Abstract
This paper is study of unstability of beam column. The change of bending curvature of steel section under axial load and alternative bending morment when the morment is less than incremental collapse morment that show privious study, are shaking down but when over that, changing curve draw steady roop, in this case the strain are increasing by reversing morment. When we discuss on the unstability of beam column under reversed load, must consider two point, the one is maximam load carrying capacity, the next is shake down load or incremental collapse load. On this point, we discussed in this paper unstability of cantilever type of beam-column. rectanguler section and H section, subjected to axial load and alternative lateral force on top of column, and conclusion are, when the slenderness ratio (λ) of column is small, the critical load of unstability is not maximam carrying capacity but incremental collapse load.
