Will 2x12 lumber forms support my concrete pier?

You have a couple of issues: 1) load that the lumber can support, and 2) fasteners that will support such a load.

1) Depending on the species and grade of lumber, a 2x12 spanning 10’ will support a total of about 3550 lbs. on a 10’ span. (Thats a No.1 grade or Select structural grade.) So, your 5 boards will support about 5 x 3550 lbs. = 17,750 lbs.

However, that’s based on “Working Stress” design in a repetitive installation. “Working Stress” means the load will be there permanently. That load can increase significantly if it’s an “impact load” like snow for less than 30 days, a deck that is occasionally used, etc.

2) You indicate the joists will be fastened with 2 - 5/8” LAG bolts. Assuming the correct spacing between bolts and correct end distance from the end of the 2x12’s, the 2 - 5/8” THROUGH bolts will support about 1,000 lbs. at each end. (For Working stress design with perpendicular to grain loading.)

That means the 5 boards would support about: 5 boards x 1,000 each end x 2 ends = about 10,000 lbs.

So, I’d increase the bolting or provide end bearing for each joist and not rely only on shear from the bolts.

Alternate Idea: You indicate the boards are 16’ long, which I assume about 6’ is being cut off each board. Is there a way the 6’ board could be cut in half and each 3’ piece installed under each end of the 10’ joists to help support it? If the 3’ pieces are installed with the grain running up and down your concrete supports, the allowable load is about double, or about 1,000 lbs. for each 5/8” thru bolt. That could add up fast with just 2 bolts in each 3’ piece.