Why do double stacked pallets tend to lean and often topple?

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Double stacking of pallets doubles the effective storage area and transport loading density so there is a massive incentive. However, incorrect double stacking greatly increases risk.

AEC Designs provides a guide to successful double stacking.

Successful double stacking relies on the base pallet load being sufficiently strong to carry the load of the pallet above. In palletised carton loads all the strength is provided by the walls of the cartons, which are only strong in compression. Any sideways forces will cause the cardboard to deform and ultimately collapse. Once one carton wall starts to collapse the other cartons are sharing a greater portion of the load, so adjacent cartons are also likely to collapse. Added to this the upper pallet load will start to lean applying further load to the already weakened side of the carton stack.

Successful double stacking therefore requires the upper pallet to be placed squarely and level onto the pallet below so that the weight is evenly shared by all sides of the bottom stack.

The problem for FLT drivers is that they cannot see to accurately set the upper pallet level before stacking. Any error in tilt angle as the upper pallet is lowered will apply more load to one side of the lower stack. Any attempt to correct the tilt once the loads are touching will impart lateral forces to the lower pallet, which is likely to cause the carton walls to deform.

The collapse of double stacked pallets can occur days or weeks after the stack was created, and without any warning. Changes in atmospheric conditions, especially humidity have a large effect on carton wall strength. Often final collapse will occur overnight when the temperature drops and condensation levels increase.

Similar stack collapse is also common with palletized bagged granular products, where unbalanced forces due to uneven pallet stacking cause the granules to shift inside the bags. The contents of each bag act like a slow moving fluid that is being squeezed by the load above. If the load is more on one side of the pallet, then the contents will flow away from this load point causing the upper pallet to lean, which increases the load to the same side of the pallet.

Guide to successful double stacking:

Correctly applied stretch or shrink wrap helps to stabilize the carton side walls as well as holding the cartons into a close stack.
It is important that only the correct pallets are used as not all pallets are suitable for double stacking.
Ensure Forklift trucks have a ForkAngles mast tilt indicator so that the pallets can be accurately stacked.
Double stacked pallets should generally not be moved as a stack. The extra forces applied whilst tilting and moving the stack can cause its collapse.
Be very careful when forming a row of double stacked pallets. If these are pushed close together there is a build up of forces down the row which can cause the whole row to collapse.  There are strict rules relating to the correct stacking and restraining of pallets for transportation due to the additional dynamic forces experienced during shipping.

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