The doors on an arriving container bulge outward, and the consignee is warned not to open them without support. That is the visible end of a decision made weeks earlier at the factory: cargo stacked to the door with nothing blocking it, no void filling, and a centre of gravity in the wrong half of the box. A container is not a room with a lid. It is a structural unit that gets lifted, tilted, shunted and rolled, and it has to be packed for all of that.
Start with the ceiling. The maximum gross mass and the tare are stamped on the CSC safety approval plate on the door, and the payload is the difference between them, so it is read off the plate rather than assumed from a table. That figure is rarely what binds in practice. Road weight and axle limits on the drayage legs are usually stricter, they differ at origin and destination, and a box that is legal on a Shenzhen chassis can be over the limit on a European one.
Longitudinal distribution is the check most often skipped. The centre of gravity should sit as close to mid-length as the cargo allows, and the working rule is that no more than about sixty per cent of the payload should fall in either half of the container. A legally loaded box with the weight bunched at one end can still overload a chassis axle, which is the fault the weighbridge finds and the trucker refuses. For dense or uneven cargo, plan the position of each item, not just the total.
Vertically and laterally the aim is a low, central centre of gravity. Heavy items go on the floor and lighter ones above, and nothing heavy sits on a carton stack rated for less than it weighs. Weight should be balanced across the width, because a laterally offset centre of gravity makes the container roll harder at sea and sit badly in the stack. For project units the centre of gravity is calculated and marked on the packaging, and that mark is what the planner and the crane driver work from.
The container floor takes distributed load well and concentrated load badly. It is rated for a forklift axle load, not for the feet of a machine tool, and a point load pressed onto a single cross-member will work through plywood in transit. Spread concentrated loads across several cross-members using timber bearers sized for the job, and run the bearers across the container rather than along it. The same principle applies to steel coils, transformer bases and anything else standing on small contact areas.
Securing is designed against real forces, not against an average. The CTU Code published by IMO, ILO and UNECE sets out design accelerations by mode, and they are not comparable: rail shunting produces the highest longitudinal figures, a sea passage the highest lateral and vertical ones from rolling, and road braking sits between them. A box moving factory to rail to vessel to truck must be secured for the worst leg it will see. Friction is a contribution to restraint, never the whole of it.
Void filling is the cheapest securing there is. Fill gaps with airbags rated for the pressure and positioned against solid faces, block and brace with timber fixed to bearers rather than to the floor, and lash to the container's own lashing points, never to the side corrugations. Lashing points are individually rated, and the rating comes from the container owner rather than from judgement. The door end gets a proper barrier, because an airbag pressed against a door is what makes doors bulge.
Two compliance items live inside the loading job. Any solid wood packaging, bearers and blocking included, needs the IPPC mark showing heat treatment under ISPM 15, and untreated dunnage has held whole containers at destination. Dangerous goods loading requires a container packing certificate confirming segregation, securing and correct marking. Photograph the load in stages, from empty floor through first tier, mid-load and door row to sealed doors with the seal number legible, because that sequence is the only usable evidence when a claim arrives.
Put the plan on paper before anyone starts. A loading plan states the order of loading, the position of the heavy items, the bearer and blocking specification, the dunnage list and the expected gross mass, and it is worth an hour of a planner's time against the cost of a shifted container. Where we load or supervise loading at origin, that plan, the stage photographs and a weighbridge ticket stay with the file, and for oversized units the lashing arrangement is drawn before the cargo leaves the factory.
