KenV, this isn't my area of expertise (I'm an engine guy). But my understanding is for Boeing built 747 Freighters and Boeing Converted Freighters (BCF, which is what the Bagram freighter was), they don't use seat tracks. The floor structure is someone unique to the freighter (and yes it is strengthened relative to the passenger main deck floor). The 747F main deck has a fairly elaborate cargo handling system which incorporates stops and hold-downs for the standard air-cargo pallets. Unfortunately those are not suitable for hauling large military vehicles, so alternate means are required.
Boeing freighters have seat tracks. The airplane cannot be flown without seat tracks. The cargo rail/lock system (made by a 3rd party who holds the Supplemental Type Certificate, STC, for the cargo systems) attach to the seat tracks. You can't see them because they are under the rail/lock system, but they are there. Removing/eliminating the seat tracks would require a very complex and expensive STC. There are rollers of various kinds (some powered) in the freighter floor panels. But they are attached to the seat tracks at their edges and dump much of their loads into the seat tracks, with some of the floor panel loads distributed to the floor beams.
Seat track on a 747 is only good for 5000lbs…You can attach a 25000 lbs chain to a D ring in a seat track but its only going to give you 5K.
Individual seat track notches are rated to 5000lbs (5Klbs). This would be a point load. Any D-ring that attaches to a seat track notch is therefore only good for 5Klbs. The overall seat track (distributed load) can handle much more. The cargo rails that attach to the seat tracks distribute the loads over the full length of the seat track.
Can you clarify what you mean by "K". I guess it isn't Kelvin in this context.
"K" is short hand for Klbs, which is shorthand for 1000 lbs.
If as indicated by post #700 those 18-ton vehicles were loaded "floating" along the centerline, the floor was likely overloaded. The pallets are designed to transfer their loads to their edges and the edges of the pallet transfer their loads to the cargo rails, which in turn trasnfer their loads to the seat track system which distribute the loads over multiple floor beams. A free floating pallet would not be able to transfer their loads to the rails and would instead transfer the load directly into the floor beams. This creates many point loads and does not distribute the load as designed. This condition is OK under 1G when loading or unloading the aircraft, but not inflight. In the event of a high maneuver or gust load, the floor would have failed, although probably not catastrophically. But it would have required a lot of expensive repairs before the next flight. It would seem that the aircraft never got a chance to experience a maneuver or gust load since it crashed seconds after take off. But with no pallet locks involved in their restraint, it would have required a whole LOT of straps to properly restrain this load, and therefore a whole lot of D rings. I don't know the specific configuration of this freighter, but I doubt there would have been enough D rings to properly restrain three 18-ton and one 12-ton vehicles.