HDGP in the name of this model stands for hot dip galvanized pipe, and on a film greenhouse it is the frame that decides whether the house is still straight after ten winters. The film itself is light, so the arches are not sized by what they carry across the roof; they are sized by the wind that pushes on the house, by the snow that may lie on it, and by the moisture that works on the steel every day. This page is about that galvanized structure: how the pipe section follows the span, how the arches are tied and spaced, and which coating class belongs in your order. The house itself is one of the models in our film greenhouse range.

Hot dip galvanizing is neither paint nor an electroplated finish. The tube is cleaned and then dipped whole into molten zinc, and the zinc reacts with the steel to form a zinc-iron alloy layer that becomes part of the metal surface instead of sitting on top of it. Because the whole section goes into the bath, the coating reaches the bore, the outside wall, the weld seam and the cut ends in the same pass, and no part of the tube is left bare for a brush to miss.

A film house asks more of that coating than a lot of steel structures do. The inside of the house is a wet space: moisture condenses on the underside of the film overnight and runs down the arches, so the frame stays damp for hours at a time instead of drying out with the weather. The bottom of every arch stands in soil or on a concrete kerb, and ground contact is the one place a coating can never dry. And the covering is a thin sheet rather than rigid glazing, so the frame is normally expected to stand through several cover changes before anyone looks at the steel again. A bonded zinc layer answers all three conditions without any repainting on site.

This model is listed at a design wind load of 0.35 kN/m2, and that figure is the starting point for the whole frame rather than a footnote to the covering. The wind pushes on the film, the film pushes on the arches, and the arches pass the load down to the base through the purlins, the bracing and the fixings that tie the frame together. The film is also stretched, and a stretched film pulls back on the frame; the stiffer the frame, the tighter the cover can be held and the less it will work loose in a gust.

Where a site also has to be checked for snow, the same structural drawing carries that check, because snow load is a property of the location and not of the model. What matters for the buyer is that both figures are written on one drawing and both are answered by the same members: the arch tube, the purlins that tie adjacent arches, the bracing between them and the channel that runs along the frame. None of those four is an optional extra on a house of this size.

The frame also has to be sized for what the grower will hang from it later. Cooling and heating, irrigation, ventilation, fertilization, shading and benches are all added systems, and they are attached to the structure rather than to the film. A house ordered as a bare shell today and equipped in two seasons time still needs the arch section and the purlin spacing that those systems will ask for, which is why the structure is specified against the finished house and not against the day it is delivered.

The house is built on an arch roof, and the span can be ordered at 6.4 m, 8 m, 9.6 m or 10 m, with a total structure height of 4.3 m to 6.5 m. Span and height are chosen together. A wider arch gives more floor per running metre and more room to work inside, but it also raises the bending the arch has to resist, so the section has to grow with it. A taller arch gives the house more air volume above the crop and a steeper roof, which helps the film shed water and snow instead of holding a flat puddle that stretches the cover and loads the frame at a single point.

The reference layout for this model is 64 m by 40 m, an area of 2,560 square metres, which is a middle-scale commercial house rather than a garden tunnel. Running length is flexible, and film is supplied cut to the length of the house so the cover follows the run instead of being pieced together from standard rolls. Adding length repeats the same arch, while adding span enlarges it, so the two changes do not weigh the same on the structural drawing.

The arch tube is rolled from our hot dip galvanized pipe range, which runs from 21.3 mm to 219.1 mm outside diameter with wall thickness from 1.5 mm to 12 mm. Read that range against the span and the shape of the job becomes clear. A short arch on a 6.4 m house works with a small tube and a light wall; a 10 m arch carrying the same wind pressure over a longer curve needs a larger diameter and a heavier wall to hold its shape. Stiffness rises quickly with diameter and also with wall thickness, while the arch has to carry its own added weight along with the roof.

Bay spacing works the same way from the other direction. Arches stand on a regular bay, and the closer they are set, the less roof area each arch carries and the smaller the tube can be; open the bay up and every arch takes a wider strip of roof. Span, bay, tube diameter, wall thickness and the design loads are therefore one decision rather than several, and they belong on the same structural drawing. The tube that comes out of that decision is the galvanized pipe we roll to those dimensions, ordered by outside diameter, wall thickness and length.

The exact section is fixed on the drawing against the span and the site, and it is confirmed in writing before production starts, so nothing in the quotation rests on a guess. Where a drawing has not yet been issued, the practical route is to send the span, the total height and the wind load, and let the arch be sized from those three before the covering is discussed.

A single arch is only a shape; the house appears once the arches are tied to one another. Purlins run along the frame and connect one arch to the next, bracing holds the run square, and the covering channel follows the same lines as the pipes. The channel is part of the structure even though it also holds the film: this design uses a special grooved channel frame to reinforce the panels against wind and weather, so the member that grips the cover is doing structural work at the same time. The film may only be held by polyethylene with aluminium clips in that channel, which is why the channel line, the clip spacing and the arch bay are drawn together.

The small parts travel with the house. Fittings, clips and the fixings that make the connections are packed as the install accessories group, so the frame and everything that joins it arrive as one shipment against one drawing. Nothing downstream can correct a frame that is out of line, because the film, the channel and every added system are fixed to it, and a cover stretched over an untrue arch stays visible for the life of the film. If you want to see how those groups break down part by part, they are set out on the parts a film greenhouse is built from.

Coating life is not a single number, because zinc is consumed at a rate set by the air around it. ISO 9223 and BS EN ISO 14713-1 rate a site by corrosivity, and the expected time to first maintenance for a standard coating follows those classes: heavy industrial air (class C4) about 13 years; urban conditions (class C3) about 30 years; coastal conditions (class C5) about 50 years; clean suburban conditions (class C2) 50 years or more without repair.

The class only becomes an order line when it is paired with a coating thickness. Thickness is what decides which of those figures a project lands on, and it is measured in micrometres with a non-destructive gauge or quoted as zinc mass per square metre. For greenhouse frame tube the usual specified average is 70 to 85 micrometres under BS EN ISO 1461, and either that thickness or the equivalent zinc mass belongs in the enquiry next to the corrosivity class. A coastal site and a clean inland site can share the same pipe and the same drawing, but they should not share the same coating line without the class being named. The way the coating is applied and measured is the same process whether the pipe ends up in an arch or in a guard rail.

One caution on reading those figures: they describe how the coating behaves, and they are not a warranty period. Coating life, the structural design life of the frame and the commercial warranty on a house are three separate statements, and a quotation that blends them into one number is not being clear with the buyer.

A frame enquiry that can be priced without a second round of questions carries six items: the span, the total structure height, the number of film layers, the design wind load, the site corrosivity class, and the coating thickness or the zinc mass per square metre. Add the running length if the house is not the 64 m by 40 m reference layout, and the destination port so the freight can be quoted at the same time.

The house ships in three groups that follow the assembly order on site: metal structures, covering system and install accessories. Galvanized pipe is supplied in the lengths the drawing calls for, and custom cut lengths are available on many of the components, so the frame arrives ready to stand instead of being cut down on site where the cut faces would need to be touched up. For general background on protected cultivation and agricultural structures, public references such as the FAO publish material that sits behind a decision like this one.

A frame specified this way suits the whole range of protected crops on the same structure: vegetable, fruit, flower and seed nursery work all use the same arches and the same span table. The house is suitable for all climate conditions and adapts to any type of landscape, and because the covering goes on as a single or a double layer, insulation performance and energy consumption are decided by the cover rather than by the frame.

The original description of this house mentions both an aluminium frame and a hot dip galvanized steel pipe frame, and the two statements describe two different jobs rather than two competing structures. The load-bearing frame - arches, purlins, bracing and the ground fixings - is hot dip galvanized steel pipe, which is what carries the wind load and what stands in the soil. The aluminium parts are the film fixings: the house may only be covered by polyethylene held with aluminium clips, and those clips lock the cover into the grooved channel, where they are not asked to carry structural load.

That split is worth knowing before the order is placed, because it tells you what to inspect on delivery. Pipe diameter, wall thickness and coating thickness are structural items and are checked against the drawing; clip profile and channel depth are covering items and are checked against the film. A house with the wrong clip size will lose its cover in the first gale, and a house with an undersized arch section cannot be rescued by better clips. The trade press that follows commercial greenhouse construction, such as Greenhouse Grower, is a useful place to see how projects of this scale are specified.

We have produced greenhouse structures and their steel frames for above 35 years, and the frame on this house comes out of our own factory rather than from a trading desk. Our engineer department reviews span, wind load and site class with every buyer who sends a drawing, and where a customer asks, we send engineers abroad to guide the installation on site. Growers and project contractors visit the factory each year to see the galvanizing line and the frames being made before they place an order. If you want the arch sized against your own drawing, call +86 191 5068 3942 or write to sales@cngreenhouses.com with the span, the total height and the load figures you are working to.

Q: What does HDGP stand for on this film greenhouse?
A: Hot dip galvanized pipe. The load-bearing frame of the house - the arches, the purlins, the bracing and the ground fixings - is made from steel pipe dipped whole in molten zinc, so the coating is bonded into the steel surface rather than applied on top of it, and it covers the bore, the outside wall and the cut ends in the same pass.

Q: How do you choose the pipe diameter and wall thickness for a given span?
A: The span sets the bending the arch has to resist, and the site wind load sets the pressure it is resisting. Between them they fix the arch section. The tube is rolled from a range of 21.3 mm to 219.1 mm outside diameter with wall thickness of 1.5 mm to 12 mm, and the exact figure for your house is confirmed in writing from the structural drawing before production begins rather than being left to the quotation.

Q: How long will the galvanized frame last in my area?
A: That depends on the corrosivity of the air, not on the name of the process. Under ISO 9223 and BS EN ISO 14713-1 a standard coating gives about 13 years to first maintenance in heavy industrial air (class C4), about 30 years in urban conditions (class C3), about 50 years in coastal conditions (class C5), and 50 years or more without repair in a clean suburban setting (class C2). Name your class and a coating thickness such as 70 to 85 micrometres average, and the figure follows.

Q: What wind load is this house designed for?
A: The design wind load listed for this model is 0.35 kN/m2, and the arches, purlins, bracing and ground fixings are sized together to answer it. Snow is a property of the site rather than of the model, so where a snow check is needed it goes on the same structural drawing. Both figures are answered by the same members, and neither is an optional extra.

Q: Can I add cooling, heating or benches after the house is standing?
A: Yes, and that is the reason the frame should be specified against the finished house. Cooling and heating, irrigation, ventilation, fertilization, shading and benches all hang from the structure rather than from the film, so the arch section and the purlin spacing have to be right for the systems you intend to use, even if they are fitted two seasons later.

Q: How does the galvanized frame arrive, and is it cut to length?
A: In three groups that follow the assembly order on site: metal structures, covering system and install accessories. Pipe is supplied in the lengths the drawing calls for and custom cut lengths are available on many components, so the frame stands without being cut down on site, where every fresh cut face would need touching up.