Valente Lab : Iron posts used in vineyard trellis systems

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Modern viticulture requires a careful and precise evaluation of all the variables that come into play when establishing vineyard systems, to achieve structures suitable for the current growing techniques, which increasingly require the use of machines for managing the processes.

From mechanical harvesting, to grass mowing and grass weeding in the cultivation belt, to adverse climatic conditions, farming systems must effectively withstand all types of vibrations and stresses, while allowing users to work based on their needs.
Therefore, the choice of materials for the vineyard system deserves to be thoroughly analysed, in particular with regard to the posts that constitute the backbone of the entire structure.
As we know, there are various types of posts used to manufacture a vineyard farming system, but nowadays the most widespread is undoubtedly the metal (steel) posts.

Steel posts are considered among the best products in the construction of modern vineyard systems, mainly thanks to the fact that this material is naturally elastic and light .
Steel, actually, is the name given to an iron alloy containing carbon in a percentage not exceeding 2.11%. Above this percentage, the properties of the material change and the alloy takes the name of cast iron. In addition to carbon, further alloying elements may be present such as copper, silicon, lead, manganese, etc.
Based on the carbon content, steels are divided into:

ULTRA MILD: carbon content included between 0.05% and 0.15%
SEMI MILD: carbon content included between 0.15% and 0.25%
MILD: carbon content included between 0.25% and 0.40%
SEMI HARD: carbon content included between 0.40% and 0.46%
HARD: carbon content included between 0.60% and 0.70%
VERY HARD: carbon content included between 0.70% and 0.80%
ULTRA HARD: carbon content included between 0.80% and 0.85%

The lower the carbon content is, the lower the mechanical strength and brittleness will be, whilst the ductility and weldability values of iron will be high.
Therefore, as the carbon content in the steel changes, some important physical-mechanical parameters change as well.
To obtain a steel post featuring the highest resistance and elasticity values, the quality of the material, the right amount of carbon and the section must be taken into consideration.
Depending on the type of steel used, different material strengths can be obtained. Please find below a table comparing steel for profiling and forming and structural steel.

We can clearly see how the first has an Rm value (mechanical resistance) stretching over a very wide range, while the second has a fixed and well-defined value.
This means that materials have either variable resistances (in the first case) or guaranteed resistances (in the second case).

As for our posts, we decided to opt for structural steel, the only one that can guarantee a defined mechanical strength value on which to perform all the necessary calculations with great precision.
As for the shapes of the posts, the profiling technique allows to obtain numerous different results starting from a flat steel strip (our posts have reinforcement ribs that guarantee high resistance, especially on the sides, but at the same time flexibility and elasticity); however, the higher the carbon content in the steel is, the harder it is to shape it according to the desired shape.
It is therefore clear that the steel used in posts production  must have the right balance between strength and workability.
Again, we started by choosing the best materials on the market (we will talk about them later in this article), and then we managed to define the guaranteed resistance values for the metal vineyard piles that serve as a calculation basis to verify their dimensional limits and to understand the distances to be left between the piles.

The larger the post section is, the stronger and the more stable the product will be. However, due to the cost of raw material, the steel post sections are still thin and therefore they may more easily move in the ground. To solve this problem, steel posts must be driven deeper into the soil than the ones in other materials.
In this regard, wind is certainly one of the fundamental factors to understand the statics of structures , and it is therefore necessary to understand all of its characteristics and then use these data during the static and preliminary calculation phases to verify the stability of a farming system.
The wind speed and penetration inside the vineyard have been thoroughly studied using the shadow measurement technique. The data thus obtained, after the engineers completed all the necessary checks, provided very interesting information.

These data are fundamental for the subsequent static verification of the growing systems because, by means of the engineering calculation, they will provide precise indications on how to use the  posts, the spacing, the lengths of rows and the size of the farming system as a whole.
The structural verification involves the use of parameters where the dimensions of the farming system, the stress it is subjected to and the spacing between the rows and the posts are identified.
They are briefly listed hereunder:

SELF-WEIGHT LOADS ANALYSIS

a specific weight of 78,5 kN/m³ (7,85 kg/dm³) is used for the ropes

Ropes were pre-tensioned with a preload equal to 2,50 kN (250 kg)

HAILSTORM

The hail load considered is 0,06 kN/m² (6 kg/m²)

WIND

The farming system may be subject to a maximum peak wind speed equal to: Vmax = 33,0 × 3,6 = 119 km/h

Farming system size: 150 meters wide x 150 meters long
Max distance between rows: 4,00 meters
Max distance between piles: 5,00 meters
Maximum farming system height above ground: 2,20 meters

Here below is the example of a calculation of a row of vineyards using the parameters described above and with the technical characteristics of our materials.

Going back to the characteristics of the posts, a widely debated question concerns their coating.The goal is that the safety factor (referred to in the FS table) is > = 1. This result would provide evidence on the ability of the vineyard rows to withstand the stresses described above.

The following summary table shows the results of the safety coefficients of the posts used in different situations and with different percentages of wind penetration.

The posts produced with this technique are also defined as “pre-galvanized”, because they are profiled using an already galvanized sheet supplied by the steel mill.
Steel, as we have seen, in fact has excellent mechanical strength characteristics, but also a great problem: oxidation. Indeed, steel subjected to atmospheric stresses tends to oxidize, and therefore needs to be properly coated to ensure its durability.
The most used treatment to protect steel from oxidation and corrosion is galvanizing, which can be carried out in two ways: by immersion (also defined as “hot” or “wet”) and/or in a continuous mode (UNI standard -EN-ISO 1461, UNI EN 10042, UNI EN 10147).

In the first case, the post is immersed in molten zinc at 450° after a degreasing, washing and preheating treatment. This technique allows to uniformly apply a layer of zinc with a thickness varying between 45 and 55 microns over the entire surface.

In the second case, however, continuous galvanizing is used to coat the steel strips where the metal to be protected flows “continuously” inside the zinc tank. The technique allows to uniformly apply a layer of zinc with a thickness varying between 20 and 50 microns, depending on the immersion speed, over the entire surface.

The debate over which the best technique is appears very heated; however, if we intend to analyse the situation from an objective point of view, we need to keep in mind some key concepts:

  • For steel that was pre-galvanized at a steel mill, the amount of zinc applied to the sheet metal to be used to produce the posts (and therefore the resulting thickness) varies according to the client’s request (in our case the posts manufacturer). However, the manufacturers of hot-dip galvanized (or wet) steel posts cannot adjust (by increasing or decreasing) the amount of galvanization (and consequently the thickness) at their own discretion, therefore, such a quantity can be considered as standard and common for all.
  • Modern steel mills can offer coatings made with stronger and more durable metal alloys than zinc alone, such as zinc-aluminium and zinc-aluminium-magnesium alloys, which give the steel greater protection and higher duration over time. Such special coatings can only be applied in steel mills.
  • The zinc-aluminium-magnesium alloys, in particular, in addition to being more resistant to corrosion than zinc alone, also have a particular self-healing characteristic that allows the coating to regenerate after profiling, covering those parts that, after being cut, appear to be free of galvanization.

Following the above considerations, and based on numerous scientific analyses and laboratory tests, for our EVO iron posts we have decided to aim for the top quality by using Evoluzinc (Magnelis®), a carbon steel coated on both sides with a zinc-aluminium-magnesium alloy.

Its characteristics, certified by the manufacturer, are:

  • Excellent corrosion resistance (at least 3 times more than galvanized steel for outdoor applications)
  • Full edge protection thanks to self-protection properties on cut edges
  • Extremely high resistance in very difficult environments (marine environments, presence of chemicals – such as those commonly used in viticulture)

Here is a short video by the manufacturer as an evidence to said the statements.

The excellent corrosion resistance properties of Magnelis® have been measured with a Preliminary Technical Evaluation of Material (ETPM) by the CSTB (Center Scientifique et Technique du Bâtiment) and certified by a number of external laboratories, including the SP (Science Partner) and the DIBT (Deutsches Institut für Bautechnik).

Furthermore, the results of our laboratory tests on materials coated with different zinc plating, give us further confirmation of what has already been highlighted.

The standard pre-galvanized post (Z275 coating) was very damaged after the tests.

The hot dip galvanized (or wet) post performed better, but was largely scattered with traces of rust.

Our EVO post was found to be free from traces of oxidation.

The tests thus clearly show the primacy, in terms of resistance to oxidation and corrosion, of the pre-galvanized post coated with the special Evoluzinc alloy (Magnelis®).
Returning to the initial focus, what conclusions can we draw from this analysis of the specific characteristics and methods of use of steel posts for vineyards?
We certainly understood that to be sure of the stability of a farming system we must, first of all, verify  the technical characteristics of the posts that support it and of the structural calculations carried out to design it.

And the choice of the best support posts must be preceded by a thorough technical evaluation of the materials and of the system to be achieved. This is the only way to make sure that the results will meet the expectations.