Sensitivity of Collapse Potential of Buildings to Variations in Structural Systems and Structural Parameters

Abstract

This paper discusses the sensitivity of collapse potential of buildings to variation of structural parameters in different structural systems. In this study, collapse capacity of a building subjected to a single ground motion is defined as the scalar ground motion intensity measure at which the building will become dynamically unstable. The process for obtaining the collapse capacity using Incremental Dynamic Analysis is illustrated. For a given building, collapse capacities of a number of ground motions are used to estimate the collapse fragility curve for the building. Two types of structural systems are considered in this study, moment-resisting frames, and shear walls. Simple mathematical models denoted as “generic structures” are devised to model moment-resisting frames and shear walls. A comprehensive database of collapse fragility curves (assuming a lognormal distribution for building collapse capacity) for a wide-range of combination in structural parameters of generic moment-resisting frames and generic shear walls are developed. Using this database, closed-form equations for median and dispersion of building collapse capacity curves are developed. Such equations not only facilitate the design and assessment processes, but also help in understanding the major trends and importance of certain parameters in changing the collapse potential of a structural system. Comparison between estimates of median collapse capacity using the close-form equations and data show that the presented equations are in good agreement with the data. It is concluded that the primary causes of collapse are severe cyclic deterioration and small plastic hinge rotation capacity of structural components. Furthermore, it is shown that P-Delta effect is the major cause of collapse especially in moment-resisting frames with a small yield base shear coefficient. Another parameter that greatly affects the collapse potential of moment-resisting frames is the ratio of column to beam strength. It is shown that increasing this parameter from 1.2 (ACI suggestion) to 2.4 could increase the median of collapse capacity by up to 90%. In this study, effect of incorporating a vector-valued ground motion intensity measure for estimating the collapse capacity is investigated. It is shown that using the scalar ground motion intensity measure for defining the collapse capacity can lead to underestimation of median collapse capacity by 50%, compared to using a vector-valued intensity measure for this purpose. At the end, a brief discussion of the methods for design for collapse safety is presented.