reentrant corners
 

reentrant corners
what is the difference between a sharp corner and a reentrant corner?

a sharp corner has a sudden change in direction, like a 90 degree corner, as opposed to a slight radius or fillet. it will have a point at the change in direction. a reentrant corner is an inside corner. if it were a slab for instance, at an inside corner you would have 270 degrees of slab and 90 degrees of not-slab, the corner points to the inside of the building. an outside corner is the opposite.
ucfse said it well. additionally, some form of crack propagtion control (jointing, reinforcing, etc.) is warranted at a reentrant corner.
another purpose of reentrant corner is to
reduce the stress concentration, which is the primary cause of crack at corner.