When a stack of containers falls over, the first suspicion is that the bottom container failed. It usually did not. Stacks collapse for reasons of geometry rather than strength: the load was not vertical, the supporting surface was not level, or the stacking features were never properly engaged. Establishing which is worth doing, because the fix is different in each case and strength is rarely it.
Vertical load is the assumption
A stacking rating assumes the load is applied vertically and centrally. In a real stack, the load above is applied wherever the container below happens to be sitting, and if that is not central the load is eccentric. An eccentric load on a stack produces a leaning moment that grows with height, and a stack that is stable at three high can be unstable at five with the same eccentricity.
Uneven floors are the usual culprit
A stack on a floor that is out of level leans, and everything above leans with it. Warehouse floors are rarely perfectly level, and a slope of a degree or two across the height of a five-high stack moves the top a noticeable distance. Where stacks are tall and the floor is uneven, the failure point is the floor rather than the container.
Stacking features that do not seat
A stacking feature works by engaging the container below, and it only does its job when it is fully seated. Partially engaged – because the operator could not see, or because the feature is tight, or because a container is deformed – the stack is held by nothing more than the container below being roughly on top. Where a stack collapses, look at whether the features were engaged before blaming the strength.
What load distribution does
A container loaded evenly presents a flat top and stacks well. One loaded with heavy contents at one end presents a tilted top, and the container above starts off-centre. Each layer adds to the tilt. Where stacks collapse, looking at how individual containers were loaded is often more productive than looking at the containers themselves.
Vibration and disturbance
A stack near a vehicle route, a press, or a conveyor is vibrated continuously, and vibration walks a stack out of alignment over time. A stack that is stable when built becomes progressively less so. Where stacks stand for long periods near a vibration source, the height limit has to be lower than for a stack in a quiet corner.
What the container contributes
Two things it can do well. A stacking feature that self-locates – that pulls the container into alignment as it is placed rather than requiring precision – removes most of the seating problem. And a base that is flat and stiff enough not to deform under the load above keeps the stack vertical. Both are specification items. The deformation side is covered in creep and permanent deformation.
Setting a height rule
Maximum height, stated in containers, on a level surface, away from traffic, with the features checked as seated. Where the rule includes the conditions, it holds; where it is just a number, it gets applied to sloping floors next to a route and then fails. Writing the conditions on the same sign as the number is the whole trick.
What to do after a collapse
Before clearing it, look at how the features were engaged and whether the floor was level. That observation takes seconds and it is the only evidence available – once the stack is cleared the cause is gone. Photographing the base of the stack before clearing it is worth the ten seconds it takes.