I just bet you will find that an awful lot of pole barns have dirt or gravel floors. I've got no pictures but I've seen two barns with concrete floors and floor drains. They were not pole barns but that probably doesn't matter. One had a single grated drain in the center and the other had two grated drains. The floors were sloped to the drains. Both had catch basins below the grated drain that were plumbed with 4" plastic pipe to a perforated concrete dry well. Dry well located approximately 25 feet from building.
The idea behind the catch basin - it serves very similarly as a septic tank. You can lift the floor grate and muck out the catch basin. It stops solids and prevents them from going out and plugging the dry well.