The frame structures are usually designed with a column over design factor (COF) to avoid undesirable modes of failure during earthquakes. In the present paper COF requirement are investigated probabilistically considering the mass irregularity of the building frame applying first order reliability method (FORM). The study has been conducted with multi storied building frame having regular configuration under given reliability level. The mass irregularity is described by the mass ratio which is the ratio of the heavier mass applied to an arbitrarily selected floor over the mass of the adjacent floor. The mass ratio is incrementally changed to observe its effect on the failure probability of the frame structure and its target COF requirement. It has been found that the failure probability of the frame structure as well as its target COF requirement increases with the increasing mass irregularity of the building frame but the effect of the mass irregularity is not so significant.
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