Almost every racking overload we are called to look at comes from the same misunderstanding. Someone reads the beam capacity, multiplies it by the number of levels, and assumes the answer is what the bay will hold. It is not, and the difference is the reason a bay can be within every beam rating in the building and still be overloaded.
What each number describes
Beam capacity per pair is the load two beams at one level will carry, distributed across the span, without exceeding the deflection limit they were designed to. It is a property of the beam section and the span between frames. Change the bay width and this number changes.
Bay load is the total the two uprights will carry: every beam level in that bay, plus the beams themselves. It is a property of the upright section, its gauge, the bracing pattern, and above all the distance from the floor to the first beam level. Change the height of the bottom beam and this number changes, sometimes dramatically.
Why they do not multiply
The upright is a column. Columns fail by buckling, and how readily they buckle depends on the unsupported length between restraints. The bracing between the two uprights of a frame provides those restraints, so the longest unbraced length is usually the run from the floor to the first beam. A bay with its bottom beam at 300 mm behaves very differently from the same bay with its bottom beam at 1 800 mm, even though nothing about the beams changed.
That is why a supplier cannot quote one bay load for an upright section. The figure only exists once the beam level positions are fixed. If you receive a quotation with a bay load and no beam layout attached to it, the number is describing an assumption you have not seen.
The practical consequence
When someone lowers the bottom beam to squeeze in another level, they have not simply added capacity. They have shortened the top of the column and lengthened nothing, which is fine, or they have raised the bottom beam and lengthened the unbraced base run, which is not. Either way the bay load has to be recalculated and the load notice reissued. Moving beams is a two-person job with a mallet; that is exactly what makes it easy to do without telling anyone.
What to ask for
Three things on the drawing, not in the quotation: beam capacity per pair for each level; total bay load for the configuration drawn; and the beam level heights those figures were calculated at. Anything less and the load notice on the end frame is describing a rack that may no longer exist.
And the unit load
One more figure worth stating. Beam ratings assume a uniformly distributed load, which for pallet racking means two pallets of roughly equal weight sitting over the frames. One dense pallet in the centre of the span is a point load, and it deflects the beam more than the same weight split into two. Where a customer stores drums, tile or battery stock, we drop the bay width rather than raise the beam rating, because a shorter span solves a point load more cheaply than a heavier section does.