Why would anyone use beams on the bottom level of a selective rack systems. After all, the beams cost more money. And by not putting the bottom load on the floor, the total load on the racking systems is higher, requiring more steel and more cost. Nevertheless some users routinely demand a bottom beam. Why?
The usual answer is that the bottom beam allows the floor under the pallet to be kept clean. This is critical in warehouse than store ingredients or finished products that are intended for human consumption, most importantly for grain based products. Spills can quickly lead to insect and rodent infestations.
But there are other reasons. In a racking system with high openings and few levels, the bottom beam improves the structural stability of the system. Also, wood pallets store long term on a concrete floor absorb moisture from the floor. Over time, this will undermine the strength of the pallet.