I live in Northern MO and I'm getting ready to pour a 25'x36' garage slab. My first question is do I need both piers and 12" footer? And will a 4" slab be OK for a 3/4 ton truck?

As long as you add reinforcing mesh/wire/rebar,etc.and as long as you have sufficient stone base.

This is a floating pad.

I used 2 X 6 for the perimeter and scooped the earth down at the edges,then stone sloping to the edge making it deeper than the body,about 12" in,I did this for a 24' X 40' pad,broken into 4 parts on the 40' run(expansion/stress lines)

My contractor want to put a 12" footer all the way around the edges plus 18" x 30" deep piers every 5' below the footer. Just sound like overload to me... Also, I am attaching this structure to a exsiting structure that has only a 4" floating slab. Does that sound like a problem? My plan is to dig and level down far enough to use 2x12's as my forms then putting in 71/2" of crushed rock packed then 1/2" rebar and 4" of concrete. And follow your advice by leaving the edges clear of so much crushed rock. Is your slab capable of holding a heavy 3/4 ton truck?

The building I made mine for holds large equipment,loaders,1 ton trucks,etc.. It's at a grain elevator.

To attach to the existing structure,I would drill and install rebar to the existing concrete,then tie the new rebar to those.

I only used wire for my reinforcement,It hasn't budged once,no cracks,no separation.

A truck's weight is so spread out on the tires,you really have little to worry about as far as weight issues, 4" concrete is more than enough.

I see no reason to go with a 2 X 12,that is overkill.Unless the ground water table is near the surface?or there is some kind of slope in the yard near this project?

Piers,, I can't see ANY reason for them, is there going to be some kind of posts in the middle, or some kind of weight bearing on the pier locals?

If no to all these, it's overkill and not in your best interest to stay with this contractor, get other bids.

There is not enough information to answer the original psoter's question.

The need for piers under the footings may the result of point loads that may be imposed from a post-and-beam structure and may be required because of the existing soil conditions.

The original poster needs a design professional to determine what may be needed along with the local code office to determine what will be required.

Connecting this structure to another floating structure with questionable construction remains suspect at best.

The original poster needs a design professional to answer his questions... and should do nothing until the design is confirmed by the local code professionals.

I'm curious about the 25' measurement, seems like building in a lot of waste wood. Or not?

I am building a three car garage with exterior walls only, there will be no center post. The reason for the 25' dem. Is that's how wide the other building is. I asked a engineer I know from TX and he said I would only need peirs under the footings if I was building on a hill or a slope of some kind to stop the slab fron moving horizontaly. The code for home construction in Pasadena, TX if 12"x12" footer around the edges and where there are load bering walls. He recommends I pour a 4" slab with thickend edges of 12" wide and 8" deep.

The key words were location, Pasadena Tx is flat sandy base which should only require footings, further north near Dallas the land has more clay that moves with the seasons makes it more prone to use piers. A 4" slab with proper reinforcement should hold up fine, the question is do you want to have just enough or a little safety zone for unseen use later? After slab is poured to help relieve spider cracks you can also cut relief lines in surface to relieve stress. Pouring later in the afternoon or evening and not in the hot sun can also help relieve
The stress letting the concrete cure longer. When attaching to an existing structure make sure the ground is firmly compacted to eliminate any shifting and stress to existing foundation. Iam in the process of repairing this exact problem made by a house builder, doing it right the first time is easier than trying to repair it years down the road!