Overheight Load and a Low Bridge
You are assigned a semi-trailer with a flat deck carrying a large prefabricated steel frame. The consignment note lists the frame as 4.1 m high on the trailer, and the trailer deck height is 1.35 m, so the combined height is 5.45 m. Your route planner has given you a route, but the driver ahead on the same job reports that a rail underpass on that route is signed 5.2 m. The load is 3.0 m wide and 12 m long, sitting on the deck with timber dunnage but no chains fitted yet. The trailer's maximum deck capacity is 30 t, and the frame weighs 22 t. You have a permit application process available, and an alternate route adds 40 km and passes a weighbridge.
1. Assess the height and width compliance issues and decide whether the vehicle can proceed as loaded.
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Height: The combined height is 1.35 + 4.1 = 5.45 m. The signed bridge clearance is 5.2 m, so the vehicle would strike it by 0.25 m. A signed clearance is a hard limit - you cannot assume the sign is conservative or that the actual structure is higher. The load cannot pass that underpass.
Width: At 3.0 m the load exceeds the typical general-access width limit (commonly 2.5 m), so it is an oversize load requiring a permit and likely escort/ signage conditions, plus route approval that accounts for lane widths, overhead lines, and bridge parapets.
Height also triggers permit conditions: even without the bridge, 5.45 m is well above general height limits in most jurisdictions, so a permit and a height-approved route are required regardless.
Decision: The vehicle cannot proceed as loaded on the planned route. Options are: (a) obtain a permit and a route survey that clears all structures at or above the load height plus margin; (b) use the alternate route only after confirming every structure on it clears 5.45 m with margin; or (c) reduce height by removing/rotating the frame, using a lower deck, or splitting the load. The 40 km alternate is not automatically acceptable - it must be checked structure by structure, not assumed.
2. The frame is 22 t on a 30 t deck. Explain the load-restraint and mass considerations before moving, and what you check on the dunnage.
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Mass: 22 t is within the 30 t deck capacity, but deck capacity is not the same as legal axle/ GVM limits. The load must be positioned so axle groups are not overloaded, and the combination must be checked against GVM, axle-group limits, and any permit conditions. The weighbridge on the alternate route is an opportunity to verify actual axle loads rather than relying on the consignment figure.
Load restraint: A 22 t frame restrains enormous inertia. Timber dunnage alone does not restrain it; it only spreads load and provides friction. You need a restraint system designed for the load's mass and shape:
- Lashings/chains rated for the load, arranged to resist forward, rearward, and lateral forces, with the number and rating calculated against the load's weight (restraint capacity must exceed the forces generated in braking and cornering, not merely equal the weight).
- Direct restraint (chains over/through the frame) where the shape allows, rather than relying on friction.
- Friction/dunnage check: dunnage must be sound, dry, of adequate cross-section, positioned under load-bearing points, and not crushed or split. Wet or soft timber reduces friction dramatically.
- Edge protection where chains contact steel to prevent cutting or slipping.
- Check after initial movement: re-tension after the first few kilometres, since loads settle.
Judgement: Do not depart until restraint is fitted and checked; an unsecured 22 t frame is a catastrophic hazard regardless of permit status.