Numerical Modeling of Delayed Progressive Collapse of Reinforced Concrete Structures

There has been ever-increasing interest over the past decade in improving understanding of the mechanisms responsible for the progressive collapse of structures. Existing design recommendations and analyses are largely limited to instantaneous (time-independent) collapse. However, recent experiments revealed that reinforced concrete (RC) structures may be susceptible to delayed collapse, prompting the consideration of time-dependent material behavior as part of progressive collapse analysis and design. A reduced-order computational model for delayed collapse behavior of RC structures is introduced here in which the potential damage zones that evolve within a structure are treated as cohesive elements. The constitutive model of the cohesive element accounts for viscoelastic deformation and time-dependent damage accumulation of the concrete and hardening plasticity of the steel reinforcement. The remaining part of the structure is treated as a viscoelastic continuum. The model is first applied to simulate a pushdown experiment on a RC frame subassemblage under displacement-controlled loading. The deformation and failure mechanisms are in good agreement with experimental observations. The model is then used to investigate the behavior of the subassemblage in a "static fatigue scenario" in which the load is monotonically increased to a prescribed level and is then held constant until ultimate structural failure resulting from the assumed time-dependent response of the concrete material. The corresponding timescale of the delayed failure is on the order of hours, a result that has important implications not only for the analysis and design of RC structures against progressive collapse but also for the safety of first responders who enter structures that may collapse within that period of time. (c) 2020 American Society of Civil Engineers.