First, from the screen shot, it appears as if your highest stress is 28.34 MPa (2.8e+1) not 2.8.
Second, why does it appear as if you are applying a vacuum to the outside surface instead of a pressure to the inside surface? (Not that it should make any real difference.)
Thanks for your reply.
I actually figured out the problem soon after I posted this. It resulted from how I was defining the shell face. Apparently to receive the correct hoop stress, you have to select membrane as the shell face.