Points to note on fencing
It is an interesting fact that a very large number of blokes who live in, or have spent some time in, the country claim expertise in farm fencing, and they cannot refrain from airing their "expertise" to the new chum. Unfortunately, the advice given is rarely the best advice available, and this is especially the case where individual circumstances are not considered. The lesson here for newcomers to farm fencing is to read the most up to date material on fencing available, from technically qualified sources, and to apply common sense to adapt this information to the specific situation. Friendly advice can be included in this process, but it should not taken as the primary source.
Recommended reading would be the most recent agricultural department publications or pamphlets and the "Waratah Fencing Manual" which can be obtained through your local rural products supplier. Much of what follows has been taken from the "Waratah Fencing Manual", which at the time of writing was the most up to date publication available.
Strainer PostsStrainer posts are the most important components in any fence. If any of these fails, the fence fails, therefore, it is most important that they are erected correctly and are strong enough for the job. The functions of strainer posts are to provide:
It is very important to consider engineering principles before embarking on a fencing project. Research undertaken in the United States into the best method of fence erection found that vertical movement of the strainer post is the most frequent type of failure. Some important findings of this work are discussed in the following paragraphs.
The following table shows exactly what sort of results occur the deeper the post is set in the ground.
These results show that by increasing the depth, in this case, by 150mm (6in), the total load carried by the fence is more than doubled while the horizontal movement has been reduced by nearly 50%, and the vertical movement by 33%. This is a lesson that should never ever be forgotten.
This next table illustrates the benefits of driving the strainer post into undisturbed soil versus placing it into an oversized hole, back filling with earth and ramming.
It can be seen that where the post was driven into undisturbed soil, it withstood a higher load, the horizontal movement was reduced by 60%, and the vertical movement by 80%. Another valuable lesson. Where driving is not an option, the next best approach would be to drill the post holes. In this case, however, the auger size should be noted and posts obtained that are only slightly less in diameter than the auger.
These findings disprove a commonly held belief that the greater the size and weight of the strainer post, the less the chance that it will move. In fact, it is quite possible a 15cm post driven 100cm into the ground to out perform a 75cm post dug into the ground. Thus, considerable savings in time and material costs can be achieved, as well as better performance, by following these engineering principles.
The US research also found that "the horizontal stayed or boxed assembly was 25% more effective than the diagonally stayed unit. However, on a material cost basis, the box assembly could be expensive and it is more time consuming to erect". One could infer from this that, if diagonal stays are used, the depth that the post is sunk in the ground is an even more important consideration. It is also important that the diagonal stay be attached about 2/3rds of the way up the vertical post, whilst the other end of the stay be placed on a base plate that is itself is bearing on undisturbed ground. The positioning of the diagonal stay is important because, unless it is correctly positioned, it can act as a fulcrum and increase the likelihood of vertical movement of the post.
© 2000 ACGA