A flat rack is a heavy-duty platform with end walls at the front and back but no roof and no side walls. It is the choice for cargo that is too wide, too tall or too heavy for a dry or open top box: excavators, transformers, steel coils and fabricated steelwork. Two things decide whether a shipment works: its out-of-gauge dimensions and how its weight reaches the floor.
20FR and 40FR dimensions
Typical values from the Stowage.id catalogue (height measured from the deck to the top of the end posts or end walls):
| Specification (typical values) | 20FR | 40FR |
|---|---|---|
| Inside dimensions (L × W × H) | 5,638 × 2,228 × 2,233 mm | 11,652 × 2,228 × 1,955 mm |
| Doors | none | none |
| Reference volume | 28.0 m³ | 50.8 m³ |
| Tare | 2,740 kg | 5,000 kg |
| Max gross mass | 34,000 kg | 45,000 kg |
| Max payload (evenly spread) | 31,260 kg | 40,000 kg |
| ISO type code (examples) | 22P1 / 22P3 | 45P1 / 45P3 |
Volume is only a reference on a flat rack because cargo often extends beyond the sides and top. The payload is far higher than a 20ft dry container, but only when the weight is spread as the rating assumes.
Fixed ends vs collapsible ends (P1 and P3)
Under ISO 6346 the type characters P1 denote a platform with fixed, complete end walls, while P3 denotes folding end walls. Collapsible flat racks are therefore often coded 22P3 or 45P3. The collapsible design lets empty units be stacked, which cuts repositioning costs, and allows long cargo to be loaded with the ends folded down. Fixed-end racks are simpler and stiffer. Always check the ISO type code and the CSC plate of the unit you receive, because ratings and post heights differ between owners.
Out-of-gauge cargo: width, height and length
Out-of-gauge (OOG) cargo protrudes beyond the profile of a standard box:
- Over-width (OW): measured per side against the standard external width of 2,438 mm.
- Over-height (OH): the part of the cargo above standard container height, roughly above the tops of the end posts.
- Over-length (OL): cargo projecting past the end walls; rarely accepted and very restrictive.
OOG cargo takes extra slots on board because the neighbouring or overstowed positions cannot be used. Carriers therefore charge OOG surcharges, often linked to the slots lost, and restrict the unit to top-tier deck positions or dedicated places in the stowage plan. Amounts vary, so check the carrier's official tariff.
Concentrated loads and lashing points
The 40,000 kg payload of a 40FR applies to weight spread along the deck. A heavy machine resting on a small area creates a concentrated load that can overstress the floor bearers. Carriers publish concentrated-load tables (for example, the maximum weight for a given length of support); ask for the table that applies to your unit.
The fix is to spread the load with timber or steel beams laid across the side rails so the bearing length increases. Flat racks have lashing points along the side rails and in the deck, each with its own rating. Use chain or webbing lashing with a suitable MSL, plus blocking against the end walls. The principles are in the cargo lashing and securing guide, and the Lashing Calculator gives a first estimate of how many lashings you need.
Worked example: a machine on a 40FR
A contractor ships a machine of 10,500 × 2,900 × 2,600 mm weighing 22,000 kg from Tanjung Priok. It sits on two steel skids, each 6 m long.
- Length: 10,500 mm fits within 11,652 mm, so no over-length.
- Over-width: (2,900 − 2,438) / 2 = 231 mm per side.
- Over-height: 2,600 − 1,955 = 645 mm above the tops of the end posts. Confirm how the carrier measures OH.
- Load per metre: 22,000 / 6 ≈ 3,667 kg per metre of deck. For comparison, 40,000 kg spread over 11.652 m works out at about 3,433 kg/m. The machine is more concentrated than the average rating, so the load must be spread.
- Fix: spreader beams over 10 m bring it down to about 2,200 kg/m. Still check it against the carrier's concentrated-load table.
- Estimated VGM: 22,000 + 5,000 kg tare + about 400 kg of timber and lashings = 27,400 kg.
A width of 2,900 mm also exceeds the 2,500 mm vehicle width limit, so the road leg to the port needs a permit and special arrangements. Work out the trailer's axle loads as well; see truck axle load and GVW and the Axle Load Calculator.
How to load and secure a flat rack
With no side walls, a flat rack can be loaded from above by crane or from the side by a large forklift. A typical sequence at the depot or yard:
- Inspect the empty unit: timber deck, side rails, end posts, lashing points and CSC plate. On a collapsible rack, make sure the end walls are fully locked once raised.
- Place spreader beams as planned before the cargo comes down, with rubber or timber between steel and deck for better friction and to protect the floor.
- Land the cargo centrally, with its centre of gravity as close as possible to the middle of the deck lengthwise and across. Cargo off to one end is awkward to lift and overloads one end of the trailer.
- Block and lash: fit chocks front and rear, then diagonal lashings in four directions, all tensioned evenly. A slack lashing carries nothing.
- Protect and record: cover sensitive parts, photograph every side and note the number and tension of lashings for the survey report.
Vibration loosens lashings on the road. On long inland legs, plan a re-tensioning check before the unit enters the terminal.
Before you book
- Measure the actual cargo, including outermost points such as mirrors, booms and ladders.
- Prepare a drawing showing the centre of gravity and lifting points.
- Send dimensions, weight and OW/OH to the carrier for technical (OOG) approval.
- Confirm both load and discharge ports have suitable lifting gear, such as an over-height frame.
- Arrange a lashing survey if the carrier or insurer requires one.
Common mistakes
- Assuming the max payload applies to concentrated loads.
- Lashing to points of unknown rating, or to fragile parts of the machine.
- Not declaring small protrusions, so terminal OOG measurements differ from the booking.
- Forgetting weather protection; a flat rack is open, so machinery needs cover from sea spray.
- Ignoring the P1/P3 code and a post height different from the unit you planned on.
- Not planning the road leg for cargo wider than 2.5 m.
Plan your flat rack shipment
Check unit dimensions in Container Sizes, then simulate the machine position, centre of gravity and OOG in the 3D Load Planner. A planner layout is a simulation and does not replace carrier technical approval, a lashing survey, official weighing or operator approval.