Steel coil securing on a flatbed trailer
You are to load three steel coils, each 8.0 t, 1.2 m diameter and 1.5 m wide, onto a flatbed semi-trailer with a wooden floor. The coils will stand on their curved surface (eye horizontal) in a single row, with wooden wedges but no anti-slip mats. The plan is to use four 50 mm polyester straps per coil, each with a lashing capacity (LC) of 2,500 daN and a preload (STF) of 300 daN, passed over the top of the coil. The trailer's certified lashing points are rated at 2,000 daN each. The route includes a 7% downhill and sharp roundabouts. The load limit is 24 t. The driver has 30 minutes to load.
1. Assess whether the proposed securing is sufficient for forward acceleration. Use the friction and lashing numbers.
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The total load is 24 t, or about 24,000 daN. For road transport, the forward acceleration assumption is commonly 0.8 g, so the required forward securing force is about 0.8 × 24,000 = 19,200 daN. With steel on a wooden floor and no anti-slip mats, the friction coefficient is roughly 0.3, giving about 7,200 daN of friction. That leaves about 12,000 daN to be provided by the lashing. A top-over strap with STF 300 daN adds friction approximately equal to 2 × STF × friction, so about 180 daN per strap at a vertical angle. Twelve straps would add only about 2,160 daN, giving a total of about 9,360 daN, far below 19,200 daN. The proposed four straps per coil over the top are therefore not sufficient for forward acceleration, even though the LC of 2,500 daN looks large; the limiting factor is the preload and the low friction.
2. What would you change, and how would you verify the lashing points?
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The load should be secured with a method that provides direct resistance, not just top-over friction. Options include certified coil cradles, loop lashings around each coil, or anti-slip mats combined with more lashings. If anti-slip mats raise the friction coefficient to about 0.6, the friction force becomes about 14,400 daN, leaving about 4,800 daN to be secured. Even then, the top-over straps add only about 360 daN each with the higher friction, so twelve straps give about 4,320 daN, which is still marginal. A better plan is to use direct lashings around the coils, with the strap angle as close to horizontal as practical, and to verify that each lashing point is rated for the load it receives. The points are rated at 2,000 daN each, so the number of straps and the sharing between points must be calculated. Wooden wedges alone are not a substitute for calculated lashing.
3. What are the consequences of the 7% descent and roundabouts for load security, and what checks should be made after 10 km?
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A 7% descent increases braking forces and can make the coils creep forward. Sharp roundabouts produce lateral forces that can make round coils roll or twist, especially if the wedges are only placed on one side. The coils must be chocked against movement in both directions and checked for contact with the headboard or other restraints. After 10 km, stop in a safe place and check the straps for tension, the wedges for movement, the coils for shifting and the lashing points for deformation. Re-tension the straps if they have settled. Check again after the descent, because heat from braking and vibration can loosen the securing. If a coil has moved, do not continue until it is re-secured to the calculated standard. The 30-minute loading window is not a reason to skip the calculation or the post-loading check.