deckingpressure-treatedshed
I'm building a backyard storage shed that will be supported by two 4x6x16' pressure treated rails laying on concrete piers. The 6" side will be flat against the pier.
At the building supply store I have seen metal 6×6 post bases that seem to be used for attaching a post to concrete:
Since I'm using pressure treated lumber, and the piers stick above the ground a bit, do I really need these? Can the rails sit directly on the concrete? I can bolt the rails directly to the piers with wedge anchors.
As with any advice you get here, keep in mind that you must stay within the bounds of IRC building codes adopted by your local municipality. Obtain a building permit and a local inspector will give you guidelines on minimum requirements for posts, beam spans, joist spans, etc...
Your concrete piers must be a set depth to go below the frost line in your area. You must set the posts on top of concrete with a post base to protect from rot.
When in doubt always make it stronger! Increase to 12" piers with 6x6 posts. Consider using 2x10 or 2x12 beams doubled or tripled on each row. Make sure that each beam span is no longer than 7' if you are worried. When you submit your plan for the permit then the inspector will point out any flaws or deficiencies that you need to address before you start.
NOTE: Any specific sizes I use are merely examples of how one can increase the strength of a structure. Nobody can say for sure without more specifics about the shed design.
Do piers always need to be at the very end? Or can there be a small overhang of wood? Ideally I'd like to have the piers away from the edges of the perimeter so they're not as visible.
When the beam extends past the last post this is called a cantilever. IRC building codes specify the maximum allowable length for live load for different types of lumber, and different sizes.
Personally, I would dig the topsoil out from under the blocks—preferably down to mineral soil so that almost all of the concrete is buried. This makes less of a step up for wheeled equipment and less space for critters and weeds to grow underneath. I put mine very close to the ground on concrete blocks with slots for 2 inch wide lumber and concrete screws through the joist into the block to keep the wind from lifting it up:
The kind of concrete block you are using is plenty strong, though I don't see anything to keep your shed from moving, for example from strong wind, heavy rain, or leaning livestock.
The strength of the joists is the next weakest link in this design. Some more support in the middle could be overkill, but it is unlikely to be so.
Best Answer
You can sit the rails (or beams) right on the piers and level them using shims. That bracket is used for vertical mounting columns.