Instability of buildings during seismic response

The issue of gravity induced instability during response to severe seismic excitation is examined, It is contended that while static instability is fully determined by the existence of at least one negative eigenvalue in the second-order tangent stiffness, this condition is necessary, but not sufficient for instability during dynamic response, It is noted that the likelihood of collapse is strongly dependent on the shape of the mechanism that controls during the critical displacement cycle and that this shape can be reasonably identified using a pushover analysis with an appropriately selected lateral load distribution, A characterization of the instability limit state based on reduction of a multistorey building to an equivalent SDOF system is presented.