Loading Dangerous Goods: Containers, Trailers and Air ULDs

This is a guest article written by Per Törnblom, a former Captain and Supercargo.

On 15 July 1971, the cargo ship Poona was being loaded in Gothenburg harbour. Steel beams were going into a hold that already held barrels of sodium chlorate and rapeseed oil from an earlier leg - two substances that should never share a compartment, since chlorate is a powerful oxidizer and needs little more than a spark to turn spilled oil into an explosive fire. A beam swung as it was lowered and punctured a barrel of each. The resulting explosion blew the hatches off, scattered burning debris across both banks of the river, and killed four people - a crew member and three stevedores. Nothing about the barrels themselves was wrong; they were simply stowed next to cargo they were never allowed to be near.

That’s the failure mode this guide is about. Dangerous goods behave like any other cargo on a spreadsheet, but get the physical loading wrong and the consequences are worse than a wasted container. Two incompatible classes sharing the same box, a heavy drum loaded off-center on a trailer, or a ULD packed past its class quantity limit are all loading mistakes, not paperwork mistakes. This guide covers what to think about when packing dangerous goods into sea containers, road trailers and air ULDs, and how to turn those requirements into rules Cargo-Planner enforces automatically.

This is not a substitute for the regulations themselves. Which classes are incompatible, how far apart they need to be, and how much of a class is allowed in one unit is defined by the IMDG Code for sea, IATA DGR for air, and ADR for road. Cargo-Planner doesn’t classify cargo or look up those rules for you - it gives you the tools to encode whatever the regulation requires into a load plan, and then packs around them.

What to think about when loading dangerous goods

Consideration	Why it matters
Segregation	Incompatible classes can’t share a unit, or need a minimum gap between them, even if neither is stacked.
Stacking and support	Some classes can’t be stacked at all, can’t go under other cargo, or can’t be the base another class rests on.
Placement and access	Dangerous goods are often required near a door for fast access, or kept off specific sides or zones.
Weight distribution	A heavy hazardous drum shifts axle load on a trailer or trim on an aircraft more than its weight alone suggests.
Quantity limits	Regulations cap how much of a class can travel in one container, ULD or trailer.
Equipment restrictions	Some classes need specific equipment - a reefer for temperature-sensitive goods, or exclusion from others.

By equipment type

Sea containers

The IMDG Code’s segregation groups are the starting point - explosives kept away from flammable liquids, oxidizers away from organic material, and so on. Within a single container there usually isn’t room for the finer stowage distinctions used at the vessel level (separated by a compartment, separated longitudinally); in practice it comes down to a binary question per pair of classes - can they share this container or not.

Road trailers

ADR sets which classes can share a trailer and how. The extra factor on a trailer is weight distribution: a heavy hazardous load positioned to one side or one end can put a single axle over its legal limit even when the trailer’s total weight is fine. Centering the load laterally matters more here than it does in a container that’s simply craned onto a ship.

Air ULDs

IATA DGR is generally the strictest of the three - tighter segregation requirements and lower quantity limits per ULD than IMDG or ADR allow for the equivalent classes in a container. Weight and balance also carry more consequence: an unevenly loaded ULD affects the aircraft’s overall trim, not just the space inside the ULD itself, which is worth keeping in mind when planning a full aircraft load.

How Cargo-Planner helps

Once you know what a shipment’s dangerous goods requirements are, there are two places to encode them, depending on how strict the requirement is.

For a straightforward “these two classes can never share equipment” rule, tag each item with a class and mark the classes as incompatible in the segregation table. From that point on, the calculation engine simply won’t place them together - no per-shipment checking required.

For anything more specific than “can’t share equipment,” custom rules cover the rest:

Requirement	Custom rule
Two classes may share a unit but can’t be adjacent	Can only be side-by-side with
Two classes need a minimum physical gap	Must keep distance
A class can’t be stacked, or can’t support other cargo	Cannot support / Can only support
A class must stay reachable near a door	Must touch side
A class must load last so it comes off first	Must be loaded before
A class is restricted to certain equipment (e.g. reefers)	Must be loaded in / Cannot be loaded in
A class has a fixed quantity cap per unit	Cannot exceed quantity
A heavy class needs centered weight on a trailer	Must be loaded along centerline

Say you’re loading Class 3 flammable liquids and Class 8 corrosives that can share a container but need at least a meter of separation. Rather than eyeballing that gap in the load plan, a single Must keep distance rule between the two classes holds it on every container the engine generates, including reloads after the cargo list changes.

Once a ruleset like this works for one shipment, save it as a load preset so the same segregation and spacing logic applies automatically the next time you plan that customer’s or that class combination’s cargo, instead of rebuilding the rules from scratch.