Cargo can pass every stability and longitudinal strength check and still damage the ship if its pressure is too high in one place. Dented tank top plating, a deflected hatch cover or a cracked barge deck often starts with a point load that was never spread. This guide explains how local strength limits are stated, how to calculate footprint pressure and how to spread loads with dunnage or steel beams.
Three ways local strength limits are stated
| Structure | Usual form of the limit | Notes |
|---|---|---|
| Tank top (hold bottom) | t/m² uniformly distributed load | For general cargo, bulk and project cargo in the hold |
| Tween deck and weather deck | t/m² uniformly distributed load | Weather decks also take green water |
| Hatch cover (general cargo) | t/m² uniformly distributed load | Usually far lower than the tank top |
| Hatch cover and tank top (containers) | Maximum stack weight per 20' / 40' stack | Load enters through sockets and corner castings |
| Ro-ro deck | Axle load and tyre pressure | See also axle load |
| Barge deck | t/m² uniformly distributed load | Stated in the barge documents; often asked for by project cargo shippers |
The exact figures differ for every ship, and even between holds on the same ship. They come from the deck load plan, the loading manual or class-approved structural drawings. Never borrow a "typical" figure from another vessel.
Uniform load vs point load
A t/m² limit is almost always meant as a uniformly distributed load (UDL), a load spread across a whole panel between supports. Deck plating is carried by stiffeners, frames, floors and girders, and a uniform load is shared sensibly between all of them.
A point load acts on a small area, such as a support leg, a skid or a wheel. The pressure under it can be many times the UDL limit even when the average over the whole hold is comfortably low. If a point load lands in the middle of a plate panel, away from the supports, the plate can be permanently deformed. So there are two questions to ask every time: what is the pressure under the contact area, and is that contact area sitting over supporting structure?
Footprint and load spreading
Principles for spreading loads with dunnage or steel beams:
- Use the real footprint. Take the area of skids, tracks, legs or saddles that touch the deck, not the plan area of the cargo.
- Put beams over structure. Lay beams across the stiffeners and over floors, girders or frames, so the load goes into the supports rather than only into the plating.
- Make sure beams are stiff enough. A flexible beam does not spread the load evenly and most of it stays under the skid. For heavy cargo, beam stiffness should be checked by an engineer.
- Add the beam and dunnage weight to the total pressing on the deck.
- Do not assume a spreading angle (for example 45° through timber) unless that method is accepted by the ship or the surveyor.
Timber dunnage also adds friction for lashing and sea fastening; securing forces can be estimated with the sea fastening calculator.
Worked example: an 80 t transformer on the tank top
Illustrative example. An 80 t transformer (see transformer cargo) stands on two steel skids, each 4.0 m × 0.30 m. The planned hold has a tank top limit of 15 t/m² (illustrative), and company policy caps use at 80% of the limit, which is 12 t/m².
| Step | Calculation | Result |
|---|---|---|
| Skid contact area | 2 × 4.0 × 0.30 | 2.4 m² |
| Direct pressure on tank top | 80 ÷ 2.4 | 33.3 t/m² (222% of limit) |
| Minimum area at 12 t/m² | 80 ÷ 12 | 6.7 m² |
| HEB 300 steel beams (0.30 m flange) | 8 beams, 3.0 m effective bearing length | 7.2 m² |
| Beam weight (about 117 kg/m, 3.5 m long) | 8 × 3.5 × 0.117 | about 3.3 t |
| Pressure with beams | (80 + 3.3) ÷ 7.2 | 11.6 t/m² (77% of limit) |
With eight beams laid across under both skids and bearing on the floors, the pressure falls from 222% to 77% of the limit. The key assumption is that the beams are stiff enough and bear evenly on the tank top; if not, the real pressure under the skids is higher. For cargo like this, the final calculation should be checked by an engineer and agreed with the chief officer. The wider topic is covered in heavy-lift cargo stowage.
A quick comparison with other cargo (illustrative figures):
| Cargo | Weight | Contact area | Average pressure | Note |
|---|---|---|---|---|
| Tracked excavator | 30 t | 2 × 3.7 × 0.6 = 4.44 m² | 6.8 t/m² | On a 3 t/m² hatch cover that is 227%, so spread it or move it to the tank top |
| Steel coil | 20 t | Line contact on dunnage | Very high locally | Needs cradles or cross dunnage and a dedicated stowage pattern |
| Transformer on beams | 83.3 t | 7.2 m² | 11.6 t/m² | Example above |
See also the cargo pages for excavators and steel coils.
Container stack weights on hatch covers
For containers, hatch covers and tank tops usually carry a stack weight limit per stack. Illustrative example: a 40' stack on a hatch cover holds four boxes of 28 t, 26 t, 24 t and 22 t, a total of 100 t. The 40' stack limit in that bay is 90 t, so it is at 111%. One fix is to swap the 26 t box with a 14 t box from a stack that still has headroom, then re-sequence heavy to light: 28 + 24 + 22 + 14 = 88 t (98%). Make sure the stack that receives the 26 t box is still within its own limit.
Points to watch:
- Stack weight is often also capped by lashing forces and stack height, not only by hatch cover strength.
- A 20' box in a 40' position on deck (or the reverse) often has a different limit and needs special fittings.
- Weight order (heavy at the bottom) also affects lashing forces; see container lashing on ships.
Barges and project cargo
On barges, project cargo usually sits directly on deck. The deck t/m² limit and the positions of frames and longitudinal bulkheads decide where skids may be placed. The guide to barge deck cargo stowage covers layout and securing, and the deck load calculator gives a quick footprint pressure check.
Where planning software fits
STOWAGE.ID Vessel Stowage checks footprint pressure and stack weights against the limits entered for the vessel and flags positions that exceed them. It is a shore-side planning and decision-support tool; the official references remain the vessel's class-approved loading computer, stability booklet and deck load plan. For the heavy cargo workflow, see heavy-lift stowage.