Building a new equipment shed

I've got a complete metal building that I took down for free years ago (they were going to demo it and gave it to me to take it down. I think it is 40x60, but I think I just want to put up 1 section (2 trusses), which will make the building 40x20. My issue is the foundation. What info I've found for a 40' free span building is a pretty large foundation. One thing I'm thinking about is putting a post in the middle of the truss, that should take a lot of stress off the end columns, but it's still a fairly large foundation. Does anybody know the cost difference of mixing your own concrete vs having ready mix delivered? I first thought I could mix it myself a lot cheaper (I have a mixer), but after pricing bags of Portland cement, it looks like I can have ready mix delivered for not much more than I can just buy bags of Portland for (not counting the sand and gravel). Maybe I'm just not shopping at the right places, but bags of Portland seem awfully expensive. 

At the end of the day, I really don't know if this "free" metal building is worth it. The tin I took off has already ruined, and buy the time I hire equipment to set the metal building, I'm wondering if I can actually build a pole barn cheaper?

Granted, you only have 4 "bases" and a pole barn would have a dozen... but the TOTAL amound of concrete should be similar ??

Doing my research on free span trusses, there is a lot of lateral load on the columns. Basically, the load is pushing down on the top of the truss, this causes the columns to try to kick out at the bottom, as the force tries push down on the peak of the truss, it makes the columns wanting to "kick out" at the bottom. It's a engineered foundation that has to be right or the building will eventually fall. I talked to a guy that builds these type buildings, he said they built a building a couple of years ago that was a record free span (don't recall how far). About a year after they built the building, somebody noticed the columns where trying to kick out at the bottom and the roof was trying to fall. Engineers looked at it and the solution was to cut a trench all the way across the building in the column line, install big cables between the columns, pull the foundations back into place, and fill it all back in with concrete. He said the fix cost more than the cost of the building originally.

The reason I mentioned the post in the middle was because that would hold up the peak of the truss, and essentially shift the weight of the building from a lot of lateral stress to more or less straight down onto the columns.    

Yes, buying premixed in the bag is very expensive. Pre-covid though, I could make my own concrete for around $75 per yard (mixing the cement, sand, and gravel myself). Now, the price of Portland cement is $113 per yard, not counting the sand and gravel. I haven't called the ready mix outfit to check the current price of concrete delivered. Looks like I need around 2.5 yards, I know the ready mix outfit used to have a 5 yard minimum or they charge you a outrageous delivery charge.   

Yes, just pouring column foundations. We don't have to worry about "frost heave" down here in TN like you folks up north have to deal with. 

I'm still weighing my options. I don't need anything fancy, just enough to keep the rain off a few tractors and equipment, but I do have to make it look decent for the beautification department (wife). I could probably get by with more if you couldn't see it from the house.

That red tight cable is the bottom chord of the truss, so cable isn't needed.

Had to go back to the 1st post.....he does say truss.

Now there are two major type of trusses, 'roof ONLY and 'roof with floor'

roof ONLY trusses should NEVER have anything stored on top of the trusses bottom chord. Well OK, maybe 3 or 4 fishing poles but NOT WEIGHT. very,very bad things WILL happen,like the truss WILL fail. BTDT next door.....

 roof with floor trusses ARE designed to,well, have a floor on them and support a certain designed weight (xx pounds per square foot ).