My limited understanding is that the sucess of the corner joint depends on the formation of a plastic hinge as the shell deforms from the product's hydrostatic pressure while the bottom expands little if any. The width to thickness ratio of the annular plate and the extension past the tank shell both make this possible. For the API method of calculating the annular plate stresses, the wider the width that will project beyond the outer surface of the shell is, the smaller is the maximum stress in the annular plates. There is also a corrosion concern...
