Wet bed, concrete blocks and eight straps
At a depot a 3-axle rigid lorry is loaded with 14 t of concrete blocks on wooden pallets. The flatbed has a 1.2 m steel headboard but no side gates. The blocks are stacked 1.6 m high. The bed is wet, with grit and standing water, and the driver has eight top-over straps with an LC of 2,000 daN each and two rubber anti-slip mats. The load is placed centrally. The front axle limit is 8.0 t, the rear bogie limit is 19.0 t, and the tare weights are 6.5 t front and 4.0 t rear. The distance from the front axle to the rear-bogie centre is 5.2 m, and the load centre is 3.0 m behind the front axle. The route is 80 km on a national road, including a 1.5 km descent at 7%. The driver says the blocks are too heavy to move and that eight straps are more than enough.
1. Using a simplified 0.8 g forward deceleration requirement, assess whether the eight straps are sufficient on the wet bed. State your assumptions and the practical fix.
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The forward inertia force is about 14,000 kg × 0.8 × 9.81 = 110 kN. On wet wood and concrete the friction coefficient may be only about 0.2, giving roughly 27 kN of friction and leaving about 83 kN to be held by the lashings. Each strap has LC 2,000 daN = 20 kN. A top-over strap at 45 degrees to the bed gives a horizontal component of about 20 × cos 45 = 14 kN, so at least 6 straps would be needed. At 60 degrees the horizontal component is only about 10 kN, so 8 straps would still be short of 83 kN. The calculation is simplified, but it shows that eight straps are marginal at best and insufficient at poor angles. Wet grit and water make the result worse because friction is lower and straps can lose tension. The practical fix is to sweep and drain the bed, fit the anti-slip mats, use the headboard properly, shorten the stack if possible, and add direct lashing or more straps. Do not depart on the basis that the blocks are heavy; heavy loads also develop large forces.
2. Check the axle loads with the load in its current position and state what must change.
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The load centre is 3.0 m behind the front axle on a 5.2 m wheelbase. The front axle share is 14 × (5.2 - 3.0) / 5.2 = 5.92 t. Front axle total is therefore 6.5 + 5.92 = 12.42 t, which exceeds the 8.0 t limit by 4.42 t. The rear share is 14 - 5.92 = 8.08 t, so the rear bogie is 4.0 + 8.08 = 12.08 t, which is within 19.0 t. The gross weight is also within the vehicle's total, but that does not help because the front axle is overloaded. To keep the front axle at 8.0 t, the load may add at most 1.5 t to the front axle. That requires a load centre at least 5.2 × (1 - 1.5/14) = 4.64 m behind the front axle. The load must be moved about 1.64 m rearward, or the payload must be reduced. After moving it, check the rear bogie, rear overhang and the headboard contact again; moving load rearward can create a new problem at the back.
3. What driving technique and checks are needed for the 7% descent with this load?
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Before the descent, stop and check the straps, the headboard, the mats and the load against the headboard. Select a low gear before the grade begins so that engine braking and the retarder can hold a steady speed. Use the retarder and engine brake as the primary retardation and avoid continuous service braking, which can overheat the brakes. Keep a speed that allows a stop within the visible distance, especially with 14 t of concrete blocks. Do not make sudden steering movements, because the high load can shift or tip. After the descent, stop in a safe place and re-check the straps and load; vibration and braking can loosen tension. If a strap is frayed, a ratchet is damaged or the load has moved, do not continue until it is corrected.