Concentrated plasticity modeling of RC frames in time-history analyses

The study aims at giving an insight into the effects of modelling decisions that are adopted in concentrated plasticity formulations used in time history analyses to model the behavior of reinforced concrete frames, by investigating the sensitivity of the estimated structural response on the assumed length of the plastic hinge region L_{pl} and the effective area moment of inertia I_{eq} of the cracked concrete section. Four frames with 2, 4, 8 and 12 stories, designed in accordance with the Italian Building Code and characterized by a flexural behavior, are taken as case-studies. Structural models are coded in the OpenSees framework adopting various formulations of L_{pl} and I_{eq} taken both from the literature and the European and the Italian codes. The results of the analyses are compared to the ones provided by a distributed plasticity formulation and evaluated considering engineering demand parameters such as internal forces and deformations, and absolute accelerations. The main differences between the predictions provided from the distributed and the lumped inelasticity approaches regard the estimates of the inter-story drift ratio and the maximum base moment, while predictions of absolute acceleration and maximum base shear are found to be more consistent; a certain influence of the number of stories is also highlighted. Eventually, the agreement between concentrated and distributed plasticity formulations can be improved by adopting an effective area moment of inertia of concrete cracked section dependent on the axial load in the structural member.