Blast and Impact Loading and Response of Structures
The partial collapse in 1968 of the Ronan Point residential tower in London led to a significant amount of research on the topic of progressive collapse of buildings. Efforts towards improved understanding of progressive collapse have dramatically increased during the past 20 years as a result of high-profile events such as the bombing of the Alfred P. Murrah Federal Building (Oklahoma City, 1995) and the collapse of the twin towers of the World Trade Center (New York City, 2001). Following the initial structural damage and instantaneous structural response, one might speculate the possibility of further damage that can lead to a delayed collapse. This implies time dependence of load-redistribution mechanisms, which may be related to various time-dependent material behaviors. In the case of reinforced concrete (RC) buildings, these could include viscoelasticity and/or subcritical crack growth in concrete and/or along the steel-concrete interface. Limitations in analytical and experimental studies on progressive collapse resulted in numerical simulations becoming a major investigative tool. Coarse-scale modeling offers promise to become an efficient method for modeling large building systems that balances accuracy and computational cost. In this research, a coarse-scale reduced-order computational model is proposed: a set of nonlinear elements is used to model the behavior of potential damage zones (PDZs) in various RC structural components such as beams, columns, walls and slabs. The time-dependent constitutive relation for the nonlinear elements are developed based on the coupled creep-damage mechanisms of concrete and steel-concrete interface and the relevant constitutive parameters are determined from detailed finite element simulations of the PDZ. Preliminary results obtained using the non-linear element concrete material, under different load scenarios, predicts continuous loss of stiffness and strength reduction due to accumulated damage over time, either under creep or relaxation. This eventually leads to premature element failure, even at low stress levels. Future model expansion will account for the time-dependent bond-slip behavior between the longitudinal reinforcement and concrete, thus allowing the development of an analysis tool that can be used to simulate progressive collapse of structural subassemblies and full scale reinforced concrete buildings. Such a tool will be instrumental not only for advancing performance-based structural design, but also to guide safe rescue measures in the aftermath of events that have resulted in partial collapse.