off point for the fence wires is used.

Looking at Table 1,  it can be seen that a three-post brace is really just a two-post brace with additional rear bracing. The tie off point is clear: it is the centre post. Three end brace designs are traditionally used; double horizontal rail, double diagonal rail or mixed rail. The double horizontal rail brace can also be used as an inline brace when the extra brace wires are added. Other (non-traditional) three-post braces are also possible. These mainly combine a single span brace with additional support from a deadman anchor or the kiwi brace.

As for two-post braces, three-post braces generally have brace posts are one diameter size larger than line posts and the rails the same diameter size as line posts.

Three-Post, Horizontal Rail, End Brace

This is the standard design for fences of more than six wires or in poor soil conditions. Three driven posts are connected by two rails and two brace wires as in Figure 6. The construction sequence is similar to the two-post sequence on:

• drive the end post, preferably with a 2 inch lean away from the fence load

• drive the center post, preferably with a 1 inch lean away from the fence load

• drive the brace post

• space these three posts using the 2:1 minimum distance rule between each post

• drill the posts and rails and pin together

• leave 1 inch of pin protruding from the brace post

• wire both panels with the brace wires

• fence line wires will be tied off at the center post

• “slack” wire the rear panel


Figure 6 Three-Post, Horizontal Rail, End Brace


Three-Post, Horizontal Rail, Inline Brace

As with two-post, horizontal rail braces, this three-post brace can also be used as an inline brace (Figure 7). Fence wires from two directions will be tied off at the center post. To resist this loading, brace wires must be installed in both panels, in both directions, forming two “X”s. As with the two-post brace, these must be free to move independently, not twisted together.


Figure 7 Three-Post, Horizontal Rail, Inline Brace


Three-Post, Diagonal Rail, End Brace

This design is very similar to the previous except the rigid rails are placed diagonally instead of horizontally between the three driven posts (Figure 8, below). Brace strength is approximately equal in both. The same points discussed on page 4 regarding two-post horizontal versus diagonal braces applies to three-post braces as well; diagonal rails are more difficult to install correctly and can be loaded in one direction only (and can only be used as an inline brace as shown in Figure 9).


Figure 8 Three-Post, Diagonal Rail, End Brace


Three-Post, Diagonal Rail, “Inline” Brace

This is actually not a true three-post brace (as only the brace strength of a two-post brace is supporting either fence wire load) but rather two, two-post braces sharing an end post (Figure 9, below). Brace strength in either direction is not equal to other double span braces. This design is not normally recommended because for about the same materials and effort a three-post – horizontal rail inline brace has greater strength.


Figure 9 Three-Post, Diagonal Rail, Inline Brace


Three-Post, Mixed Rail, End Brace

This design is a mixture of the previous three-post braces. The center post is still the tie off location. The forward section has a diagonal rail and the rear section has a horizontal rail (Figure 10). Disadvantages are once again construction difficulty and one direction loading. This brace has strength equal to the previous three-post braces discussed.


Figure 10 Three-Post, Mixed Rail, End Brace


CHANGE OF DIRECTION BRACES

The previous braces have been at either the end of the fence or along a straight line section. Those braces had loads applied directly in line with the brace.

A change of direction, however, produces reactive forces that are not inline with the brace that can cause leaning or complete brace failure. This is commonly seen at fence “dog legs.” The amount of force is proportional to the degree of angle change and acts through the center of the corner (Figure 11). Although a small change of direction produces a small force, a standard brace is not usually able to resist it. At a 90° corner, the brace is better able to resist it due to the alignment of brace members.



Figure 11 Forces in a Brace Assembly at Changes of Direction



The post at the change of direction must be supported and, although the loads are similar, different solutions are available for different situations. In the following discussions, the angle of change of fence direction is:

• defined as the angle between the line of sight of one fence to the other fence line

• can be estimated by the measurements given in Figure 12.



Figure 12 Estimating Fence Line Angle of Change


Change of Direction: At Tie Off Points

If it’s appropriate to tie the fence wires off a separate end brace can be constructed for each fence section as in Figure 13 below. Rather than ‘share’ a common tie off post, each section is tied off to a separate end post producing no forces out of line with the braces. This requires an extra driven post per corner and ‘slack’ wiring the opening between posts but ensures a stable corner.

• for corners less than 60° use either separate corner posts, Figure 13, or brace the corner post, Figure 15

• for 60° to 90° corners use a standard shared-post corner brace as in Figure 14


Figure 13 Separate Braces at a Tie-Off Corner ( changes less than 60°)



For changes of fence direction from 60° to 90°, a normal shared-post corner will be effective (Figure 14). This brace can be used at a tie off corner or used between tie points with the fence wires continuous around the brace (in this case special stapling techniques are used to reduce wire-to-post friction around the corner post ensuring flexibility of the wires when loaded).



Figure 14 Shared-Post Brace at a Corner ( for changes greater then 60 degrees )



Change of Direction: Between Tie Off Points

Often the fence change of direction occurs when it is not appropriate to tie off the fence wires (i.e. less than the 660 feet or 1320 feet rule for tensioning wire). Rather than have many short sections of fence, the wires can continue around the change of direction post provided the post can be adequately supported and special stapling methods are used. Figure 15, illustrates three possible braces.

For changes of direction greater than 60° use the brace in Figure 14.


Figure 15 Change of Direction Braces (for changes less than 60°)


The post at the change of direction must be supported and, although the loads are similar, different solutions are available for different situations. In the following discussions, the angle of change of fence direction is:

· defined as the angle between the line of sight of one fence to the other fence line

can be estimated by the measurements given in Figure 12,