Collapse Indicators for Existing Nonductile Concrete Frame Buildings with Varying Column and Frame Characteristics
Siamak Sattar, Abbie B. Liel
Many existing reinforced concrete moment frame buildings in high seismic regions around the world, including the U.S., were built without adequate seismic detailing requirements. Identifying the level of collapse criticality of these buildings in terms of a set of quantitative parameters, known as collapse indicators, can facilitate efficient identification and classification of particularly collapse-prone buildings. This paper investigates three collapse indicators: (1) the column-to-beam strength ratio, (2) the ratio of lateral (shear) strengths in adjacent stories, and (3) the ratio of column flexural to shear strength. Each collapse indicator is varied in an idealized building, and nonlinear simulation models are dynamically analyzed for a suite of ground motion to quantify the collapse performance. For flexural failures, i.e. those associated with column yielding and weak story mechanisms, the significance (i.e. magnitude) of the deficiency is important, but the distribution or location of the deficiency does not significantly affect response. However, when shear failure modes dominate, the location of shear critical columns, as well as the degree of shear criticality, are critical. The results also show how these observations can be used to define efficient collapse indicators.