|I want to build a loft above my shop for wood storage; vertical wood
|posts, beams, joists and T&G plywood for the floor. The floor of the
|shop is a slab on grade. I have plenty of height so there is no problem
|I'm not sure what to do at the connection of the vertical posts and the
|slab. Obviously there are no footings there to take posts. Can I just
|butt the posts up to the slab and hope the weight doesn't screw up the
|floor? What are other options? I'd like to avoid sawing the slab and
|putting in footings unless I can't avoid it.
A lot of needed data is missing. Nevertheless, your concern seems to
be the load bearing capability of the concrete. If your slab was
built to code, the code requirement will tell you the specified
compressive strength requirement. Usually in the 3000 psi
neighborhood. This is a "28 day" requirement. Concrete continues to
gain strength with age.
I am not a structural engineer, so take what I say with a grain of
salt, but for a rough estimate, you can calculate the weight of the
structure and estimated loading and divide the total by the number of
load bearing columns and figure out the loading on the individual
columns. The psi is of course this number divided by the area of the
All said and done however, here's how I look at things like this. I
ask myself, "Self, if I want to punch a 4" x 4" hole in a slab of 4"
thick concrete using a wooded 4 x 4 post, how hard do I have to push?"
(Or 6x6, 8x8 or whatever)
And the answer is of course, that unless the substrate is mush and you
failed to marry the concrete with steel reinforcing (concrete without
steel is like a day without sunshine), the post will fail before the
concrete. Common structural Douglas Fir for instance has a
compressive strength of about 1200 psi parallel to the grain. (ref:
American Civil Engineering Practice, Vol III)
So your concern should be how many posts and of what size. And when
you get that figured out, you'll probably find that the horizontal
members are the weakest link ;)
It is good practice to put some steel between the columns and the
concrete so if you're still worried, make the plates bigger than the
columns and spread the load over a bigger area.