A better workshop foundation?

[Malcolm McLeod]

How so Malcolm?

I was hoping no one would ask...

For a typical, small, open-floor-plan building, sitting on piers, the overwhelming majority of the weight is the roof and it's related structure. This is transferred to the ground thru the walls - - and they are ALL at the perimeter. So, the perimeter piers bear this weight. If you put piers in the middle (I did), they have relatively little weight on them. Even with a shop full of tools, the combined weight (live load) is generally much less than the structure (dead load). (....I'm talking averages here - - not the guy with a 24' 152,000lb hammer forge in his detached garage.) This weight bias towards the perimeter is compounded if you put your tools on wheels and roll them out of the way. ...On the wall, right?

So how is this bad? My clay soils and 12 ea. 24" deep 18"x18" piers allowed the perimeter to sink. Yes, 'that' perimeter - - the one with all the weight on it. The center piers never moved. Welcome to the Himalayas, and a close personal relationship with the friendly neighborhood foundation repairman. ...And you thought mistresses were expensive? ROFL

Have your eyes glazed over yet...?

To go the whole 9-yards as I understand it, you need to know the compressive strength of the soil. How many psi will it bear? Estimate the total weight of the building (or have a Civil Engr SWAG it based on his books and 30-yrs experience). Let's say the soil will bear 10psi and the building weighs 10,000 lbs. So, you need 1000 sq inches of something in contact with the soil to evenly distribute the load. Plus some factor of safety for when you roll all the big iron to one end.

If it's not evenly distributed then there's another set of equations - to try to keep the independent movement of the piers to a minimum. And they will move.

IF you use piers, my humble suggestion would be to use them at the perimeter only, and make sure that a beam, properly sized for the expected load, spans between each pier. Not perfect, but you won't need ropes and a climbing harness to retrieve a dropped socket from the corner. ... I mentioned never, didn't I?

I will use a slab.