The subject of failure mechanism is of much importance not only for structural reliability analysis but also for structural design. In the present paper the preferable and undesirable failure modes in aseismic design of steel frame structures are analyzed, and the failure probabilities of the story mechanisms of frame structure are investigated through both computational and theoretical analysis. The story mechanisms are classified into three types: upper collapse pattern, middle collapse pattern and lower collapse pattern. It is found that: the upper story mechanism with a relatively smaller number of failure stories is less likely to develop; for middle story mechanisms with an identical number of failure stories, the reliability index increases with lhe increase of the number of intact stories below the failure stories of structure; and the occurrence probability of lower story mechanism is always higher than that of middle story mechanisms with the same number of failure stories.
