What a 40-foot open top can carry, and what actually limits it
Every project reaches this question sooner or later: how much steel goes into one container. The owner usually expects a tonnage. The honest answer is a shape, and the tonnage follows from it — not the other way round.
What follows is how we actually pack, with the numbers off the boxes we load.
The envelope you actually have
The containers we load are 40-foot open top: about 11.85 m internal length, 2.30 m internal width, and roughly 2.58 m of usable height up to the top rail, with a payload of 28.4 tonnes.
Read that as three separate constraints, because they fail in three separate ways:
- Length sets the longest single piece you can ship. Anything past it needs a splice, a different packing pattern, or a different method entirely. This is the constraint that decides more designs than any other.
- Width is the least flexible. Members either fit across the floor or they do not, and there is no clever way around it once the frame is fabricated.
- Height only becomes useful because the box is open. On a closed container the ceiling is a hard stop; here you load from above, so height is a function of your lifting plan and the tarp, not of the steel.
One detail that surprises people: the door opening at the end is narrower and lower than the box itself. Anything that has to be slid in rather than lowered in has to pass that opening, and that is a different number from the internal width.
Open top is a loading method, not a box type
The reason we use open top is not the extra height. It is that the pack can be assembled on the ground, lifted in bundles from above, and lashed as bundles. A closed box forces you to load member by member through a door, which is slower and puts more handling on every piece.
What that changes in practice:
- The tarp becomes part of the shipment. It has to be intact, lashed to proper points, and it has to survive a sea crossing. A tarp that flaps is a tarp that fails, and water in a bundle of purlins is a claim, not an inconvenience.
- Lashing points matter more than the load. Bundles move differently from single members, and a bundle that shifts inside the box arrives as damaged steel and a bent frame — before anyone has looked at the site.
- The crane at origin is part of the price you are not quoted. Loading from above needs a crane with the reach and the capacity, and that is arranged at the port, not at the factory.
The payload figure is not the road limit
28.4 tonnes is what the box and the vessel accept. What the road from the port to your site accepts is a different number, and it is usually lower.
This is where shipments go wrong. The steel is packed to the container's limit, the box arrives, and then the trailer that has to move it cannot legally carry it — because of axle limits, because of a bridge on the route, because the last four kilometres are a gravel access road that was never designed for a loaded trailer at all.
Two consequences worth designing around:
- On some projects you deliberately ship light. Paying for air inside the box is cheaper than paying for a re-handling operation at the port, and it is definitely cheaper than a trailer stuck at a weighbridge 200 km from site.
- The destination's road limit is a design input, not a logistics detail. If the road caps you below the container, the packing plan has to split differently — and that changes how the frame is fabricated, because splices have to be where the site can make them, not where the drawing preferred.
Ask the destination forwarder for the road limit before the packing plan is fixed. After is too late, and it is a question with a free answer.
The free days at destination are a buffer, not storage
At the destination port we can apply for around 21 days of free container time. That is enough to schedule a trailer, arrange a crane, move the box somewhere else, and return it empty — which is exactly what a project site with a bad access road needs.
Three things to understand about that window:
- The clock starts at discharge, not at your convenience. It begins before your foundation is finished, before your crane is booked, and often before customs has released the box.
- It is a scheduling buffer, not a warehouse. Using it to store steel on the quay means you are paying for the days after it, and those days are billed to whoever holds the box.
- It buys you the thing you cannot buy later: the ability to sequence the arrival around the site instead of the site around the arrival.
The practical use is to book it whenever the site is not ready to receive at the exact discharge date. It costs nothing to apply for and it converts a hard deadline into a soft one.
What nests, what gets cut, what travels separately
The packing pattern is not a warehouse decision. It is decided at the fabrication stage, because it determines where members are spliced.
- Columns and rafters nest. They are the bulk of the tonnage and the reason a box reaches payload before it reaches volume.
- Purlins and girts bundle. They are volume, not weight — a bundle of purlins fills a box long before the weight is anywhere near the limit, which is why mixed loads beat uniform loads.
- Cladding is the volume trap. Panels occupy space with almost no mass. A shipment that is mostly cladding will cube out at a fraction of the payload, and the freight per tonne looks terrible for a reason that has nothing to do with the supplier.
- Bolts, base plates and connection plates travel separately — and must be marked by erection mark. Unlabelled small parts turn into a week of site labour sorting through crates, which is the most expensive week in the whole project.
One rule that saves real time on site: pack in erection sequence, not in drawing order. The first steel off the truck should be the first steel up. If the ridge members are at the back of the box, the crane sits idle while the site crew unloads everything to reach them.
What to tell a supplier before they pack
Five things, and the first two are almost always missing from the enquiry:
- The road limit at destination, and whether the last stretch can take a loaded trailer at all.
- What lifting is available on site. A mobile crane, a hired crane, or a truck with a boom — each one changes the maximum bundle weight you can ship.
- Whether you want one shipment or phased arrivals, and what has to be on site first.
- The site's access: gate width, turning space, and whether a 12-metre bundle can physically enter.
- How long you can hold a box at the port, so the delivery sequence matches the free time.
None of these are structural questions. All of them change the structure.
The mistake I see most
Planning the building and planning the shipment as two separate jobs, in that order. The building is adjustable; the container is not. Design to the envelope and the freight becomes predictable. Design first and fit later, and you spend the difference on splices, rework and site labour — with no line item anywhere that says so.
If you take one thing from this: the container is the only fixed object in the project. Treat its inside dimensions and its road limit as design constraints, the same way you treat span and eave height.
Written by Johnny, who advises owners and project managers in Africa and South America on steel buildings. If you have a frame designed and no packing plan yet, send me the member list and the destination port and I will tell you where it will not fit.
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