Global Collapse of Frame Structures under Seismic Excitations

Global collapse in earthquake engineering refers to the inability of a structural system to sustain gravity loads in the presence of seismic effects. This research proposes a methodology for evaluating global incremental (sidesway) collapse based on a relative intensity measure instead of an Engineering Demand Parameter (EDP). The relative intensity is the ratio of ground motion intensity to a structure strength parameter, which is increased until the response of the system becomes unstable, which means that the relative intensity - EDP curve becomes flat (zero slope). The largest relative intensity is referred to as “collapse capacity”.

In order to implement the methodology, deteriorating hysteretic models are developed to represent the monotonic and cyclic behavior of structural components. Parameter studies that utilize these deteriorating models are performed to obtain collapse capacities and quantify the effects of system parameters that most influence the collapse for SDOF and MDOF systems. The dispersion of collapse capacity due to record to record variability and uncertainty in the system parameters is evaluated. The latter source of dispersion is quantified by means of the first order second moment (FOSM) method. The studies reveal that softening of the post-yield stiffness in the backbone curve (post-capping stiffness) and the displacement at which this softening commences (defined by the ductility capacity) are the two system parameters that most influence the collapse capacity of a system. Cyclic deterioration is an important but not dominant issue for collapse evaluation. P-Delta effects greatly accelerate collapse of deteriorating systems and may be the primary source of collapse for flexible but very ductile structural systems.

The report presents applications of the proposed collapse methodology to the development of collapse fragility curves and the evaluation of the mean annual frequency of collapse.

An important contribution is the development of a transparent methodology for the evaluation of incremental collapse, in which the assessment of collapse is closely related with the physical phenomena that lead to this limit state. The methodology addresses the fact that collapse is caused by deterioration in complex assemblies of structural components that should be modeled explicitly.