SEISMIC STABILITY ASSESSMENT OF STEEL MOMENT FRAMES AND IMPLICATIONS FOR DESIGN

Although it is clear that a building must be capable of carrying gravity loads while developing large inelastic deformations and associated lateral displacements during a large earthquake, achieving this performance objective in day-to-day practice still represents a major challenge. Current code-based consideration of seismic P-Delta effects is generally based on simplistic elastic models, and despite major advances in seismic systems and analysis techniques, no simple and reliable design methods for seismic stability are available. Specific to steel buildings and the design framework in the United States, the current fundamental approach for stability design was developed and calibrated for non-seismic scenarios where the structure has modest overstrength and the ultimate strength (stability point) of the structure occurs prior to significant inelastic deformation. However, in a ductile steel seismic lateral force-resisting system (LFRS), the design-level forces and resulting nominally-elastic deformations are not consistent with the ultimate strength state of the system, which corresponds to significant overstrength and inelastic deformation. Despite the vastly different behaviors expected in wind-dominated design vs. seismic-dominated design, the same stability design approach is employed. This stability design approach is nominally based on second-order elastic analysis (i.e., in the structural analysis model, equilibrium is formulated on the elastic deformed position and inelastic response is not considered). However, in seismic design it is not rational to consider P-Delta effects at elastic deformation levels. The results described in this paper are part of a comprehensive study that is seeking to identify the most critical LFRS parameters that affect seismic stability and to develop a rigorous yet simple methodology whereby these parameters can be considered in design. This paper focuses on a set of steel special moment frames that is designed with or without consideration of stiffness reduction due to inelasticity, elastic P-Delta effects and drift limits. The moment frame designs are interrogated using nonlinear static and dynamic analyses to assess their collapse potential and to identify the most important parameters for design. The results from this paper will be combined with similar assessments for other types of steel seismic LFRS to propose design provisions that will enhance safety and economy for future design.