Don't know your Z28 from your Z35?
Tunnel talk from WET

Well-Pict European Tunnels (WET) has always focused on giving their customers the best advice necessary to successfully grow protected crops; this dedication to providing superior customer service has highlighted the need to expand the 'WET' range of products to include a selection of permanent tunnels.

Over the last two years WET has concentrated mainly on the development of their Spanish tunnel range, when it came to permanent tunnels the WET engineering team decided to move away from the trend of simply strengthening Spannish tunnels to double up as permanent greenhouses, and developed a range based on other proven designs. Below left and middle is the vertical sided greenhouse; these are available in either single or multispan, in widths of 6, 8 and 10m. Suitable for wide range of crops the side and roof venting come as standard. The Hoophouse (right) has proven to be an extremely popular design throughout Europe, the superior design and build quality make it ideal for the trellised vegetables and fruit and ensure it can withstand wind speeds in excess of 65mph, it is available in widths of 6, 8, 5 10 and 12m.

The introduction of WET into the Uk market brought an end to the monopolised pricing structure that has been in place since Spanish tunnelsl took off in the UK some years ago. With this levelling of the playing field, UK based tunnel manufacturers/suppliers started using the steel quality, strength and specification in their marketing and introduced catch phrases like Z28 and Z35 in an attempt to impress the customer base with engineering jargon and regain market share.

From its inception, WET has made it a policy to make use of the highest quality materials and has specifically selected specification certified material best suited to the given applications. There are numerous standard organisations around the world and each has a different specification code for any given product. Further to this, once you get your head around the specification code you must then attempt to understand the engineering terms used to describe these codes.

The properties of steel are determined by its chemical composition. By varying the amounts of carbon, silicon and other elements, the steel producers manufacture stell that has various physical and mechanical properties, all of these suited to different applications.

There are many physical and mechanical properties that can be used to compare one steel to another. The two mechanical properties that stand out in determining steel strength are 'Yield strength' and 'Ultimate Tensile Strength'.

Yield strength is an indication of amount of force required to get the steel to 'yield' i.e. how much force is required to bend the steel? The higher this value the most resilient the steel is to permanent deformation / damage.

Ultimate Tensile Strength is an indication of amount of force required to get the steel to 'ultimately' fail i.e. how much force is required to break the stell.

In the case of steel used in the production of tube that WET and other tunnel manufactureres use to produce Spanish tunnel components we again have various specifications. For example, the steel used in the production of some Pre-Galvanised tube has yield and ultimate tensile values than that use din the production of 'black' tube used in the production of Post-Manufacturer Hot Dip Galvanised components. Due to the high nature of the Pre-Galvanising process, high specification steel cannot be used and hence the lower mechanical strength of this tube.

Z28 and Z35 are specification codes given to a Hot Dip galvanised tube. This tube can easily be identified by looking at the internal weld seam. Hot Dip galvanising completely coats the steel with Zinc (both inside and outside). If an unprotected weal seam is visible then the tube is NOT Z28 or Z35 but rather a Pre-Galvanised tube with a lower mechanical strength.

WET makes use of a high strength black steel to manufacture its Y Legs, Anchor Legs and table top Legs and then Post manufacture Hot Dip Galvanises all of these components. This process provides a complete protective coating both inside and outside and leaves a layer of zinc on the stell that is a minimum 4 times thicker than the zinc on the Pre-Galvanised Tube.

The use of Pre-Galvanised tube by other manufacturers in the production of their legs not noly results in mechanically weaker leg, due to the lower yield and ultimate tensile values of the tube, but the welding process burns the thin layer of zinc around the weld leading the weld unprotected. The use of a zinc spray is purely cosmetic and provides limited protection to the outside weld area and not protection to the inside weld area. WET makes use of the highest possible strength Pre-Galvanised tube for the production of its struts and hoops. The mechanical strength of othis steel is significantly higher that other Pre Pre-Galvanised tube available in the UK.

November 2005

Crop PRO-TECH Ltd the UK leading supplier of Spanish Tunnels, Polytunnels, Greenhouses, Bed formers, Poly-layers, Poly winder, Horticultural Machinery, Table Tops, Growing Systems, Agricultural nettings, Agricultural Polythene.