Instability of Buildings Subjected to Earthquakes

A method to check safety against dynamic instability of two-dimensional buildings is presented. The method is based on the reduction of the multistory structure into an equivalent single-degree-of-freedom (SDOF) system and on the derivation of statistical expressions that yield minimum base shear coefficients to prevent instability as a function of key structural and ground motion parameters. The results indicate that the minimum strength (base shear capacity) needed to withstand a given ground motion without collapse is strongly dependent on the shape of the controlling mechanism but insensitive to the initial elastic stiffness. On the basis of the latter observation it is contended that safety against inelastic dynamic instability cannot be ensured by restricting the drift computed in a linearly elastic model of the structure. It is shown that the minimum design base shear required by the recommended provisions of the National Earthquake Hazards Reduction Program may or may not provide adequate safety against instability, depending on the available overstrength and on the shape of critical mechanism.