Looking for standard or design specs for a turned-down footing or foundation. It's understood that associated with an at-grade pad, there must be at least 2 #4 rebars, one at the top and one at the bottom; and that the footing must rest on stable, undisturbed substrate. But what about its thickness relative to the pad? Need there be an arc to the turned-down outside edge? Are there any practical or practiced options to its construct?