Repair or replace floor in a Cub Cub-o-matic

[itaylor]

The flip-over floor in our Cub has an issue.The side closest to the main trailer body has detached - when you stand on it it flexes down. One of the rivets in the centre is missing and when I replaced it the new one didn't hold at all. I am thinking that there is some corrosion or rot in there. What is the structure in there, before I think of pulling it apart?