This tube is square and rectangular hollow section, hot rolled to an ASTM grade and galvanized after forming so the zinc covers the outside, the bore and the cut ends. Hollow section is used where a member has to resist bending in one direction and take a fixing on a flat face, which is why it appears in frames, rails and gate structures as well as in the heavier parts of a greenhouse. The same stock goes into the frames we supply, so a site can work from one grade.
Round pipe does most of the load carrying work on a farm, but it is not always the right section. Where a member has to take a fixing on a face, or has to resist bending in one direction and stay slim in the other, hollow square and rectangular section does the job that round cannot, and it does it without a bracket welded on to make a flat surface.
A flat face is the practical reason. Rails, gate frames and the members that carry cladding or trellis all need a surface to bolt or screw into, and on a round tube that means adding a plate or a clamp. Hollow section gives the same load capacity with a face already there, which removes a joint and a place for water to collect.
The other reason is stiffness in one plane. A rectangular section is deeper in one direction than the other, so a member can be sized to resist bending where the load actually comes from rather than being made heavier all round. That is why the pitch of a frame is often set by the section chosen for the rails rather than by the uprights. Frames using this stock are shown on the film greenhouse range page on this site.
Galvanizing is done on the tube rather than applied afterwards, so the zinc reaches the bore and the cut ends rather than sitting on the face that can be reached with a brush. On a hollow section the inside is a closed void, and it is where an uncoated tube fails out of sight, so a coating that covers it is the difference between a member lasting and a member being replaced.
Outside, the coating does the work at the two places a frame fails first: the joint, and the point where the member stands at ground level or passes through a wall. Cut ends are bare steel, so they are treated at the time the tube is cut rather than at the end of the build.
The tube is supplied to be worked on site, and hollow section is straightforward to cut and drill. It can be bolted, welded or joined with a mechanical connector, and the flat faces make a bolted joint simple because the mating surfaces are already square to each other rather than needing a shaped bracket.
Two points are worth holding to. Welding burns the coating back along the tube, and on hollow section it also opens the end void to moisture unless the joint is sealed, so welded joints are the ones to inspect at the first season check. Where the tube is drilled for a bolt, the hole is treated at the time it is made, because a bare hole in a wet frame is where the rusting starts.
The table below is the standard stock. Section, wall thickness and length are set to the drawing.
| Type | Round, square and rectangular hollow section |
| Standard | ASTM |
| Technique | Hot rolled |
| Surface treatment | Hot dip galvanized |
| Alloy or not | Non-alloy |
| Jointing | Bolted, welded or mechanical connector |
| Uses | Structural frames, rails and greenhouse members |
| Delivery time | Set out in the quotation for each order |
Tube is delivered in bundles, so unloading and stacking are part of the job. Bundles are lifted with slings rather than forks wherever possible, because a fork that slides under a bundle drags the coating off the bottom layer, and the coating is the reason the tube was bought galvanized rather than painted.
On site the stock should be kept off the ground on bearers and covered, so water does not sit inside the open ends through the wet season. Hollow section holds water in the end void if it is left open, and on a finished frame the ends that sit at ground level or face upward are the ones to seal. A round tube on the same grade is described in the heavy duty hot dip galvanized steel pipe.
This tube is used for greenhouse rails and members that carry a fixing, for gate and fence frames, for shelter structures and for general construction work on a farm. It suits growers building or extending a frame from a drawing, contractors who need one grade of hollow section across a job, and operations that want a structure they can repair with a section rather than replace as a set.
Research and extension material on horticulture and protected crop production is published by Michigan State University through its horticulture department. Production and technical reporting for commercial growers is published by GrowerTalks. The section the frame is built from is set out on the structure greenhouse page.
When the member needs a flat face to fix to, or needs to resist bending in one direction more than the other. A rail or a gate frame would need a bracket added to a round tube; hollow section already has the surface.
The bore. The zinc is applied to the tube rather than sprayed on afterwards, so the inside void is coated, and on a hollow member that closed space is where an uncoated tube fails out of sight.
A galvanized steel frame is rated for 15 to 20 years under normal conditions, with fixings and any cut or drilled ends checked as part of the season service.
We provide a 10 year structural warranty and a 5 year anti-corrosion warranty on the galvanized steel. Details are set out in the contract for each project.
Send the sections, wall thicknesses and quantities you need along with the application and the delivery port. We reply with a quotation and the delivery schedule.
