Box Section

What Is Box Section and Where Is It Used?

Box section is a hollow structural member formed from coil steel into a tube on a cold-forming line, crushed into a square or rectangular cross-section and closed with a longitudinal weld. A hollow cross-section gives far higher bending resistance than a solid bar of the same weight; hence load-bearing frames built from box section.

Fence posts and horizontal rails, gate and leaf frames, canopy and pergola frames, shelving and workbench legs, handrails, container and cabin carcasses are the most common applications. The sections we hold in stock have a galvanized finish and are sold by the kilogram. We sell from our base in Ostim and ship Türkiye-wide, and an order can be combined with ready-made systems such as panel fence.

Unlike hot-rolled sections, box section is cold formed and can be made with thin walls. That brings light weight and economy; in return, a thin wall dents more easily under a single impact. Where vehicles pass, surround the posts with a guard element or step up to the next size. A dented box section loses much of its load capacity, because its cross-section is distorted.

Stock Section Sizes and Technical Table

We hold three sizes. The per-metre weights in the table are calculated from the theoretical cross-sectional area.

Rounded corners can put the weight on the scales a few per cent below the theoretical value, while the galvanized coating adds a little back. Length and batch details are confirmed when ordering.

From Metres to Kilograms: The Weight Formula

Box section is sold by weight, but a project is measured in metres. To convert, first find the cross-sectional area, A (mm²) = 2 x wall thickness x (short side + long side - 2 x wall thickness). Then multiply that area by 0.00785; the result is kilograms per metre.

For 40x40x2.0: A = 2 x 2 x (40 + 40 - 4) = 4 x 76 = 304 mm², weight 304 x 0.00785 = 2.39 kg/m. For 20x20x1.5, A = 3 x 37 = 111 mm², weight 0.87 kg/m. For 40x60x2.0, A = 4 x 96 = 384 mm², weight 3.01 kg/m. The formula subtracts the inner void from the outer rectangle; that is why the wall thickness is deducted twice.

Worked Example: Kilograms for a Fence Frame

Say you are enclosing a 40 metre perimeter with 2.5 metre posts at 2 metre centres and two rows of horizontal rail. The number of posts is 40 / 2 + 1 = 21; 21 x 2.5 = 52.5 metres of 40x60 section. That weighs 52.5 x 3.01 = approximately 158 kilograms.

The rails call for two rows x 40 metres = 80 metres of 40x40 section; 80 x 2.39 = approximately 191 kilograms. The order comes to roughly 349 kilograms, or around 366 with 5% added for cutting waste. It works in reverse too: a one-tonne order of 40x40 section is 1000 / 2.39 = approximately 418 metres.

Do not forget gate and access openings; they change both the post count and the running metres of rail. On sloping ground post lengths are not constant either, as embedment depth and free height shift along a line running downhill. There, working from an average length plus an 8% allowance is quicker than sizing every post one by one, and safe enough.

Choosing Wall Thickness and Section

The choice turns on the free span and the load coming onto it. 20x20x1.5 is the light-work section: cage infill, partitions inside a frame, shelf bays, light lids and wire tensioning frames. It should not be used alone over long free spans, as it deflects easily.

40x40x2.0 is the general-purpose carrier, the standard for horizontal rails, gate leaf frames and medium-duty frame work. 40x60x2.0 gives a higher moment of inertia vertically, so it suits posts, columns and members taking wind load. Set a rectangular section with its long side parallel to the load; fitted the wrong way round, a 40x60 is barely better than a 40x40.

The second factor is how the post is fixed. A post seated on concrete with a base plate is stressed more at its foot than one set into the ground; there, thickening the plate and strengthening the anchor layout beats going up a section. If the post is set in, leaving roughly one fifth of its length in the concrete is common practice.

Welding, Cutting and Looking After the Galvanizing

Cutting and welding galvanized section burns the coating away in the heat-affected zone. Zinc fumes must not be breathed in; always ventilate an enclosed space. Sealing the welds and cut ends afterwards with a zinc-rich repair paint is the single step that most extends outdoor life. In practice, rust almost always starts at a weld.

Close off the open ends of hollow sections too. Water entering an open end freezes in winter and stresses the section from inside; a plastic cap or sheet-metal cover is enough. On infill and cladding work, seating sheet products such as perforated sheet into a box-section frame gives a neater, vibration-free result than screwing straight through.

Bolted connections expose one drawback of a hollow section: tightening the bolt collapses the thin wall inwards. Fit an internal sleeve or reinforcing bush at the hole, or take the connection through a plate. This detail decides long service life in systems that will be dismantled and re-erected.

The Difference Between Box Section, Steel Tube Post and Rebar

Three different members can do the same job, but the results differ. The square cross-section of box section, with its flat faces, makes welding and bolting straightforward; panels, sheet and wire frames sit on it comfortably. A steel tube post meets wind load much the same way from every direction, and its circular section resists twisting; hence its preference as a fence post.

Rebar, is made to work inside concrete, so using it as an exposed structural frame is wrong. In gate fabrication the frame and leaf are mostly box section; for anyone after a ready-made solution we also carry products such as our double swing gate.

What Determines the Price of Box Section?

The first factor is the current tonne price of galvanized coil; the section is priced by adding forming and welding labour on top. The second is wall thickness: a thin-walled section uses less material per metre, but line output drops, so cost per kilogram never moves in a straight line.

The third is order tonnage, length preference and delivery distance. Coating quality and surface requirements also feed in. All our prices include VAT and exclude carriage. Current figures are in the price table on this page; because raw material moves, we do not write price figures into the body text.

Section sold by weight but used by length is a trap when quotes are compared. Two suppliers may quote the same price per kilogram, yet if their wall thickness tolerances differ you pay for a different weight over the same metre. Always compare on the same section and wall thickness, converted into a cost per metre; that habit alone makes a real difference.

Ordering, Site Survey and Delivery

When ordering, tell us the section, the wall thickness and the total weight in kilograms. If all you have is the running metres, we can work the tonnage out together with the formula above; the commonest mistake is assuming an order was placed in metres, then being surprised by a quantity invoiced by weight. Site survey is free within Ankara and is the quickest route to the right section.

Delivery is always charged, including within Ankara; carriage is added to the order separately and is never built into the product price on any consignment. We tell you the amount in advance, from weight and distance. Installation is not a standard service; our business is selling and shipping product, and our teams attend site only on perimeter projects of approximately 1000 metres and above.