I need help in knowing how to place a 16 x 20 portable building on piers. For years I could turn to my husband, the last few he was bed bound but did drawings for me and told me how to do it. Now he is gone and didn’t get the drawings completed for my cash register station for our greenhouse business.  I am building a 16 x 20 portable building to be placed between two greenhouses that are on a sloped property. In other words, one side of the building will be at ground level the other side of the building will be about 3 foot off the ground. I was planning on using the sonotubes, drilling 4 foot holes (our frost line) with the post digger, then using the tubes above ground. I’ll use 12″ tubes with rebar)  The building deck will be on 4 x 6 runners, with 2 x 6 floor joists. I could have 10 customers in the building at a time, I don’t want the floor to be spongy. Is 3 rows of piers on the 16 foot side, by 4 piers in each row on the 20 foot side overkill? If I use that pier configuration are 2 x 6’s enough to span  8′ or should I bump those to 2 x 8s. I have attached a photo of the shed design. Thanks in advance.