Dynamic P-delta effects for flexible inelastic steel structures

Inelastic seismic response of steel moment resisting frame structures, which are usually quite flexible and subject to large displacements under severe ground motions, may be severely influenced by the structure P-delta effect. For structures in which this effect induces a negative postyield story stiffness, as seen from a pushover (story shear-story displacement) curve, the response becomes very sensitive under ground motions that are sufficiently strong to drive the structure into this negative postyield stiffness range. The response under such conditions may be highly nonsymmetric with progressive displacements to one side, which increases the collapse potential of the structure in a sidesway mode. This paper focuses on two case studies that (1) illustrate P-delta sensitive behavior; (2) demonstrate the sensitivity of predicted behavior to ground-motion characteristics and analytical modeling; and (3) contribute to the development of engineering guidelines for protection against excessive P-delta effects. A relatively simple procedure is proposed for identifying P-delta sensitive cases, and recommendations are made for improving the P-delta response of flexible steel frame structures.