Four Straps, Six Steel Plates
You are carrying six steel plates on the flat timber deck of a rigid tray truck. Each plate is 2,400 × 1,200 mm and 600 kg, so the load is 3,600 kg, stacked flat in a single pile about 300 mm high. You have four webbing straps with a lashing capacity of 2,500 kg each, hooked to rated points on the deck and run over the top of the stack. The yard hand tells you four straps is generous because 4 × 2,500 = 10,000 kg, which is almost three times the weight of the load. He adds that the drop sides will also help hold the plates in. The performance standard requires restraint for 0.8 g forward, 0.5 g sideways and 0.5 g rearward.
1. Is the yard hand's arithmetic a valid way to check this load? Explain what the 0.8 g requirement means and work out the forces involved.
Read the worked answer
The arithmetic is not a valid check. Lashing capacity (LC) is the rated strength of a strap, not the restraint it actually delivers to the load. Adding up LC values and comparing the total to the load mass ignores tension, geometry, friction and how the load behaves as a stack.
What the load actually demands:
- Forward (0.8 g): 0.8 × 3,600 = 2,880 kg of restraint
- Sideways (0.5 g): 0.5 × 3,600 = 1,800 kg
- Rearward (0.5 g): 0.5 × 3,600 = 1,800 kg
Why the strap total overstates the real restraint:
- Only the component of each strap's tension acting in the direction of the force counts; straps running vertically or at an angle deliver less than their rated tension horizontally.
- Tension is limited by what you can physically tension and by the rated strength of the deck anchor points, which may be far lower than the strap LC.
- Friction between steel and timber helps, but it is unreliable if the deck is dusty, wet or oily, and it must be assumed low rather than generous.
- The plates are a stack: individual plates can slide out from under the straps, and the stack can tip as a block.
- Drop sides are not load restraint. They bow, spring open and are not designed to hold a load in a 0.8 g event.
2. What do you check and change before the truck leaves the yard?
Read the worked answer
Checks:
- Rating of each deck anchor point - the weakest one sets the limit for the whole arrangement.
- Strap condition: cuts, fraying, UV damage, damaged hooks, legible labels.
- Deck condition: clean, dry, free of grit and oil so friction is as high as it can be.
- Whether the plates are sitting flat and square, or whether they can slide and tip as a group.
Changes to make:
- Open the Load Restraint Guide method and calculate the restraint properly for the 0.8 g / 0.5 g / 0.5 g requirements rather than totalling LC values.
- Run straps squarely over the stack, positioned so each plate is held and the stack cannot slide as a block, with edge protection at the plate corners.
- Use chocks or a headboard to take the forward load if the geometry allows - direct restraint against a rated structure is far more reliable than relying on strap tension.
- Add straps or change to a rated rack/a-frame if the calculation does not clear 0.8 g with margin.
The judgement: if you cannot show the arrangement meets the standard, the truck does not leave. A four-strap arrangement that has not been calculated is not proof of anything, and the yard hand is not the person who will be driving it down a hill when a car pulls out.
3. Two hours into the trip you stop for fuel and find one strap frayed through several fibres near the hook, and the plate stack has shifted about 50 mm sideways. What do you do?
Read the worked answer
Do not continue the trip as loaded. The strap is compromised, and the fact that the stack has already moved tells you the restraint was inadequate - the same restraint will not hold in a 0.8 g stop.
Actions:
- Park in a safe, level spot away from traffic and secure the area before working on the load.
- Take the damaged strap out of service and tag it; a strap with cut or frayed fibres must never be reused.
- Check the deck, anchor points and remaining straps for damage or movement.
- Re-stack the plates squarely, apply the restraint arrangement you worked out at the yard, and add the extra straps or chocks that were needed in the first place.
- Record and report the incident to your operator, including the strap failure, so the gear is replaced and the loading method is reviewed.
The key point: a load that has shifted once is evidence the method failed, not an invitation to re-strap it the same way and press on. The 50 mm shift is the warning; a hard brake or a corner could turn it into a plate through the cab or onto the road.