Analysis of thin shell element for Buckling in Ansys

Hi,

i'm trying to analyse a thin shell hollow cylinder with both sides open for buckling  failure under external pressure. the problem i'm facing is that the model is only bending at the sides and not showing real buckling as it should. Can i obtain buckling results in simple structural analysis? What should be my boundary conditins... and what type of shell elementy should i use, and how should i apply normal pressure uniformly on the outer surface. As i am new to Ansys please guide me wioth suitable steps.

Thank You,

aabhas