A container on a road trailer experiences hard braking. A container at sea experiences weeks of continuous motion in three axes: rolling, pitching and heaving, plus the occasional heavy weather event. Cargo secured well enough to reach the port is routinely not secured well enough to reach the destination.
The reference document is the IMO/ILO/UNECE Code of Practice for Packing of Cargo Transport Units, universally called the CTU Code, published in 2014. It is a code of practice rather than a convention, so it is not directly mandatory everywhere, but it is the recognised standard, it is referenced in carrier contracts, and it is what an expert will be measured against if a claim is disputed.
The forces you are designing against
The CTU Code sets out acceleration coefficients by area of operation, as a fraction of gravity. For sea transport, the design forces are broadly:
- Longitudinal, fore and aft: around 0.3g to 0.4g depending on sea area
- Transverse, side to side: up to 0.8g in unrestricted ocean service
- Vertical: the load can be pressed down at up to 1.0g on top of its own weight, and it can also go light
Read the transverse figure again. A 2,000 kg machine can push sideways with a force equivalent to 1,600 kg. That is what a couple of ratchet straps thrown over the top are being asked to hold, and it is why they do not.
The four ways to restrain a load
- Blocking. Filling the space so the cargo has nowhere to go. Always the first choice, because it does not rely on anything staying tight.
- Bracing. Timber, steel or board structures transferring force from the cargo into the container walls, corner posts and floor.
- Lashing. Webbing, chain or wire under tension. Splits into top-over lashing, which works by increasing friction, and direct lashing, which takes the load in tension along the line of the strap.
- Friction. Not a method on its own, but a multiplier on everything else.
Friction is doing more work than you think
The friction coefficient between the cargo and the floor determines how much of the restraint is free. Bare timber on a steel or ply container floor gives roughly 0.3. Anti-slip mats raise it to about 0.6, which can halve the number of lashings required.
Anti-slip matting is cheap, fast to place, and consistently the highest return securing measure available. If you do one thing differently, do this one.
Dunnage
Dunnage is the filler material: timber, honeycomb board, foam, airbags. Two rules:
- Fill the voids, especially at the door end. The most common single failure in container packing is a well built stow with a gap at the doors, allowing the whole load to walk aft over a five week voyage. It arrives against the doors, and it falls out when they are opened, which is also a serious safety problem for whoever opens them.
- Wooden dunnage is regulated. Scrap timber cut at the loading bay is wood packaging material under ISPM 15 and needs treatment and an IPPC mark. Inspectors look for exactly this.
Dunnage bags
Inflatable paper or polywoven bags placed in voids between pallets. Rated by the void width they suit and by burst pressure, and available in single and multi-use grades. They are excellent for filling the gaps between pallet rows and useless for restraining heavy point loads, which need direct lashing. Place them against flat, load-bearing surfaces, never against a single carton face.
Lashing points and their limits
Containers have lashing points along the top and bottom rails. They are rated, and the ratings are lower than people assume, often a few hundred decanewtons for top rail points. Do not assume a lashing point will hold a tonne. For heavy items, spread the load across multiple points, use the bottom rails which are stronger, and consider whether the cargo should be on a flat rack lashed to the platform instead.
A practical sequence
- Plan the stow before anything moves, so gaps do not exist by design.
- Heavy low, light high, and distribute weight evenly fore and aft and side to side. Uneven loading can overload a road axle even when the total is legal, see container weight limits and VGM.
- Lay anti-slip mats under anything heavy.
- Block and brace against the walls and corner posts.
- Direct lash anything heavy, wheeled or unable to be blocked, in all four directions.
- Fill remaining voids with dunnage bags or honeycomb.
- Close out the door end properly with a bulkhead or bracing.
- Photograph the finished stow before the doors close. It costs nothing and it is the only evidence you will have.
General guidance. Heavy, high value or unusual cargo should be secured to an engineered lashing plan prepared by a competent person, not to a general checklist.
