Market report - Polytunnels

Only one polytunnel is designed and built to British and European standards, Sally Drury discovers.

ROBUS is designed and built to British and European standards - image: NP Structures
ROBUS is designed and built to British and European standards - image: NP Structures

You would think we would be used to extreme weather events. Nevertheless, I had a heart-in-mouth moment last week as I stood in a multispan polytunnel with the wind blasting at 110kmph and rain lashing so hard I could not even hear myself shout.

I stuck my head out of the door. Visibility was less than 100m as the wind drove the rain, and anything not tied down, across the field. I dashed back inside. But would the polytunnel blow away too? I need not have worried. This tunnel, at Sooty's Plants, between Southport and Preston, is a ROBUS and has been designed and manufactured to British and European standards. At the moment, it is the only one.

It seems odd, but polytunnels are not subject to building regulations, nor do they have to be compliant to any British or European standards. Tunnels have evolved since the 1970s and multispans from the 1980s, but it has all been a bit ad hoc and largely based on experience, needs-must reasoning and, of course, cost.

Cost issue

With increasing competition in the polytunnel market and as grower margins are squeezed, cost has become a central issue. That has led, inevitably, to specifications being scrutinised and many cases trimmed. High winds have been a worry for polytunnel owners but then last winter's heavy snowfalls caused a number of structures to buckle and in some cases completely collapse.

In 2013, the Construction Product Directive means that when buildings are manufactured they will have to be CE marked. But it is not clear whether this will apply to polytunnels or greenhouses.

When we think of making something stronger, thoughts mostly turn to "beefing" it up, adding more, or using thicker, metal. But that is costly. Looking for another solution, the research and development department at NP Structures of Colne in Lancashire undertook a herculean project to design a polytunnel that would stand up under wind and snow loads as directed by the British Standards Institution and the European Committee for Standardisation. The team included mechanical engineers, a structural engineer and an architect.

The process involved value engineering and used software that identifies parts too weak or too strong so they can be adjusted accordingly. The result is the NP Structures ROBUS - a multispan structure to replace the company's Split Link. The new build is stronger yet does not cost as much.

"We found that construction of the classic multispan polytunnels can be very time-consuming, complex and costly. Our aim was to cut down on complicated assembly and standardise on fittings and fasteners," says NP Structures director Ian Wolfenden. "We used computer-based stress analysis packages to optimise every member in the new structure."

The value engineering identified points of weakness in historical polytunnel designs, allowing the inclusion of extra bracing from columns to hoops in those areas and so maximising material efficiency and reducing costs. New fittings were developed to aid easier manufacturing and construction. The hoops are 60mm-diameter Z35 galvanised tube with 50mm tie bars and 32mm-diameter bracing in each arch.

At Sooty's Plants, the column thickness is 60x60mm Z35 galvanised box section - specified to suit the conditions of the location - with NP-designed channel foundation posts. The wider aluminium gutters are standard on the ROBUS.

Structural assurance

Wolfenden is confident that using a British Standard-compliant structure is the way forward for growers. "We can give assurances that the structures will resist the dead (self weight of structure and equipment) and live loads (wind, snow and crop loading) imposed on them, as specified in the relevant standards," he says.

Clearly, the loadings are site specific and each structure accompanied by structural calculations. With vast differences in the conditions found in, say, Oxford compared to a coastal location, each ROBUS is built for the particular site's altitude, average snowfall and wind speeds. The result is a structure that is safe for staff as well as the crop. It may also result in lower insurance premiums in the future.

Wolfenden explains: "We foresee a hardening of the stance taken by insurance companies, to the point where they may ultimately refuse to insure polytunnels unless they are BS/EN compliant."

NP has also taken health and safety into account. The ROBUS has been designed to negate the issues associated with working at heights. The main frame, made of pre-galvanised steel to reduce the need for hot-dip galvanising, is built at ground level and then raised into position. And for cladding and maintenance tasks, the provision for a fall-arrest system has been incorporated. The gutter has a channel extruded that allows a buggy system to be used to clip a harness to and move freely along the length of the structure.

The ROBUS has been designed in line with the following:

BS 6399-2:1997 Loading for buildings. Code of practice for wind loads.

BS 6399-3:1998 Loading for buildings. Code of practice for imposed roof loads.

BS 5950-1:2000 Structural use of steelwork in building. Code of practice for design. Rolled and welded sections.

BS 5502-22:2003 Buildings and structures for agriculture. Code of practice for design, construction and loading

BS EN 13031-1 Greenhouses. Design and construction. Commercial production greenhouses.

The first things you notice about the construction at Sooty's Plants are the height, the volume of air and the amount of light. The framework offers eave heights of 2.7m with eight 6.5m bays, hoops being spaced at 2.5m centres. Fans provide the conditions needed for bedding production. As I stand deafened by the wind, I know it has been designed and built to cope with the weather a few kilometres from the Irish Sea.

Gothic Solo built for strength

Designed for extended or full four-season cover, the new Gothic Solo Series from Haygrove Tunnels of Herefordshire combines strength with large air volume.

The tunnel can be used for trellised crops such as tomatoes, cucumbers, peppers, strawberry tabletop production, raspberries and bramble production. It has also been designed to provide a robust "cold frame" structure for nursery and ornamental growers.

Said to be capable of managing snow events and built to be strong in the wind, the Gothic Solo structure is specified according to location requirements.

The foundations are created by screwing 0.8m steel anchors into the soil. From these, the main hoop is erected to form the arch of the tunnel. The standard specification is based on the hoops spaced at 1.4m centres and uses the highest grade of extra-strong galvanised steel. The Gothic comes complete with adjustable telescopic steel cross trellising to enable crops to be trellised directly onto the structure.

For good tractor and machinery access, the vertical walls are 1.2m and the apex of the tunnel is built to a height of 4.3m - enabling large-tractor access. Length can be as long as practically required.

As with all other Haygrove Tunnels, the Gothic Solo can be fully vented by lifting the polythene from the sides using the Haygrove venting tool and venting clip system. The air volume for a 2.2m section is 53.5cu m. This is 19 per cent more than the Super Solo and 45 per cent more than a standard Solo tunnel.

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