Floor stacking – boxes stacked directly on the floor with no pallet – saves the cost and the cube of a pallet per stack, and it is standard practice in a lot of plants. It works up to a height, and then it fails, and the failure is always in the bottom box rather than distributed through the stack.
The bottom box carries everything
In a pallet stack, the load path goes through the box walls to the pallet deck and the pallet spreads it. On the floor, the bottom box’s base carries the entire stack load and transmits it to the floor at whatever points are actually in contact. If the floor is uneven, or if there is swarf or a small object under one corner, that corner takes a point load it was never specified for, and the base cracks from the inside.
Floor condition is part of the specification
Floor stacking is only as good as the floor. A smooth, level, clean industrial floor will support a five-high stack of loaded boxes without complaint. The same stack on a floor with a damaged expansion joint, a chip out of the surface or a patch of swarf will fail at three high. If floor stacking is the plan, the floor condition is a specification input, not a site condition to be tolerated.
Interlock, and cap the height
Rotating alternate layers by ninety degrees ties the stack together and stops the slow lean that is the usual precursor to a collapse. Set a height cap and paint it on the wall: for most loaded PP corrugated boxes on a sound floor, four to five high is the practical limit, and above that the bottom box is being asked for more than any reasonable base will give. If the cube is needed, use a pallet and go higher rather than pushing the floor stack.
Where floor stacking is the right call
Short-duration staging – a stack that will be broken down within a shift – and light loads. For anything standing more than a day, or anything above about forty kilograms per box, put it on a pallet. The pallet cost is trivial next to a collapsed stack, and a collapsed stack of machined parts is a bad afternoon for everybody involved. The height limits follow the same reasoning as how high a stack can safely go. The height limits follow the same reasoning as how high a stack can safely go.
Marking the floor is not optional
Floor stacks drift. A stack put down in a marked position stays in the aisle it was meant to be in; a stack put down wherever there was space ends up in a walkway, in front of a door, or in a fire route. Painted floor positions with the box footprint marked on them cost almost nothing and they do two jobs: they keep the aisles clear and they make an overloaded or over-height stack obvious, because a stack that does not fit its marked footprint has been built too high or turned the wrong way.
Mark the maximum height next to the position as well. A painted line on the wall or upright at the height cap is the only reliable way to enforce a stack limit, because a written limit is a rule and a painted line is a fact. It also removes the argument at the end of a shift when someone wants to get the last few boxes off the floor quickly.
What changes when the floor is not sound
Most advice on floor stacking assumes a smooth level slab. Where the floor is not that – and in older plants it often is not – the stack height cap has to come down, and it has to come down by more than feels necessary, because a single high point under one corner of the bottom box turns the whole stack load into a point load.
The practical test is to set a loaded box on the floor and rock it: any movement means the floor is not bearing evenly and the box is already taking a point load before anything is stacked on it. Where the floor cannot be repaired, use a pallet or a deck even for low stacks, because the pallet bridges the irregularity. Where stacks must go on an uneven floor, cap the height at two and accept the floor area cost – it is cheaper than a collapsed stack of parts.
What we can do
We build PP corrugated turnover boxes with base constructions specified for floor stacking where that is how the box will be used, and we will tell you the height cap for your load rather than leaving it to be found out. Tell us the floor and the dwell time and we will set the base for it.