Collapse indicators for existing nonductile concrete frame buildings with varying column and frame characteristics

Many existing reinforced concrete moment frame buildings in high seismic regions around the world, including the U.S., were built without adequate seismic detailing. Quantifying the relationship between a set of quantitative parameters, known as collapse indicators, and collapse risk metrics can facilitate efficient identification, classification and retrofit of the most collapse-prone buildings. This paper investigates three collapse indicators: (1) the column-to-beam strength ratio, (2) the ratio of lateral (shear) strengths in adjacenot stories, and (3) the ratio of column flexural to shear strength. Each collapse indicator is varied separately through a parametric study of 36 idealized buildings, and nonlinear simulation models are dynamically analyzed for a suite of ground motions to assess the collapse performance. The results establish the relationship between the variation of the collapse performance of the building as a function of each collapse indicator, and identify the range over which variation of the collapse indicator significantly influences collapse performance, as well as upper and lower bounds beyond which the indicator is not significant. The results show that the deficiency associated with the column-to-beam strength ratio and the ratio of shear strengths in adjacent stories can be defined efficiently as the average of the associated values in the components at critical floor/story. For the collapse indicator related to the ratio to column flexural to shear strength, the collapse performance is strongly related to the average values in the critical story, and also the location of the most shear critical columns. The study also shows that the gradient of the collapse performance with respect to the three collapse indicators is similar.